Johnson Viking Thunderbolt

This has been a 2+ yr project and am pretty close to making my first signal with it, but as Im not too good with user manuals, having trouble figuring out what to set the knobs to and what to look for with current on the meters.

This ones been modified from AB2 to AB1 somehow that I am not sure of, but the manual says once you hit a certain
point the amp normally transcends to that mode anyhow. Its a typeset manual from 1959, so theres a bit of a generation gap
for sure.

I have a bunch of pictures here : http://www.mediafire.com/folder/7plc...vz/THUNDERBOLT
(as well as a schematic in PDF)

Its using an external Heathkit KS-1 KV power supply , which originally went with the Chippeawa matching amp. I have
a seperate 220V line for it, and confirm its putting 3486Vdc into the amp. Word has it that the stock transformer in the
Tbolt was the bare minimum to drive the 4-400A finals, so this was someones idea to get more power from the amp, and
apparently it worked wit no arcing on the plate tuning caps, which was I guess a big concern.

Right now, I can power it up - fans run, all filaments light, had to replace 2 dial lamps but that was no big deal.
I have not measured the screen voltage yet, but there is an external bias cable that has to have 2 pins shorted for it to be in standby. The manual says to use a T-R relay, which I am doing, so that part seems all set. Im told with NO plate voltage applied, I ought to have no meter movements for any current - and that has been confirmed. When I apply plate volts, the meter of course peggs, because its a 2kv max meter movement, but when I choose curent, it shows about 63mA during standby. Curious if this is normal before I go further and try to load up. I'm told my plate ought to be 700mA to 1A with
the higher KV from the external PS. The bias cable I guess gives the final tubes a little negative bias to keep it turned off,
so I am thinking maybe the 63mA is normal as its I guess nothing compared to the 700-1000 mA the engineer told me ought to be seen during normal operation.

I'm no expert, but friends call me Tube Man for a reason, I used to be a walking tube manual. It's been a while, so having lost my tube manuals in transit I rely on scanned pages on the Internet. Yep, that's where to look for Everything You Ever Wanted To know About Tubes But Were Afraid To Ask. https://duckduckgo.com/?t=ffsb&q=4-4...cations&ia=web (Just my twist on a very funny 1972 Woody Allen movie.)

You're headed in the right direction, Johnson skimped on everything since the original Viking transmitter, IMO the only good thing besides the matching VFO they ever made. I see so many shortcomings in that amp that I would do a major rebuild to bring it up to my standards beginning with those cheesy 500pF 15KV B&W TV doorknob caps and that pathetic fan that just have to go. There are proper .001uF doorknob caps available, (Johnson paralleled 2 to make .001uF) and I'd give particular attention to the plate bypass cap on the cold side of the RF choke, if that's another TV cap I'd replace it with a .01uF doorknob.

Pressurizing the underside of the chassis with a squirrel cage blower made for the purpose will keep the critical base seals temperature down. If you can fit recommended ceramic sockets with mating glass chimneys for the fire bottles do it, but I have my doubts, they look too close together and too close to the cabinet. At least there are much needed finned plate caps on them, anode seals are also critical.

Oh BTW, plate and screen voltage under load is what you really need to know, but you won't until you fire it up into a dummy load. 1A is WAY too much under ANY circumstances, as voltage goes up current goes down (V X I = W). For 2 tubes at 3500V plate, 500V screen, and -170V grid you get 500mA DC plate current. Back in the day 1KW DC input was FCC maximum, typical RF output is 500W with a well designed Class AB1 amp. I hope you have one that can handle a killerwatt, your fellow hams will appreciate it.

Since you are almost there, whatever you do don't get anxious to put it on the air and hurry to completion, remember haste makes waste and GOOD LUCK. As a boat anchor man who unfortunately had to downsize before turning to paper I like what you're doing, keeping vintage technology ALIVE. One last comment that will save you from a killer electric bill, unless you need to overcome an AM Gangsta's high noise floor more than 100W is wasted. Operating K2PG when I lived in West Creek, NJ I was Phil's DX magnet using his Drake TR-4. The first trick in the book is patience, tune around listening for his CQ and jump on it before the dog pile finds him. By the time the DX is posted on an Internet reflector there is a heavy dog pile already in progress, ignore the reflectors. Once that happens all is lost unless he's working split and you have dual VFO capability. Find the guy he's in QSO with, that's your TX frequency, as he signs step on him, it's called coat tailing. (Nobody will hear such a dastardly act, hi.) Don't ever try to be top dog in a dog pile even if you have your killerwatt at the ready, you'll NEVER break the California Kilowatt Curtain. On 20M I tried with a Collins 30S1 into a 3EL tribander up 60ft (optimum for that band) and failed miserably. That one time taught me not to waste electricity. ('->) If it makes you feel better the left coasters have the New York Kilowatt Curtain to contend with... heh, heh, heh.

Warren

Thanks for the encouragement and suggestions. I was told the doorknob caps were a bit off when I bought it, so I got a couple of NOS ones
and put them in. They arent much better but are original Sprague 20KV of the same kind. Apparently they are the "padder"caps in the tank.

Last night I wanted to get some voltage readings on the screen so opened the back, and had trouble. Realized that a metal tab I bent back into place on the cover to help guide the long screws to the front, interferes with one of the tube anodes when you have the linear on its side, which I so to make it easier to get the cover off. I was too late as the tab hit the anode, cracked the glass right off the top - so am down (1) tube.
Fortunately the owner threw in (2) spares of the same kind, Eimac 4-400A's. These are identical except the plate shape is a rounded crown
not like the others with have a sharp square crown. I read the sharp square crowns dissipate heat better, but for now will have to go with what I have.

When I powered up things seemed ok then a loud HUMM from one of the xfmrs prompted me to shut down. They are using an octal socket populated with (2) diodes in place of V104 and it had shorted. That supplies volts to the VR75 neon regulator tube that I believe puts the standby bias on J102 so that when you short the "block" to "grid" pins the tube has some bias. I have read that not having the block
pin tied causes stress on the tubes, as per this post:

http://www.qth.net/pipermail/johnson...il/001006.html

So that could be what happened, somehow I was operating in the wrong state and overloaded the output line of VR104. I replaced both diodes,
and powered up briefly to see all the tubes lit up again.....so all was ok. Didnt have the bias plug in but the test was very brief as to not load things down too long. That article does say the version II placed a resistor on the control to ensure proper bias always, but as its a value for the new kinds of tubes, I am not 100% sure that would be the ideal value to put in my version.

Hi again Stephen,

Here comes an old buzzard transmission, don't fall asleep, OK?

Before I begin I'm putting out a CQ for a little help here, and a suggestion that you join the AM Gangstas here http://amfone.net/Amforum/index.php These guys really know their stuff, many are also broadcast engineers from the daze of tube transmitters. I know a LITTLE about Collins 1KW AM BC transmitters having repaired one re-tuned for 160M. Then the crazy dude moved to Pennsylvania and put a Collins 10 KW transmitter on 160... Can you hear me now?

OK, an uh oh, you were told those caps were a little off because they're the wrong caps and doubling the working voltage does nothing for the capacitance value, THAT'S what IMO should be changed. Not absolutely necessary because they total to the correct .001uF value, but using the right tool or part for the job is a little quirk of mine. I'd have a look at that blocking cap on the cold side of the plate RF choke, it should be .01uF. Since you have a no load supply voltage of 3.5 KV in round figures filter caps in the HV circuit should be 5.25KV or more, and ceramic doorknobs 7KV or more because RF voltage doubles at 100% modulation. Your guess about padder caps is as good as mine, but I hope there aren't any because I had so much trouble with them in 100W Johnsons, and here we're dealing with 2,000W P EEK! I mean peak aka PEP.

The worst I worked on were Valiants, padders were bad enough, but they used one of those 500pF 15KV TV caps for the plate coupling cap. Putting 3 6146s in parallel gives 500 ohms or thereabout impedance that calls for .05uF at 1800KHz the bottom of the 160M band. If that's not bad enough the pi network matching to a 50 ohm load has an impedance ratio of 10:1 which makes for lousy harmonic attenuation or in other words, a dirty transmitter. Like I said, their first transmitter, the Viking 1 was their only good transmitter. They loused up a good design with the Viking 2 by reducing the capacitance of the loading air variable and making up the difference by switching padders in parallel with it. OK until fixed caps start shorting and the rotary switch arcs over melting contacts.

Kinda funny you should mention those long case screws, all but one of the Rangers and all of the Valiants I worked on had them and most of the short case screws missing. No rocket science here, Johnsons are so problematical previous owners repaired them so often they got frustrated and threw the screws away! That made for a mighty CLUNK when the transmitter was keyed, it was the magnetic pulse from the HV transformer momentarily pulling the top of the case down. I don't know if that will happen to the Thunderbolt on key down, but removing the long bolts and bending the tabs flush will prevent shorts and broken tubes.

Somebody gave you a bum steer with the anode crowns, they contribute nothing to cooling. If they did all of them would be flat, and in fact all of the 4-400As I saw, all 12 of them had rounded crowns. The fins do most of the cooling, but then 4-250s have no fins and glow just as orange as the 4-400s. I spent many a Saturday with the Chief Engineer of WERA in the transmitter building before the station went dark. The main transmitter was an RCA BTA1R 1KW unit operated at half power, and every other month the BTA5R 500W standby unit became primary to level wear. All 4 of the 4-250s glowed as they should, unlike receiving tubes that have an internal silver (barium metal) flashed getter that prevents the tube from getting gassy, the getter in these transmitting tubes is the grey coating on the anodes (plates) that only works when heated red to orange. When a 1KW transmitter with 4-400s is operated at reduced power only the modulators glow. Apparently Bob, that CE hadn't read The Care And Feeding Of Power Tetrodes published by Eimac, when I told him to switch the modulators with the finals when the main was standby I had to explain why. (;->)

Some guys are overly touchy about operating boat anchors made to operate from 110 and/or 220V mains on modern 115/230V mains, I never gave it a second thought. They're just as afeered (;->) of replacing vacuum and mercury vapor rectifiers with solid state, I never gave that a second tought either and I never saw the magic smoke escape. Neither did the CE of WMTR, K2PG acquired the old Collins 20V transmitter with home made solid state plug in cards in it. Mercury tubes have a bad habit of flashing over tripping the HV circuit breaker/switch and the CE got tired of late night trips to the transmitter plant to turn the bloody thing back on. Phil's Collins 30S1 KW amp also had commercial plug in solid state rectifiers, I preferred to make my own as needed. I used 1N4007 1,000PRV 1A diodes shunted by .001uF 1KV ceramic caps and 470 ohm 1/2W resistors, as many of these assemblies in series as the circuit voltage required. The caps suppress spikes and the resistors equalize voltage, good protection for the diodes.

VR tubes are rated by the VR number, the VR75 holds voltage at a constant 75 volts as part of the bias circuit I believe. The operating bias of -175V is made up by grid excitation rectifying some of the RF to supply the additional 100V. I'm not sure what that post is all about being small difficult to read print with 70 year old eyes. (;->) It said something about a Ranger, they put out around 60-75W that is barely enough to excite the Thunderbolt, actually it's the mating exciter to the Desk Kilowatt that also has 4-400As in it, only it's a modulated final that gets audio excitation from the Ranger's modulator. Johnson skimped on THAT power supply too, at full power HV flagged causing audio to sound muddy, half power OK. Back to the thundering box of bolts, like any amp it must be interlocked to the exciter, ALC fed back to keep SSB under control. The Ranger and Valiant used grid block keying for CW, I don't know about extending it to an amp, but in all cases an exciter and an amp work together as one.

There are always mods for boat anchors, some factory and some "aftermarket". Putting a resistor in series with a pot is common as pebbles on Pebble Beach. 4W drive pots in Rangers and Valiants kept burning out, we not only replaced them with 10W pots, we lowered the current by putting 10W wire wound resistors from the cold side to ground, plenty of drive and no more burnouts. What new kinds of tubes? If your amp has PL-175s instead of 4-400s not to worry, it's a direct replacement that in Class AB1 operation can provide 20% to 40% greater output. The suppressor grid operates at zero voltage, and is terminated at the base where it is grounded. The PL simply is the manufacturer, Penta Laboratories. https://duckduckgo.com/?t=ffsb&q=pl1...e+specs&ia=web

YOUR version? I can barely get my brain around Johnson's version, now you say you have yet another version. I made my own versions of two ham rigs, I stripped an Eico 720 down to the bare chassis and built the RF deck for a 100W AM transmitter operating on 1340KHz on it. Then I stripped down a Globe 65 and rebuilt it as a 50W exciter operating on 107.9MHz. I had the push-pull 4X500A 1KW final almost finished when the FCC told us if we want to broadcast get ham licenses, we did.

Hi Warren

Long but good read, thanks for all the feedback, it will come in handy for sure.

When I replaced the 2 diodes that make up the new dual diode tube, thought about the snubber caps and resistors across them. I have heard they protect the diodes from surges and also help reduce ringing which probably also helps lessen IMD.

Turns out another Ham in Texas KD5WT tested his Tbolt and he does have a "hair" of needle movement on the mA meter for the plate in TUNE.
Perhaps not as high as mine, but I am gonna figure with the higher plate KV and the possible mod from AB2 to AB1 perhaps thats normal for the TUNE position.

Tried to resume testing today but the KV supply was being overloaded, and it threw the breakers in the panel. At least I know the breakers in that box work now. I was bummed that something else went out on me - however it was the safety interlock providing the dead short on the
HV line the way I guess Johnson intended it to be. Makes me wonder how often people did that back in the day, which for sure puts wear and tear on the Thunderbolts PS.

The HV supply, a Heathkit KS-1 has (2) round glass type 15A fuses on the outside of the chassis - they are the kind used in older houses AC line fuse boxes back in the day. When they blow you can see the broken metal line inside. These have been hard wired short at the base pin to the screw thread. Id like to put breakers in there, but I hear those kind are unreliable.

After defeating the interlock, I powered up just the filaments with the bias plug in and no HV just to make sure all is well, and its looking ok again. Tomorrow will apply the HV in standby and maybe get brave and apply an input RF signal.

Hi Stephen, Peter Greeder the meter reader & other readers,

Another long one, but you like reading.

The last go around was a bust thanks to the (insert expletive here) WiFi here that kept switching channels that causes glitches. It must be gremlins, more on that later.

I used 1N4007 rectifiers in series as needed. The suppressor caps bypass spikes that would short the P-N junction putting excessive load on remaining diodes leading to cascade failure. What protects against surges, especially hard start inrush current charging the filter caps is a low value, about 3 ohms, 1 or 2W resistor in series with the common point(s) in a full wave configuration. I never profess to be an expert, I'm not a hated know-it-all either, the only ringing I am familiar with is in L-C filters in analog audio equalizers. IMD short for InterModulation Distortion is a product of poorly designed power output stages in amplifiers, in this case a nonlinear linear amplifier, or "leenyar" in CB lingo characteristic of Texas Star and Dave made pill boxes. (RF power transistors = pills)

Vacuum tubes being voltage devices conduct more at higher voltage, that's why your resting current is higher. This one I had to look up because I'm not the walking tube manual I used to be:
Class AB1: Class AB1 is where the grid is more negatively biased than it is in class A. In Class AB1, the valve is biassed so that no grid current flows. This class of amplifier also gives lower distortion than one running in class AB2.
Class AB2: Class AB2 is where the grid is often more negatively biased than in AB1, also the size of the input signal is often larger. In this class grid current flows during part of the positive input half-cycle. It is normal practice for the Class AB2 grid bias point to be closer to cut-off than occurs in Class AB1, and Class AB2 gives a greater power output.
Credit to: https://www.electronics-notes.com/ar...-f-g-i-s-t.php

Ah, you know the breakers work as intended and are far less fussy than overload relays most hams would trip every time they tune up. I blew the finals in a Drake TR4 because the owner never removed it from the case and cleaned the dust out of the final compartment. He wasn't happy with me, but when general replacement transistors gave him 20W more RF he was happy with that. I didn't have the heart to tell him it made not a needle's width of difference to the receiving S meter.. (;->) I'm not surprised Johnson's cheap and dirty safety interlock shorts the HV line to ground taking the place of the Jesus stick in old tube type broadcast transmitters, Jewish engineers call it a shorting stick. ('->) Collins on the other hand employed the same system as their broadcast transmitters, the safety interlocks removed AC mains power leaving the bleeder resistors to drain the LV and HV caps. Then if the stick made a BANG when shorting a DC line there was an open bleeder resistor to find and replace. Drawing on a bad experience in my feckless youth it's better to put wear and tear on the power supply than on the operator or technician!

I grew up with those glass Edison base fuses and the unreliable screw in CB replacements with a reset button in the middle. They're unreliable, especially in summer heat because high ambient temperature lowers their trip current considerably. Since I had a source where stray single hole mount push to reset breakers followed me home (also Walmart sells them) I made good use of them in mine and transmitters in my basement repair shop. Good old Johnson did it again, two fuses in a non polarized line plug and another in a clip under the chassis difficult to get at. I replaced the line cords with 3 wire cords having U ground polarized plugs that also followed me home and appropriately rated breakers, if one pops again it's time to look for the short.

Since I have a SERIOUS dislike of defeating any safety device PLEASE don't do that again, we'd like to have you around here for a long time to come! In my feckless youth I put my guardian angel through some serious changes, I wonder how I survived. Then I grew up and left Children's Band for Amateur Radio, my angel breathed a sigh of relief. Oh they do breathe when they solidify and they don't have wings, they're pictured that way because they come from the stars. Being they work undercover they must pass for human. A wise man passed on a wise saying to me when he said "Always be kind to strangers, you never know when you're talking to an angel.". Without getting TOO preachy on you, atheists don't know what they're missing, my best friend Scott W2SJW married an angel and she turned his life around.

Oh, here's a gremlin. If you see one in your radio room kill it before it destroys something!

Attachment 17481

Hi Warren

Ok good stuff on AB2 / AB1, very understandable the way you explained it.

Got brave and tried loading up on 80m today and adjusted to 300W carrier as that's the most the balun can handle in the Windom.
Checked into a net, did 2 rounds, got good reports, so its a start in how the controls work and am happy. :o)

I was told the amp may work on 160m (even though its not designed as such) if more capacitance and inductance is added. So
I thought I would try it with my Dentron MT-3000A tuner as its big and has lots of inductance and capacitance. Unfortunately I guess not enough. I could not get a dip and the tubes were cherry red most the time fiddling around, so I guess for now I can do 80-10m which is fine.

I have ran into some stuff online where a relay was added in to switch in the extra inductance and capacitance to get it to work on 160m.

Now that I think about this, I did change the door knob caps (C19, C20) which are in parallel to form 0.001uF which you had mentioned prior.
The replacements were new old stock, but when tested , are better than what was in there, but still not too close to 0.001uF when you do the math. These capacitors lower in value as they age apparently. I am wondering if these were bought closer to the intended value, maybe my tuner would be able to pick up the slack.

You had mentioned TV style , versus another style. These are I guess the TV style. I have a photo of the originals here:
http://www.mediafire.com/view/qgrel4...dders.jpg/file

They are referred to as the padder caps.

A big Kermit T. Frog YYYAAAYYY!!! If you want to squeeze another 3dB (1/2 S unit) out of it and make your electric meter spin faster there are plenty of balun projects to be found. BTW I found the ex WERA transmitters pretty much destroyed by clods who are clueless that they could have been re-tuned to 160M without butchery. Like Scotty said about his overloaded warp engines, "Me bairns! Me poor, poor bairns!" That's Scottish for babies.

What did you do to the Dentron trying to get the T Bolt to tune up on 160? That can never cut the muster until you pad down the plate and load caps in the amp, then if your signal spitter is up to it things may work for you, sort of. A dipole type antenna will only have a low vertical takeoff angle for good DX when it's up at least 1/2 wave, but you'll only find out what you have until you taste it and see. (Only an expression, RF burns like hell.)

The only caps I know of that lower in value as they age are old electrolytics, they crap out when they dry out. You have to be careful when using single caps to pass RF current, it's not too high in the plate circuit where impedance is high, but it gets touchy in the load where impedance is low and several amperes may be encountered. Phil K2PG found that out the hard way when his Collins 20V 1KW B'cast XMTR tripped out, I found bits of exploded doorknobs in the bottom of the ATU cabinet near the base of his tower used as a grounded vertical folded monopole, an antenna used in AM broadcasting shorter than a series fed tower thus saving expensive steel. I fixed that problem when I found 3 doorknobs totaling the one that exploded, each shared 1/3 of the current. He's a broadcast engineer who goes by the book, I'm a ham who went with what works. Went past tense, now where I can't set up a station I'm a papaer ham, but can still help a little, not much, a little.

I don't know who fed you misinformation, those doorknobs CAN BE USED as padders, but their original purpose when Johnson cheaped out on everything was high voltage filter caps in early B&W televisions. Notice the value is in micromicrofarads rather than today's picofarads. From around 1960 (?) onward picture tubes themselves became HV filter caps, not much capacitance was needed to smooth the 15KHz pulsing DC from the half wave HV rectifier. Yeah, at one point I was a TV repairman struggling along when the partnership broke up and I closed the doors on today's throwaway society.

Well, this looks like the end of my Johnson adventure, have fun with it, but in closing let me remind you of something out of The Care And Feeding Of Power Tetrodes by Eitel McCulloch aka Eimac. Those anodes are supposed to glow cherry red to bright orange under normal operation. The dull coating is the getter, a chemical that "gets" vestiges of gas out of the tubes. If they don't glow eventually they'll get gassy and crap out. Good luck friend and happy hamming to ya!

Warren

Ok good info once again, thanks.

From what I hear of the 4-400A's they will glow a somewhat cherry red / orange right at or near their max comfortable operating conditions.
So in my case for sure it looks like a bias problem as in the meantime (2) of the gas regulator tubes, a 105 and a 150V type had white film
inside the glass , indicating a bad seal. Today I have got some NOS replacements, plus a new 5U4. Though my 5U4 is ok, its also a 1959 date code... (back when sunspots were supreme) so figured may as well because of sheer age.

Also pondering the 160m mod in the back of my mind. Am told there is a Jan 1991 issue of electric radio that covers a mod like this to the Johnson desk KW amp and its design circuitry is similar. Would be interesting to read that, however looks like it would have to be purchased as a back issue.

I am thinking that my plate choke L7 needs another 25-50 uh or so. Its a National R-175A I am told, and apparently if I can find a way to add some more turns in line with it , enough to cover the extra L, keep it shorted with a relay until needed, would be the goal.

Attachment 17501

I also think we need more coupling capacitance, and the coupling switch SW2 seems to be a good spot to put it.
I have seen others using HUGE vacuum capacitors in their mods, but I think thats kinda overkill here and since I have a few of the TV doorknob type caps, I could put them in series to get another 170pF or so with some jiggling around. They are overkill at the rated 20KV, but I may be able to finagle them onto L6 which shares a common connection. The relay connection may be a little tricky but as long as I remember to engage it first BEFORE keying up, that ought to greatly diminish the potential for sparks, flames and or smoke....

Hi again Stephen,

Here's another long one to read in your spare time. You can publish my works not copyrighted, make some money to finance your boat anchor restorations. Those big vacuum caps command big bucks proving once again Amateur Radio is a rich man's hobby except for us, champions at the sport of Dumpster Diving. Phil K2PG, Tubewormius Splatasaurus, invented New Jersey Mud Diving... a long story in itself.

I thought the project was finished, I now see not. Thinking back to my daze hanging around WERA and observing how the RCA BTA1R 1KW operated at 1/2 power behaved here's the scoop on the 4-400As. There were 2 in parallel RF FPA modulated by 2 P-P, the modulators glowed because the DC voltages were in normal operation while the audio voltage was half. The finals operated at the 500W output level (BC transmitters are rated and operated at specified RF output) didn't glow which prompted me to tell Bob the CE to swap them when the BTA500R was operating every other month. It BTW used 4 4-250s and all glowed. So the bottom line is such tubes should show color when operated at their normal operating voltages. Since I only saw them in CCS plate modulated Class C service at WERA (also ICAS in a Collins 20V as the K2PG 160M transmitter) I can't say anything about Class AB2 SSB/CW service with only a 20% duty cycle, but likely with a couple of WA1HLR atomic YYYAAAYYYLOOOs they'd glow momentarily. (;->) Timtron is living proof that genius and insanity go hand in hand.

Yep, that white film is barium oxide, it used to be silver barium metal flashed on as the getter. You're right, the seals are broken and air entered the rarefied neon or argon atmosphere. Since there's no way to test them I know of my test was if they light up inside the cathode cylinder they're good. Ah yes the 5U4 full wave vacuum rectifier the previous owner used in place of the stock 5R4. My best guess why they were in so much Amateur gear is they were dirt cheap JAN (Joint Army Navy) surplus, but the 5U4 common in TV sets is actually rated higher than the 5R4. I was awash in tubes I scrounged, tested, and sorted, I could afford destroying them in my power supply with the output shorted. ('->) It doesn't matter when a tube was made, unlike electrolytic caps and carbon composition resistors they don't go bad in storage. Old Timers will tell you that 1957-1959 was the highest sunspot peak and the best DX in radio history. FYI 1957 was IGY, International Geophysical Year that saw a massive spike in what today is known as Earth Science, good reading if you can find what's left of scientific papers from the studies.

"Also pondering the 160m mod in the back of my mind."
Are you pondering what I'm pondering Pinky?
Uh, I think so, Brain, but we'll never get a monkey to use dental floss.

Electric Radio, in the Victorian age it was the age of coal and steam. Naturally G1AVQ like all other radio hams built stations along the Radio Steampunk design, and this one is fully operational. Here's where things get sticky, I don't know if you're building in another Frankenstein Laboratories where I built an ATU for a friend out of parts from a 10KW AM transmitter, or if your grandfather was Sweeney Todd The Demon Barber of Fleet Street. (;->)
"I am thinking that my plate choke L7 needs another 25-50 uh or so."
Why?
"Its a National R-175A I am told"
If it looks like the picture you were told right.
"and apparently if I can find a way to add some more turns in line with it"
Not on that form, there is no room, and the tab on the mounting bracket is where the plate bypass cap goes. That's where Johnson effed up again with a single 500mickymike (that's what we called them before puffscofarads came along) TV doorknob, it should be .001uF at LEAST.
"enough to cover the extra L"
If you think it's necessary you can sky wire (remember there's high voltage here, hi) a 50uH single layer solenoid wound choke in series. In a way I miss the old days when I had scads of them and other salvageable parts clipped from discarded TV chassis.
"keep it shorted with a relay until needed, would be the goal."
Relay not needed, shorted not needed, this is not a resonant circuit, just 2 chokes in series. (145uH + 50uH = 195uH) I lost count of the finals I built up to 100W using 1mH 3 pie & 2.5mH 4 pie RF chokes, they're good up to about 900V or so and wasn't concerned about distributed C in low power low frequency circuits. The R-175A choke is a revised version of the R-175 choke. The revision has made the reactance of the choke high throughout the 6 meter band as well as the 10, 15, 20, 40 and 80 meter bands. The R-175A choke is suitable for parallel-feed as well as series-feed in transmitters with plate supply up to 3000 volts modulated or 4000 volts unmodulated. Inductance 145uH, distributed capacitance 0.6 pf, dc resistance 5 ohms, dc current 800ma, voltage breakdown to base 12,500 volts.

You're right, you will need more coupling capacitance, perhaps another 500 puff TV doorknob. (;->) Here voltage is less of a consideration because the circuit is designed to match to a 30-600 ohm load, 50 ohms if you have a 1:1.5 or less SWR at the output terminal. Here is where RF current rises sharply, not having a BSEE I can only guess somewhere between 5 and 10A. This is why there are several small values in parallel to distribute current between them like I used to replace the exploded cap in an ATU. Right again, at SW2 to keep them all together. I absolutely ABHOR butchery, adding another switch on the front panel is butchery, changing SW2 to a 6 position shorting type ceramic rotary switch is a neat and clean job, but a bit more difficult than at first glance. Another of Johnson's bad habits was double front panels with everything mounted both on the front and on the sub panel behind it. Yeah, either way you'll have to disconnect everything from the front panel and remove it to get at the sub panel behind it. (:-<) <groan> Another idea is your relay (preferably ceramic insulated for low loss if you can find one today) operated by a switch on the rear chassis skirt, far less obtrusive than cutting a hole in the front panel. Mounting the switch on the front introduces problems of its own like a bowed in panel unless you mount it the same way as everything else and it still requires removing the outer panel. Bearing in mind we're dealing with fairly large circulating current here I'd use a DPST NO relay with contacts rated at 10A and wire the 2 sides in parallel, that should do it. With the rotary switch set with all of the caps in circuit you'll have to experiment with the value of the extra loading cap so the variable tunes up at or close to center rotation, it's good to have a little wiggle room on each side. From Warren's Hints & Kinks, an air variable of known maximum capacitance connected temporarily makes taking a good guess at the final fixed value easy. A lyric from KC & The Sunshine band says it all, "That's the way (uh huh uh huh) I like it.". Irony has it that when that amp was made before KHz it was KC, so let the sunshine in. (Another stolen lyric from The 5th Dimension.)

"The relay connection may be a little tricky"
Not at all, it's eminently practical, it only takes a small hole drilled in the chassis to mount a 115VAC relay and some 14ga bus wire to connect the contacts, then wire the coil and switch to the mains hot and other side of the coil to the chassis. If the mains input is wired correctly with neutral to chassis you'll get 115V from hot to chassis regardless if the transformers are wired for 115V or 230V primaries. Unfortunately there is no DC of the correct voltage or low voltage AC available, so if the relay buzzes flip the switch and it should quiet. AC relay coils are shaded pole like small motors, but the armature doesn't always land solid.

"but as long as I remember to engage it first BEFORE keying up, that ought to greatly diminish the potential for sparks, flames and or smoke...."
The rationale of a killjoy. (;->) That would not allow for an interesting sign on, "With a flash of flame, a shower of sparks, and a puff of smoke KA2PTE is ON THE AIR! Hello CQ CQ CQ, hello CQ, how are you today? CQ CQ CQ...". The only thing missing is The William Tell Overture, you may recognize that as the intro to The Lone Ranger radio play and later TV series. QSL Kimosabe?

Attachment 17502 Attachment 17503

Once again Warren thanks for the very useful info.

Turns out a new set of hurdles has arose. So plans for the 160 mod are postponed.

Apparently one of the set screws in a 4-400A heat sink was loose, so that explains why one tube was more eager to glow red
under low power levels - starving for plate current most likely. So I resolved that unusual issue I believe. But before I noticed that,
had attempted a tune on 40m, and encountered some xfmr humm and other not sounding nice noises, so shut down.

Apparently part of some kind of mod, this NP 600V / 0.5u F cap leaked oil -

https://www.mediafire.com/view/olwny...l_cap.jpg/file

It was hooked from ground effectively in parallel with C117 near the bias plug jack. Measures a bit high ESR, so tossed it.
Someone speculated its an anti-humm mod on the screen supply but it is connected on the output of the VR75 regulator tube which I think delivers -75V to the system in certain modes.


As this Amps been modified, I set out to update the drawing and have an updated version in this schematic:

https://www.mediafire.com/file/c59nb...2PTE_.pdf/file

As it stands now in CW or LINEAR mode the voltage regulator tubes go off, which means they are
encountering an overload. I get just about the entire supply volts as a drop
reading on R115 (4k) which concurs the tube string is seeing too much load
or perhaps one or more of the VR tubes is weak.

Default voltage readings as per the manual are in spec, except when you
place tubes V1 and or V2 (4-400A's) in place.
Somehow with them in the sockets, standby current is drawn from the VR tubes, but
cant trace back to see which way. With this modded chassis, they are actually measuring
cathode current on the meter now, not plate. At 75mA or so this seems awfully odd.


Trying to figure out the mode switch is sort of painful - its got (3)
commons - 1, 5 and 9.

All tests by default are with only the filamant xfmrs powered, no HV 3400V
applied.

Even if the screen volts is making it to the tube pins 4 or 2, cant see how
the plate mA meter is reading like 60mA.
They modified the plate meter to be reading cathode current by lifting the 2
common heater connections from ground.

From what I have read, with no HV applied the screen is more or less a sub for the plate
and some current flow through is expected. I'd think perhaps not as much as I am seeing now
as there has never been this kind of current reading at idle ever. I have (4) tubes, and they
all read the same thing so I dont think there is tube failure.

The original owner of the amp said this of the mod:

"Please note that the 4-400A parameters have been rejiggered for class
AB1 operation. The original design operated class AB2, grid current on
voice peaks approximately 2 ma. Absent an input signal the class AB2
mode resulted in substantial resting plate current. After the internal
power supply zorched itself, I was gifted a KS-1 power supply which
puts out 3000V. The original PS ran around 2400V. This was an
opportunity to switch from AB2 (grid current, distortion, etc) to AB1
which for all practical purposes runs at zero grid current and requires
less grid drive power while reducing distortion. The input to the
T-Bolt gives a choice. Either use the tuned circuit or a 300 ohm
termination resistor as seen on the diagram. In AB1 the driver doesn't
suffer from the grid current induced non-linearity of AB2.

Results: Grid bias increased and screen grid voltage increased."

I seem to concur seeing this when I had ok results testing the amp before
something went wrong. Though for some reason, the screen current never ever
registered anything at all, till this odd situation.

Now its got that "substantial" resting plate , er cathode, resting current
he described for AB2 possibly. Could something have changed the config back toward AB2 ?
That oil cap perhaps?

Also another question is, the new resistors I found (R116A-B) in the updated schematic,
do they look like they are modding to AB1 operation?

Apparently you can swap out the VR75 tube with a VR105 and even a VR150 and that lowers idle current
in the factory AB2 config.

UPDATE:

I concluded the NOS VR tubes must have just went over the cliff on me and cant supply much current under any load anymore. Supplier is taking them back for an exchange, but I have more on the way from another source just in case.

If the tubes do not work, the raw filtered DC supply volts is applied directly tot he screen. The tubes only rated at 600V for the screen max,
and I imagine theres likely 700-800V once the filter caps do their thing from the tube full wave diode. Its now acting like the plate and makes
current draw on the meter of about 70mA or so. The meter is actually the cathode current meter , as per part of the mod, in the updated dwaring...so makes sense. Also tells me the transformer is able to put out ample current into the tube diode, up to that amount of current at least.

Oh boy, trying to finger out a modified amp I'm getting in a bit over my head, like you not knowing just what was done or why we can only give it a best guess. This is why I ALWAYS documented every step of a build or modification so there was NEVER any guesswork involved. Then there is The Golden Rule of Boat Anchordom, never irreversably modify a classic, for if you do its collector value drops to zero. Now you have a decision to make, whether or not to reverse all the mods based on the fact that electrical engineer(s) designed it to meet FCC certification standards, an unknown changed it. By "unknown" I mean was he an engineer or a hack? The big question is will it meet FCC spectral purity requirements or is it a Dave Made cee bee leenyaar? Hell's bells, it doesn't have any pills!

Pills = CB lingo for RF power transistors. Jokingly we called them helicopter transistors. Back to hamspeak......

That cap was an oil filled paper "bathtub" cap of the type that was common in power supplies, especially HV supplies because they never go bad under normal conditions, and they're far more compact than a 450VDC electrolytic string with associated equalizing/bleeder resistors. Johnson actually used one in the choke input HV supply in the Viking, the only transmitter they didn't cheap out on. BTW you really went over my head with ESR, I had to look up that theoretical equivalent value for inductors and caps I never had need to learn not shooting for my BSEE. (;->) C117 being part of a pi section RF filter to prevent leakage into the power supply cable and being radiated I have only a guess why that cap was in parallel with it, a lazy repair. Instead of replacing a damaged C117, just shunt it with anything in the junk box. That being the case I suggest replacing C117 with a .005uF 600V ceramic disc cap like the original. Johnson was famous for those pi section filters on everything except RF connectors, even panel meters in steel shield cans. Oh, that's the bias supply input to the 75V regulator tube, why the dropping resistor is in the supply instead of at the tube socket I have no idea.

"As it stands now in CW or LINEAR mode the voltage regulator tubes go off, which means they are
encountering an overload."
This is where I get lost reading the schematic, the eyes aren't good like they used to be, the rotary switches is where they fail me. Just one of those Mumbai Mumbles helpless desk questions, are the VR tubes all in the correct sockets?

Whoever changed the multimeter from grid and plate current to grid and cathode current royally goofed. In a grid driven RF power amplifier, cathode current is total tube current, grid, screen and plate. Often in Class C CW and plate modulated AM operating grid bias is derived from fixed bias plus rectified grid drive, at rest the fixed bias is protective bias keeping idling current down to a dull roar. Johnson engineering wired the meter to read grid and plate for a reason! <moan>

"Though for some reason, the screen current never ever registered anything at all, till this odd situation."
Screen current? You lost me there, I don't understand the metering logic at all. Ordinarily when tuning up you tune grid for maximum and adjust drive for proper grid current, then dip plate current while adjusting loading for proper plate current. Last if you use CW you should have the correct operating parameters, for SSB you adjust drive so voice peaks barely twitch grid current. That amp is modded weirdly so I wouldn't know what the readings are supposed to be now.

"Could something have changed the config back toward AB2 ?"
I think those poor 4-400s are as confused as I am.
"That oil cap perhaps?"
No, it didn't belong in there, and where it was had nothing to do with anything really.

"Also another question is, the new resistors I found (R116A-B) in the updated schematic, do they look like they are modding to AB1 operation?"
First, I have no idea why they have 3 resistors in series totaling 7.6k off the -75V regulated line. Forget what the guy who "rejiggered" it said (he sounds very unprofessional) and try to find out what Johnson did when upgrading to this model.

Changing the resting bias from -75V to -105V and even more drastic a change to -150V with a corresponding increase of screen voltage is driving them toward Class AB1. Without knowing what the TOTAL grid bias under operation is there is no way of knowing where on the curve family the tubes are operating. Your best bet is downloading a data sheet and taking measurements with the beast fired up in CW operation into a dummy load. I found that modern FETVMs are RF sensitive, older VTVMs less so, and my trusty Simpson 460 VOM was immune and at 50k per volt only loaded down AGC lines badly, a VOM would be a good instrument to measure T Bolt operating voltages. Just stay away from the 3KV without a WELL INSULATED X10 multiplier probe.

I hope I'm not confusing you TOO badly working from 40 year old memories foggy at best. Working from pdf with the + sign clicked on helped my bad eyes, but scrolling around is one PITA. (;->) BTW I doubt Covid 19 is related to tRump Derangement Syndrome, I survived the virus maybe because I never had a case of tRump anything?

Attachment 17509

Quote:

---

Originally Posted by kb2vxa

...Then there is The Golden Rule of Boat Anchordom, never irreversably modify a classic, for if you do its collector value drops to zero...

---

World + Dog would take major exception to me, the FCC and the NSA for some irreversible modifications we've done to gear over the years.

A couple cases in point:

I have two Drake R-7s which came from two different field offices. One (Allegan, MI) even had paperwork from the station chief included in its document set. Both have had 50KHz IF buffer boards and corresponding rear-panel outputs installed (my ex-Drake engineer buddy claims by the factory itself).The Allegan unit now sports a dual-mode squelch, inspired by Mark Mandelkern's published designs. OEM knobs and board interconnects were used. Drake could have included something like this in the consumer offering; the real estate for the extra board was already there (and unused).

The Icom R-7000 - famous for its role in taking down Pablo Escobar (so the story goes). NSA bought the things by the pallet load for domestic use, as they didn't have to meet the EMI requirements imposed when operating in foreign embassies - particularly, former ComBloc countries. For that, Watkins-Johnson got the nod - at many times the price. I've seen all sorts of factory and aftermarket/spook-shop mods for these receivers. Most of mine have better (CFJ vs CFW) IF filters fitted, which is something of a one-way process as IF Unit modifications including board surgery are required. Icom could have included higher quality filters from the factory as the form factors between the two series really aren't that different but the cost certainly was. Two of mine have internal SGC ADSP units fitted. I did an article back around 2011 on this site for both mods; too bad the ADSP is out of production as it really adds to the usability of the radio. The other things I'm thinking about doing to a couple of the several I own are replacing the wide FM filters (230/180KHz, respectively) with 110/82KHz parts for improved FMBCB DX performance. A variable BFO, AGC defeat switch and tunable IF notch round out the nice-to-haves, and the PLL/VCO unit's phase noise can be knocked way down with some changes to that assembly. I have a couple non-working boards to repair, modify and test.

Back to the glow-in-the-dark project.

/out

I can take a fair guess about you, however I'm reasonably sure neither the FCC nor the NSA sold boat anchors to collectors. I bought a National NC-303 with a sweet AGC mod and matching speaker from the Bell Labs ARC when they upgraded their station. It was fully documented with a partial schematic, and was reversible, but I didn't because it improved SSB and CW reception. BTW I completed the package with the luckiest ever hamfester find, the matching converter cabinet with all 3 converters, 2M, 1.25M, and 70cM. It was a steal because the 1.25M converter is rare as hen's teeth and alone costs many times what I paid for the package. Until I moved and had to downsize I kept my entire collection in perfect physical and electrical condition, like the TV show the price was right both buying and selling. No, I'm not like some hamfester sellers that think their shit is made of gold when it's really made of shit. I also restored 2 R-390s and had to replace the open circuit selenimum rectumfryers with silly cone dynodes, but they are for the DC relay coils and are hidden inside the shield under the power transformer module where nobody knows (except us now) there is anything in there. The actual mod in one is very noticeable but easily reversed with no tools, 2 tubes with 6V heaters replaced with 12V versions and the extremely rare 3TF7 ballast tube replaced with a jumper plug, the tube kept with the receiver so it wouldn't get lost. The idea was should it burn out a replacement costs the proverbial body parts IF you can find one not sold out. The crystal heaters can be switched off for fixed operation in a heated operating position, and the ballast tube was used for 24VDC mobile operation and not needed on our stable 117V mains. Leave it to the Army to call anything like the mating BC-610 transmitter that could only be moved with a forklift mobile equipment.

Oh no, every citation CBers got for "shootin' skip" came from the Allegan Point, MI monitoring station, why only that one we never did understand when "skipland" was only Texas. Well on channel 18 the Black channel they all were in Noke (Newark, NJ) when most were in Hillside and a few in Rahway.

Ah yes, the Icom R-7000 was legendary being legally sold only to specially identified government agencies and employees as well as industrial that showed proper ID creds. The first one I saw was in an FBI monitor van at the infamous Waco incident, a news camera caught it through the open back door that was quickly closed. The second I saw in operation up close and personal while talking to an Atlantic Electric engineer tracing sources of RFI in West Creek. The engineering truck was well equipped to say the least! Thanks to a complaint from Phil K2PG that day he found 2 broken insulators, 2 arcing tree branches, a burned out HPS lamp, and touch control lamps in 2 neighbors' houses. They were the worst noise makers spitting out static and harmonics of the touch plate oscillator, one quickly resolved and the other we had fun with. One so easily resolved by trading the touch lamp for one with a switch, that one got smashed to bits, and I set about finding "usable" harmonics from the uncooperative neighbor's lamps. Those fell within the CW portions of a couple of ham bands, clear the decks for action! Come nightfall we saw the lights on and Phil tuned his Drake TR-7 to the best "usable" harmonic and sent CQ in slow CW, the house looked like Frankenstein's lab with the lights flashing low, medium, high, off, repeat. ROTFLOL! He unplugged the lamps, no noise for a few days, then the cycle began again and again each time he plugged in the lamps. It took a couple of months but he FINALLY got the message, get rid of the lamps.

IIRC analog FM channels are 100KHz wide and transmissions occupy a bandwidth of 75KHz with a 12.5KHz guard band on either side. A 110KHz filter would do the trick, 82KHz better, but now I'm wondering what the attenuation curves are, just what do they look like? If you attenuate sidebands you'll hear annoying distortion leading to listener fatigue. That's extremely undesirable these days since the half hourly ID with callsign requirement was eliminated and there are countless stations using The Hot 92 logo. You can listen for hours and not figure out what station you're listening to, that's why I gave up on broadcast DX years ago.

"Back to the glow-in-the-dark project."
Some think real radios glow in the dark and even wear T shirts with it silk screened on. So why don't you sell T shirts with my version on them? 12 volts is for wimps, real radios can KILL you.

Watkins-Johnson at many times the price I'm sure, they made the best general coverage HF marine radios on the planet. I remember a few years ago several fleets upgraded their radio rooms and used WJ receivers went on the market. I could hardly believe old receivers going for tens of thousands of dollars... EEK!!! I imagine a few hams were missing arms and legs.

Ok so got in a nice set of NOS 150V reg tubes with GE logo, made in Holland. They look almost new,
and was told they are a matched set.

Upon putting them in noticed not all lit. Swapped a couple around, then they all lit. That made me inspect the sub chassis
they were mounted on. Most the 1959 solder looks still great but Johnson used stiff buss wire on everything here and one wire was straight against the sharp edge of the terminal of the wire next to it , on 2 of the sockets. The heat from that pin
over the decades, melted a groove into the insulation, and the terminal was loosely touching the exposed wire as shown in the arrows

Attachment 17511

Now all the tubes stay lit very solid, but still I had the odd idle current on the cathode of the 4-400A's in CW or LINEAR mode and the VR tubes would extinguish in those modes as I measured a little over 100mA draw on the dropping resistor
which the tubes being only 40mA rated clearly would not tolerate.

Removing the tubes restored proper screen voltages. So clearly the tubes were biased, though my standby voltage was being generated ok, but is it making it to the control grid?

Sure enough , no.

After tracing why, I found L8 , a 2.5 mH RF choke open.

Attachment 17512

The wire on one end is completely vaporized - I guess a souvenir from the unstable condition that oil cap had , perhaps
making the grid bias too unstable.

Without at least -150V on the grid, I guess these tubes can still behave like a forward biased diode from screen grid to cathode.

Now to locate a new or good used choke. The Tbolt used these same ones in 3 different parts of the chassis.

Now you see the value of visual inspection, by scrutinizing What Is And What Should Never Be (a cool Led Zeppelin song) you isolated two problems, odd behavior of the VR tubes, and the open RF choke. I've seen plenty of 4 pie 2.5mH chokes, but never one sprayed with Cosmoline. That BTW is that brownish stuff that looks like spar varnish the military moisture proofed radios with. You wouldn't want to work on one with that gunk covering everything under the chassis, BELIEVE ME! I see the wire is blown open, melted on the end, but what made the pie become displaced when it was "varnished" in place I can't imagine. No big deal wondering how or why, all that matters is you need a new choke. No problem there, ePay is littered with them, 3 and 4 pie with and without Cosmoline. Oh, in addition to pushing the shorted wire off the socket connection opening the short I'd lift R112 well away from the chassis, it looks like a short waiting to happen. If there's one there is more, time for search and destroy, kill them all and let God sort them out. (;->)

Ok Warren roger that. I did order a few new ones soon to arrive.

Was told that usually when these chokes go theres a bad HV tank cap somewhere. Since I replaced all the doorknob plate tank caps, set out to check C10 (0.001 / 2.5-5KV) and it was breaking down under load. So were the rest of the coupling caps that SW2 shorts out to help you tune up. Was wondering why I had to have it always set to MAX coupling to get 80m to dip. With the switch fully shorted, only had about 395pF max capacitance on the array. One was only wrapped on the switch solder terminal, no solder - and you could tell that connection was overheated quite alot.

So as these are obsolete, I am gonna use doorknob's because also the KV rating will be much overkill but that makes me feel ok because this is
now a modded amp, with 3400V, not the factory 2400.

I started dry fitting a bracket nearby the switch to hold the door knob caps, and it ought to work real nice, as the leeds will only be a little longer to reach the new location.

Attachment 17514

Theres also 1 new hole you can see now available on the bottom of the bracket because the old capacitor array was using it as a mount. Theres also another up higher in this photo - so I could place 2 more door knobs possibly, to add more coupling if I need to try and mod for 160m.

Attachment 17515

So possibilities look good over there, however in the fan compartment where C10 is, its alot tighter, and it means I probably have to mount (2) door knobs in parallel to get 0.001 uF.....but its possible.

I believe all these caps that are shot probably contributed to the issue I saw as I was trying to load on 40m just before the choke blew. There was for sure not enough capacitance coupling, and C10 being almost invisible, probably does something bad for the tubes output plate capacitance by the looks of it.

OH @#$&(#!!!!!!!! I have WiFi problems you wouldn't believe in addition to it being barely above dial up level (damn cheapskates) and severely overloaded even before adding a few med carts that bogs it down even worse. "They" changed several channels bungling it more, now I get disconnected several times a day and struggle to get reconnected, meanwhile I lose what I was doing, this time my nearly completed reply.

Trying to pick up a few of the pieces, you're on your way for the most part, so I suggest whipping it into shape before you even THINK of a 160M mod that for one thing isn't needed, 100W is quite enough to get you where you want to go. Working under supervision not the least bit needed except for legal reasons I worked from 160M up through 6M with 100W, less above that, and worked the world. Remember that 160M is MW like AM broadcast (HF begins at 3MHz) and is clear channel bottom to top and coast to coast. 100W is like BS in business, a little spread thinly goes a long way. Just for fun I worked Georgia, USSR on 10M AM with a re-tuned CB set, no BS.

You're running those 4-400s like they're supposed to be run with a proper power supply, and better when you bias them for Class AB1 or Class AB2 linear and Class C CW operation like the factory specs call for. I hope you downloaded a pdf page from the RCA Transmitting and Industrial Tube Manual I had once upon a time, or the sheet from Eimac that will guide you on the path that you must follow. I noticed a couple of transformers that need attention, T103 LV & bias is connected for 110V mains because that was nominal when the amp was made, put it on the 120V lead because that's today's nominal. Then there is only one way of dealing with T102 powering the 4-400 filaments that require 5V +/- .25V because they're very fussy about that. Being run out of spec shortens the life of a couple of very expensive tubes, price them, you'll flip! I suspect voltage to be a bit on the high side with higher mains nominal now, so if it is first remove the tubes and meter one of the secondaries no load for reference. Then try wiring the unused secondary winding in series with the primary and measure again to see which way it goes, up or down. If it goes up reverse the winding, in phase boosts, opposite phase bucks. The idea is to buck out some of the primary input to reduce secondary output, with a bit of luck you'll wind up with filament voltage under load within specs. If not it's time to get out the big guns, but we'll hold that idea for later only if needed.

Why do you think you need doorknobs to replace C10 that maybe shorted and put the full output of the bias through L8 and blew it open? All you need there is a new 2.5mH 4 pie RFC with or without Cosmoline (;->) and a .001uF 600V ceramic disc cap. Looking at the 4-400 spec sheet the maximum voltage to be expected at that point assuming under load in AB1 plate 3500 and screen 750 is -145, and AB2 plate 3500 and screen 500 is -75V so the 600V cap should be sufficient. With those parts replaced and before waking the sleeping dragon it's a good idea to check for shorts, something else may be providing a DC path to ground.

"I believe all these caps that are shot probably contributed to the issue I saw as I was trying to load on 40m just before the choke blew."
AHAAA, so it was YOU who blew the choke! Then maybe it's not because C10 shorted, but no harm in replacing it anyway, now it's looking tube related, maybe a grid short. That's easy enough, with voltages all wrong grid dissipation shoots skyward, they can overheat and sag against other electrodes, fizz, zap, BANG! BTW there's a mistake on your schematic, that dot at the filament common point needs a ground symbol.

"There was for sure not enough capacitance coupling"
Possibly correct, it could also be a parasitic VHF oscillation problem.
"and C10 being almost invisible, probably does something bad for the tubes output plate capacitance by the looks of it."
No, it's a bypass cap that grounds the cold end of the grid tank for RF while leaving it float for DC bias voltage. At the end of the day you'll probably have to neutralize any parasitics by adjusting C2, you'll need the manual for that. Parasitic oscillation can cause tube overheating, and some really weird tuning and loading effects, like maximum output on one side of the dip in plate current for one thing.

Until next week, same bat time, same bat channel. Don't try it, only bats can sleep upside down.

Attachment 17516

Hi Warren



Ok interesting comments. Yes, they lifted that ground on the heater middle connection between the 2 tubes, and ran a
black wire down to the plate mA meter - see diagram again.

Also updated the drawing, as they put 0.01 Capacitors to ground where that connection to gnd was.The meter already had
one, plus the terminal they used had C105 already there so you have (4) caps in parallel across the meter. Not sure if they are figuring cathode resistor bypass cap(s)?

While I was in there saw one of the terminals the control grid goes to on one tube was not soldered, and was really loose
so fixed that.

C10 was an original Sangamo molded mica, 2500WV / 5KV max. It was mounted inside the fan compartment, so maybe they thought the heat on the tube sockets would shorten its life and they went overkill?

I like using a doorknob for it. Yes way overkill but since this is no longer factory plate voltage and is AB1, I rather have higher rated in case of a catastrophe.

Yes its neat to ponder the 160m mod while I am in there. Today while tearing up an old desktop pc for parts, found a nice
metal bracket they used I think for shielding that fits way better than my other and is stronger.

Attachment 17517

Can easily fit 6 or more doorknob caps on it plus the octal relay in case I can do the 160m mod later.
The one doorknob in the lower right corner near the bottom is dry fitted on the bracket that held the string
of bad molded mica output coupling caps. I can mount 2 there at 500pF and make a new C10 of 0.001uF
as original. Turns out it goes right to the huge neutralizing cap thats very close by, so thats a great spot for it.

I read neutralization was rarely needed on the Tbolts, but as I am into new territory here, would be worth looking into
and try to avoid the parasitics.

I'm not happy with that meter modification, you can get a dip in cathode current when adjusting the tune variable, but you can't read plate (anode) current with screen current added to it. It would also be nice to read screen current after all those mods to be sure it's the proper value, at least until operating conditions are settled on and everything is proper. At the end of the day (except for neutralization that may be needed now) you can throw away the manual and write a new one, that's no longer a Johnson ANYTHING you have there, more like a KA2PTE Firedrake (fire breathing dragon). If you can make a sticker, or better yet a decal and put this in that blank spot above the tuning control it would make a mean finishing touch. JIGGITY JIGGITY! Yeah, in addition to an airline pilot Glenn Quagmire is a ham fond of boat anchors with tubes in his basement. Heh, Godzilla finally set King Ghidorah on fire! ('->)

I see the metering change, confusing at best, I can't make sense out of what it was originally, the switch selects "screen" & HV, but in Class AB1 it was originally set up for max screen current is 28mA for 2 tubes, but why measure it? I'm used to tuning the grid circuit, so grid current was metered, I'm lost here. Then there's a watts scale on M2 lower scale with plate current on the upper, why I have no idea because to get plate DC input you multiply the upper scale by M1 lower scale (KV) provided calibration is correct and that depends on R101-105 being all within 1% so there could be no more than 5% from nominal worst case scenario. Would Johnson spring the bux for precision resistors when they cheap out on everything else? Collins on the other hand used a single 1% multiplier in the 20V I repaired, but there's a gulf as wide as The Grand Canyon of Mars here. (;->) You can't have enough bypass caps though, the Ranger and Valiant had them up the wazoo and there was a shield can on the meter, but when they designed the Valiant they cheaped out on the most critical parts, the modulator and final. Mod iron on both was puny, and the Valiant used 6146s that suck as audio tubes when 807s shine. Then they paralleled 3 6146s in the final and at the voltage and current it ran at the plate impedance is only around 500 ohms. That leaves a 10:1 typical transformation ratio in the pi network, not high enough to adequately tune out harmonics and spurs, plus it was a BITCH to neutralize, more shit going up the pipe to the signal spitter. UGH! That's where my Drake TVI filter came into play, without it my TV went nuts, likely the neighbors' too. Oh, if your TV goes BSOD when you key up the Firedrake there is another Drake that can fix it, they made a KW filter too. Cathode resistor bypass caps? No, there are no cathode resistors, it's not cathode biased, it's an RF output amp, not an audio power output stage.

OH RATS! The music stopped, the bloody WiFi disconnected AGAIN! Since "they" added several more mobile laptops to this already overloaded wireless network it has been acting funky. Then instead of upgrading it like should have been done a long time ago the cheap bastards changed several channels around and made it much worse. They've also created a medical situation in the process, the nurses' meds carts mobile laptops aren't working, oh lovely. I wish they would do like I do, I have an iron clad rule; IF YOU DON'T KNOW WHAT YOU'RE DOING DON'T F***K WITH IT!!!

There, finally I got it connected again, and just in time for one of my favorite songs on one of my favorite albums, I Talk To The Wind on the King Crimson Court of the Crimson King. It's such a beautiful song and in my key too, I loved harmonizing to Robert Fripp before I lost my voice. I'm pretty sure you'll like Radio Caroline too, Europe's top pirate radio ship gone legal; http://www.radiocaroline.co.uk/#home.html Rant mode off, so ON WITH THE SHOW.

"While I was in there saw one of the terminals the control grid goes to on one tube was not soldered, and was really loose
so fixed that."
Keep looking, again you were reminded of the value of close visual inspection. If I found a terminal without solder in a Collins 32V2 transmitter (the sweetest audio I ever heard in CB in our AM Gangstas group) I was repairing I can't imagine how many you'll find in a Johnson. (;->) At least your job is pretty easy, Collins is infamous for being technician and broadcast engineer unfriendly.

"C10 was an original Sangamo molded mica, 2500WV / 5KV max. It was mounted inside the fan compartment, so maybe they thought the heat on the tube sockets would shorten its life and they went overkill?"
No, just a convenient place to mount it. I had dozens of those things and put them to good use replacing fixed loading caps in those miserable Johnsons. I've heard the expression among hams "Happiness is a big Johnson" and I'm sure they mean the ones they were born with.

"I like using a doorknob for it. Yes way overkill but since this is no longer factory plate voltage and is AB1, I rather have higher rated in case of a catastrophe."
You're in love with those TV doorknobs, but I'm sure you'd like a cat ass trophy addition to your shack, you can order one on line. (;->)
https://www.etsy.com/market/cat_ass_trophy
Jim N2EIY, W2RXR, and whatever he changed it to now, overkill is his middle name. I needed a power supply so he gave me one he had kicking around, a Tripp Lite 12V regulated 60A unit that could run 3 2M FM rigs transmitting continuously without getting warm, overkill. Then there's the bracket scrounged from an old computer, an octal plug in relat, and MORE DOORKNOBS, overkill. One of my favorite scrounge spots was behind the local TV repair shop and besides tubes that I tested and added to my collection, quite naturally I collected doorknobs. When I built a medium size Tesla coil I saved lots of work making the HV capacitor out of glass plates, aluminium foil, and glue I wired al of my doorknobs in parallel.They sorted themselves when I turned it on, some had a defect or another and exploded like the one in the ATU.

"I read neutralization was rarely needed on the Tbolts, but as I am into new territory here, would be worth looking into
and try to avoid the parasitics."
It's not a matter of new to the territory, I'm more of an egg spurt than an ex spurt, it's a matter of the amp no longer being a Thunderbolt, Arthur Pendragon had a wizard named Merlin who changed it into a Firedrake. How's that for a new kingly tale a bored knight named Lance A Lot came up with? Don't mind me, this f***ed up WhyFry is driving me mad...........

Here's a thought to ponder, there's a grounded grid mod for the wee beastie. With a grounded shield between input and output you don't need neutralization, just 80-100W to drive it vs. IIRC 5-10W grid drive two tubes.

Attachment 17518

Attachment 17519

Hi Warren

Yes, good idea on the new name.

I finished up the new coupling cap assy bracket and mounted the doorknob caps to it, wired it all up - looking pretty good:

Attachment 17522

C10 is in the lower right part of the pic with the large copper braid putting 2 door knobs in parallel to give is about 800pF. Not exactly the 1000pF per stock design, but we arent exactly factory anymore as you indicated. (2) tan colored doorknobs are in parallel on the bracket near the left side to form about 840pF, and its tied to the last position of the coupling switch so in max level thats whats beig added in. Each other red door knob is about 600-650 pF or so which is close to the 620pF in the original spec.

Fired up last night and today for some tests and things are pretty stable! Aside from the tubes being that active red color all the time, which seems strange but I am told is normal by you and others - as long as nothing is going nuclear I guess we are golden.

Into a dummy load, the most I can read on the PEP vector meter was about 800W with a constant CW carier and the linear in LINEAR mode. I am doing some tests now on the air with about 500W or so and we are drawing 380mA on the "hacked"
plate current meter.

Another who had restored a factory Thudbolt, said you can up the 105V regulator tubes to 150 and you wind up with
600V for the screen volts. 600V is the max rating for the screen on these tubes as per the spec sheet. I have (1) 105V
tube with (3) 150's for about 550V on the screen. He said the more screen volts the better and I guess under a load you would have less than 600V on the screen when under load if you went with all 150V tubes.

After some more emails , he said the -75V tube is not enough with the 550V screen setting I have, so I will have to wait till I get another 105V tube to try to bring the bias up.

But clearly some very stable progress here since the new choke and caps are in. At some point wanna get that T103 connection in order so its on the 120V taps.


UPDATE:

So I learned the goal with a 3KV supply for AB1 is 750 screen voltage with a grid bias of -137v. Thanks to KB1VWC (Steve) who runs the same PS but on a Chippeawa that puts out nearly legal limit with the same tube setup.

The previous test used -75 as the grid bias and 550v for the screen. KE7TRP who had restored a Thudbolt to original said that's not enough tube bias at -75V. Which probably explains why the tubes were glowing red, even with only 50w of amplification going on.

After some doing, I wound up putting in all 150V type tubes as the regulators, and have I think made a step in the right direction. While I can only get about 550 or so watts into the dummy load now, not 800 , the tubes are clearly barely struggling with only a mild plate glow at 500w.

Idle current is way down as well, as both of them indicated it would be. The -137V spec on the datasheet says to adjust that level for desired idle current. So looks like I have more figuring to do on the spec sheet to figure out the right spot.

The Chippeawa has a bias knob on the front to set the idle current, makes this part alot easier.

Hi Steve,

The more you change voltages around the more I feel like Oblio and his dog Arrow exiled from the land of Point and wandering in the Pointless Forest. (Harry Nilsson - The Point, the song Me And My Arrow used to advertise the Plymouth Arrow) The name Firedrake may take on another meaning if it goes up in flames, keep the tubes at their proper operating parameters, 50V below max is too close for comfort. I didn't say that red glow all the time is normal, it's only normal all the time in an AM broadcast transmitter or CCS, they may only glow a bit on peaks in CW and linear operation. Both modulators and finals glowed in the K2PG re-tuned Collins 20V when transmitting, only the modulators glowed in the RCA BTA1R at WERA operated at half power. I never saw them in an Amateur amp where the duty cycle is only 20% unless an AM Gangsta pushes them to 100% and then they still have to be throttled back to 20% allowing for carrier expansion. This is all rather new to me so I'm mostly relying on the tube specs and a lot of good old common sense. A bit of that tells me unless you have enough loading capacitance it won't load down enough on 80M making the tuning dip broad and shallow as well as pushing the tubes too hard.

"as long as nothing is going nuclear I guess we are golden."
The Chernobyl Exclusion Zone won't be safe for human habitation for 10,000 years, but without people wildlife has returned and is doing remarkably well. "We are stardust, we are golden, and we've got to get ourselves back to the garden." Joni Mitchell - Woodstock, 1970

"He said the more screen volts the better"
Hoo Hee? Something tells me he doesn't have a tube spec sheet or most likely doesn't know shit from Shinola when it comes to tubes. That philosophy only applies to triodes! (Now think on it a moment and tell me what that means, hi.)
"and I guess under a load you would have less than 600V on the screen when under load if you went with all 150V tubes.
Not quite, control grid voltage has nothing to do with screen voltage per se, one does not affect the other. All voltages are measured under load, with dropping resistors they change depending on load conditions. Grid, screen, and plate voltages need to be adjusted for the class of operation, that's why the original design changed the fixed bias according to linear (Class AB1) or CW (Class C) on the mode switch, I don't know how the tune position lowers power so you don't send the tubes into orbit. Taking a good guess from my old 60W RCA VHF base station I modified for 2M, the tune switch lowered the screens in the 6146s to a very low value. Coincidentally it was the same make and model police transceiver in the original 1958 The Blob starring Steve McQueen.

Showing my ignorance here, what is a "PEP vector meter"? I'm familiar with an RF watt meter like the trusty Bird 43 and a peak reading meter, but not that one. Then there's that hacked plate current meter reading cathode current. How do you know what your plate and screen currents are when you're reading both together? That's a hairy brain idea if I ever saw one.

So THAT'S who has the hairy brain. Running with the screens 150V over maximum he's burning them up, when a grid short develops he'll be wondering how that happened. I wonder what +750V to ground on the control grids will do to a Chippewa besides make him wonder "Who peepee in my teepee?".

" KE7TRP who had restored a Thudbolt to original said that's not enough tube bias at -75V."
What makes him think he knows better than the engineers at Eimac who designed the 4-400 and those at E.F. Johnson who designed the amp around them? The tubes were glowing red at that low level because you're feeding the tubes much more voltage without changing other parameters accordingly. Remember with a 20% duty cycle my best guess is they should only glow on peaks when they're drawing appreciable current.

"After some doing, I wound up putting in all 150V type tubes as the regulators, and have I think made a step in the right direction. While I can only get about 550 or so watts into the dummy load now, not 800 , the tubes are clearly barely struggling with only a mild plate glow at 500w."
Interesting, the 4-400s in the BTA1R showed no color at the 500W RF output level. Since you're designing an amp from the ground up, at this point I only can tell you is watch those electrode voltages and make sure all of them are proper for the intended class of operation. That means defeating the safety interlock and testing with the amp upside down while you keep an eye on the voltages and currents. I guess you also know that means careful where you put the meter probes and keep one hand in your pocket, we'd like to have you around for a while. Oh, I said upside down because Eimac says base up or down, never sideways or you'll have sagging grids and something going BANG.

Last but not least, the higher the fixed bias the closer you're driving the tubes toward cutoff, so naturally current goes down taking power with it. You know the score, RF output depends on DC input, the idea now is to get all of the voltages right according to the tube specs and you'll have a good working amp... I hope. (;->) Oh BTW, don't worry if the glass turns or has turned a slight smoky brown with age, that's normal. All of the pulls from the WERA transmitters were like that and still had plenty of life in them. Back in the day the Chief Engineer of a radio or TV station changed all of the tubes on a schedule and all of the pulls were perfectly good, that was standard industry practice. I had more 7025 low hum and noise 12AX7 twin high mu triode equivalents than I knew what to do with, the audio board in the main studio was bristling with them. They ended up in my stereo amp and other audio equipment. Broadcast quality home stereo? YOU BETCHA!

Well been a while, but now getting back onto this epic project.

I did find the neutralizing cap (C2) was dirty with a film of oil burner soot inside, and there were lots of ding marks in there from zorchings:


Attachment 17608

So I cleaned all that up , and shined the aluminum with scotch brite and sanded down the burrs from the zorching. Also noticed the center movable electrode that moves in to that cup,
was a little crooked. Turns out if you replace the doorknob caps, you stand a risk of mis-aligning that part of the cap, and looks like thats what happened also - as I had to replace those.

I also made up a cheaper L8 with (5) 560uH axial half watt inductors, totaling a bit over 2.5mH but I am hoping 2.8mH is not much a problem from 2.5mH.
Decided to put the plate on-off switch back in on the front panel so that in case of problems, its easier to throw it off, and kill the plate supply if the tubes somehow loose bias again.
Had to route 115v back thru one unused pin of the input power jack, back into the Heathkit supply to the plate relay for that, but seems to be working as intended.

I also have the spare set of 4-400A tubes in there now, as the last choke failure had one of the tubes sparking I think in the grid area. I put filament on and have all VR150 type tubes in the
screen supply, with a VR105 acting as the operating bias tube, and when I apply plate, things appear to be ok / normal for idle once again.

So with my makeshift L8 choke in there ran more tests with very low excitation in the tuned input mode. Things appeared stable at about 100W out of the amp, except the right side tube was glowing a pretty good RED color, while the other was not. I put in another that I felt was a working tube, and that glow went away and I gradually worked my way up to about 500W output, and made several long ssb transissions on 80m last night. The tubes showed barely any red at all after all that work, so I believe things are stabilized.

L8 may have been getting too hot because testing the chassis without the proper air flow for the compartment fan may have been asking too much. So I limited my tests to strictly the TUNE mode and short transmissions no more than 300w. Then I put the covers back on, made sure the fans were working and upped power levels.

Also painted the cover flat black:
https://www.mediafire.com/view/r8gw3...t_blk.jpg/file

I went in and put the plate switch back into the front panel and filled the lamp lense with a NE2 neon lamp and dropping resistor. The switch now takes 115Vac, and switches it
back through an unused fuse holder through the main input AC receptacles non-used pin, which leads back to the plate relay in the power supply. So now its easier to shut the plate supply on or off, which comes in handy when not being used, and if something goes wrong, that switch is very close by.

Couple of other things still need to be done, like placing the 115Vac on the 115 primary winding for T103, instead of the 110 winding, and perhaps some of the decoupling caps
on the 400A sockets ought to be replaced. They seem to be drying out because of the heat nearby and that usually means they will eventually short.

I may also try another pi-type L8 and ditch my home brew sub, but I am tempted to leave it as-is because things are working well.