Johnson Viking Thunderbolt

[KA2PTE]

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.

[kb2vxa]

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.

[KA2PTE]

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.

[kb2vxa]

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.

[KA2PTE]

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.

[kb2vxa]

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!

gremlin.jpg

[KA2PTE]

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.

[kb2vxa]

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!

[KA2PTE]

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.

tbolt_160m.jpg

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....

[kb2vxa]

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?

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