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.
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