This makes life easier on the tube when used for AM. Simple rule is to use one diode for every 100 Volts of anode supply. A series sting of 30 diodes will hold the zero-signal current of a 3-500Z down to 10 or 20 mA with no drive. A series string of rectifier diodes wired for forward bias gets you about 2/3 of a Volt per diode. We put fixed bias on any Z that runs from more than 2200 Volts. The other version with the 80-10 meter band switch and the two Z tubes had a transformer produced between 27 Volts DC. Black Cat amplifiers are more about steroids and less about longevity. Never mind the spec sheet for that tube says max 500 Volts. This kept the anode voltage for that tiny driver tube down (!) to 1100 Volts. That one was meant for the 'low-drive' version that had a single 8417 driver tube. One of them provides only 2200 Volts DC from the HV. There are two JB2000 transformers that I know of. (been a looong time since I have had my probes in one of those) PS: He also noted that the Plate Voltage was around 2200vdc, which on an SB220 transformer would be the low CW tap, the SSB position should be closer to 3000vdc.(+/-) Think the old JB2000 transformer should have been similar. Hard to tell from the partial picture, maybe he will post further pictures, Does the picture also seem to show the output coax attached to the end of the tank coil (?) maybe a P-L net? Hard to say since I am guessing this a partially disassembled project. The cooling fan looks to be a way too far away from the tube to give good pressure around the glass and socket base unless there is a shroud missing to focus the air across the tube and under the socket. Not sure why this is eating transformers. Interesting since either of those should have been adequate to run a Single with headroom to spare. He said this is the 3rd transformer in this amplifier(SB220 first and JB2000 last) both of which went up in smoke. I thought he said this was a single tube 3-500z? The partial picture of the RF deck looks like a single tuber.
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