UPDATE: 11/26/2022 Power Transformer Replacement
It happened again and this time I gave up on using the transformer that I "floated" and replaced it with another large transformer. 3700 VAC into a SS bridge (Goodby beautiful MV rectifiers...). The original transformer arced again and I suppose went from secondary to case to primary. Time to recycle 50 pounds of copper and steel. Plate current "kick" is not present and it seems to be working perfectly with approximately 1400 watts PEP output.
UPDATE: 11/23/2022 Power Supply Modification
There were comments made about the plate current meter making a huge jump upscale to the right when plate voltage was removed. I agree that is a serious concern. I build an outboard box (no more room in the amplifier cabinet) where I mounted a 120K, 200 watt bleeder resistor and a 0.16 uF 8 KV capacitor connecting the capacitor across the 8H choke and the resistor to ground. This completely stabilized the amplifier with no more "kick" in plate current and the 4KV bleeds off in 20 seconds. I am not certain why this worked so well but it did solve the problem. I can only assume there must have been a momentary parasitic instability in the amplifier as high voltage was removed. Bias had to be raised more negative to about -110 volts to set the plate resting current at 50 mA. Furthermore, the orange spot in the plate of the tube went away without modulation and power output increased about 200 watts.
ORIGINAL MESSAGE:
I had a high voltage secondary winding short to the case in a high power RF amplifier power supply. The transformer was intended for a much lower voltage power supply, in the 750 to 800 volt range but I ran the output of 1800 VAC into a voltage doubler to 4000 volts. This 4KV is seen by the secondary winding and it finally shorted thru a dried and cracked insulator. Since I do not have or know where to get another transformer like I need, I did all I could to remove the short and then tested for any leakage between primary and secondary windings of the transformer to the case, which turned out OK but then insulated the metal case of the transformer so it would "float" above ground before reinstalling it in the amplifier.
6 окт 2024