Having changed my power supply to use solid state rectification I now see a blue flash inside the 71A plate on start-up, and although so far it continues to work I am sure this is not good in the long term
I think what is happening is that the driver tube's plate voltage rises beyond its normal operating value at startup because its cathodes are cold. So the plate voltage of the driver tube rises to ~ 300 volts before falling back to its normal value of 150 volts. Evidently the output tube's grid doesn't seem to like this
So I am wondering if installing a high voltage diode from the output tube's grid to the top of the cathode resistor would work, striped end (cathode) to the tubes cathode? I think what should happen is that the excess plate voltage of the driver will bypass the grid until the tubes are warmed up. Then, because the output tube's cathode voltage is higher than the grid voltage, the diode would be reverse biased and no longer conduct. Seems simple and should work, but keen to see what other more knowledgeable people think. Thanks
