Bottlehead Forum
General Category => Technical topics => Topic started by: vetmed on February 09, 2014, 06:28:24 AM
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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 :o
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
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Why not install a couple of switches. Shunt the B+ to ground until the filaments are warm and the B+ has stabilized, then flip the switches on. You could be fancy and use relays but switches work nicely.
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Shunt the B+ to ground until the filaments are warm
That will blow the fuse immediately.
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The diode should work. I have heard people say that it damages the sound very slightly, but then you hear a lot of nonsense on the internet ... :^) Easy enough to try, and should be perfectly safe.
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Thanks Paul. I have tried it and I am happy to report no more blue flashes or pops at turn on 8) Nor does it seem to have any effect on sound quality, not that I expected that it would, being very sceptical about magic wires and other audio fairy tales ;D