Re: 120W PP EL34 amplifier re-born.
Think I've found the problem - a crashing 6.3v PSU that powers the heaters of the gain stage and LTP, it's still an odd fault though.
Test sequence =
1) ECC83 in socket, power up, take DC readings, they were ok. Next 1khz sine on the input = bigger sine out, as per load line calcs. All as should be.
2) Hot swop ECC82 into LTP base, IE with HT etc applied, whilst monitoring 6.3v rail = 6.3v crashes to 1.2v, LM317 heatsink getting V hot, but not too hot to touch. Result both IP and OP of ECC83 attenuated. Is the ECC82/LTP acting like a diode?
3) Repeated # 2 a few times, it did it again, but other times not - odd. Is LM317 oscillating?
4) Powered up both valves in place a few times, all well, odd. Can't replicate fault of yesterday.
Tested all passives, all seem ok. Will test PSU further.
Andy.
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