UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - 120W PP EL34 amplifier re-born.

Diabolical Artificer · 26th Sep 2018, 11:54 am

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.

26th Sep 2018, 11:54 am	#10
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,662	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. __________________ Curiosity hasn't killed this cat...so far.