Indeed, all I did was set the bias on the four output resistors to 12mV and the DC offset was fine, no further adjustments needed. I have run a good few hours soak test now and all still seems fine. Nothing is over heating and other than the hiss the sound is really nice. The previous owner had this amp wired up as mono, one channel was blown and they just jumped over the other channel, probably just to sell it as working on ebay. This is not mine, but an collector that asked me to fix it up. I do own an really nice leak stereo 30 plus fully rebuilt with stereofetic tuner running a few vintage philips 22RH426 speakers(easy to drive), The guy I am fixing this amp for gave me the leak's for a repair job, love the sound of the old vintage British amps, The R.S.C super 30 iii I have sounds really nice too after recap, bought for 10 sterling ! I had a few quads in for repair, but the price is just gone crazy for these.
Do you mean this one? https://www.americanradiohistory.co...less-IDX/IDX/70s/PW-1971-11-OCR-Page-0012.pdf Jings, that brings back memories!
Yes that is the one, surprising how good it sounds, tried it with a few different speakers, the vintage philips, and newer Bower and wilkins DM602 (with new film caps) and it's a great little amp, I have no case for it, but I will make one for it at some stage. Don't think it will drive my infinity Qb's but it drives the B & W without issue to high volume, Very clean sound.
Docs for the MkI model are on HFE.
After wading through the schematic / parts list (an absolute chore in this separate form), I have a few suspicions as to why this one might be noisy.
Is it still on its original electrolytic caps or have these already been replaced? Things could get quite hissy if all of C39, C35, C26 and C20 were wide open, as the main amplifier is on a zener-regulator stabilized supply. (I wonder why they did that, T10 doesn't have that much beta. I wouldn't be surprised to see substantial current limiting under load.)
"Not enough beta" seems to be a general theme in this amp, actually. it seems like all the preamp stages are run at 4 mA of Ic or so while featuring typical 100k pots and the like. Beta spec for BC171Bs is 330 at 2 mA, with only a modest increase above that. Input current has got to be quite substantial... around 10 µA perhaps. That's 1.8 pA/√(Hz) worth of input current noise density from shot noise alone. Clearly not ideal when there are impedances in excess of 10 kOhms around and total gain appears to be >40 dB.
This circuit could clearly benefit from either some DIY Darlingtons (with a resistor of maybe 5.6...6.8 kOhms so the input Q runs at a more sensible ~100 µA, which then is not a challenge for BC550B/C or 2SC945 or similar) or even JFETs. If there is any interest I could simulate the whole shebang and determine which ones of T3..T6 are the most critical.
The whole circuit also appears to be littered with "H. S. Carbon" resistors (is that carbon composition or carbon film?), for that extra dose of excess noise as a number of them are in the signal path while carrying substantial current. R20-22 and R29/30 may be the best candidates for metal film, but R42, 50/51 and 64 won't hurt.
After wading through the schematic / parts list (an absolute chore in this separate form), I have a few suspicions as to why this one might be noisy.
Is it still on its original electrolytic caps or have these already been replaced? Things could get quite hissy if all of C39, C35, C26 and C20 were wide open, as the main amplifier is on a zener-regulator stabilized supply. (I wonder why they did that, T10 doesn't have that much beta. I wouldn't be surprised to see substantial current limiting under load.)
"Not enough beta" seems to be a general theme in this amp, actually. it seems like all the preamp stages are run at 4 mA of Ic or so while featuring typical 100k pots and the like. Beta spec for BC171Bs is 330 at 2 mA, with only a modest increase above that. Input current has got to be quite substantial... around 10 µA perhaps. That's 1.8 pA/√(Hz) worth of input current noise density from shot noise alone. Clearly not ideal when there are impedances in excess of 10 kOhms around and total gain appears to be >40 dB.
This circuit could clearly benefit from either some DIY Darlingtons (with a resistor of maybe 5.6...6.8 kOhms so the input Q runs at a more sensible ~100 µA, which then is not a challenge for BC550B/C or 2SC945 or similar) or even JFETs. If there is any interest I could simulate the whole shebang and determine which ones of T3..T6 are the most critical.
The whole circuit also appears to be littered with "H. S. Carbon" resistors (is that carbon composition or carbon film?), for that extra dose of excess noise as a number of them are in the signal path while carrying substantial current. R20-22 and R29/30 may be the best candidates for metal film, but R42, 50/51 and 64 won't hurt.
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