Valve amplifier service

[Nick]

s1 = stage 1, srpp stage
s2 = stage 2, cathode coupled phase splitter

[simon]

D'oh!

[simon]

Thinking about this again this morning now I'm fresher

You will need to disconnect the existing s1-l -> s2-l and s1-r -> s2-r so its s1-l -> s2-r and s1-r -> s2-l

This is what I did with clipleads

And also swap the feedback over as well, I forgot about that.

I didn't do this. I think the feedback is R18//C10? If so do I disconnect them from the ground side of C2 and reattach to the ground side of the other channel's C2?

I can't work out why the other channel's feedback network would cause the loud feedback through the speakers though.

[Nick]

Because the feedback will not be related to whats going on in the amp, so wont reduce gain as you expect.

[bostod]

It has developed into way more than a simple service. Im thankful for all the advice so far and hopefully it will help others track down a similar fault in the future.

[simon]

I'm sorry this is taking so long Dave. A combination of me not fully understanding what's going on and my dodgy health. I'll try to get back to it as soon as.

[Mike H]

It's obviously some sort of fault, and not 'just needs a service'.

With the swapping the input stages over, then also have to swap over the NFB connections as well, is, as Nick said see if the hum moves to the other channel.

If it does, the input stage has the problem.

If it doesn't, it's not the input stage, so they're both fine.

Hope that makes sense.

[simon]

Thanks Mike

[simon]

I've had another poke about inside the amp this afternoon. Had to have two goes at swapping the feedback over as the amp has been modded from the stock layout and the layout on the tag strip is a bit confusing. Anyway, the hum did change channels, which points the finger at V1, the SRPP.

It was interesting to revisit this after a fair break. The hum is greater on the right channel than the left. Connected to 18" Fane drivers which will be around 97/98dB the right channel is low but audible. Back in the day, many of our amps hummed like this and we were quite pleased with them . The left channel needed my head to be quite close to the speaker to hear the hum over the PC fan. So overall I would say the hum isn't bad. But it is there and needs fettling.

[simon]

Here's the circuit again for reference
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20230614_142819.jpg (135.36 KiB) Viewed 7566 times

[simon]

I don't think I've replaced any of the resistors in the first stage, this is what it looks like
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20231231_170518.jpg (105.06 KiB) Viewed 7565 times

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20231231_170552.jpg (93.37 KiB) Viewed 7565 times

I could replace some resistors if that's the next thing to try?

[Nick]

If its now about low level hum instead of the possibly ghost signals on the scope, I would look at what seems to be an input coax and I assume first stage next to what looks like mains connections.

[Mike H]

Simon, I assume R2 is close to the socket pin, so you could short the input end of that to 0V to eliminate the input path. (?)

[simon]

If its now about low level hum instead of the possibly ghost signals on the scope, I would look at what seems to be an input coax and I assume first stage next to what looks like mains connections.

As it's had been a while since I'd looked at the amp I thought I'd start from the beginning checking the hum and started with my phone as input via a cheap USB C I2S/DAC to RCA phono converter with the amp plugged into the big Fanes. Well, the amp really didn't like the phone input. All sorts of hum, worse when the phone was locked. Slightly perturbing. It was quite a relief when switching to a proper DAC was much better.

It was quite a reality check as I'd just assumed that any hum audible on the inefficient test speakers would be loud on proper speakers.

[simon]

Simon, I assume R2 is close to the socket pin, so you could short the input end of that to 0V to eliminate the input path. (?)

Yes Mike, it's the blue resistor soldered on the pin in two photos above. Good idea, easy to cliplead.

I'll try rewetting the input coax joints too and see what happens. I really don't want to have to rag the wiring out from the input phonos as it's horrid.