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