Help for amp section noise

[Alex R]

I was thinking maybe it's really spiky feedback and it foxes the meter.

[otto]

First check - get hold of a new preamp valve and swap it for each one in there in turn, see if it goes away.

I've swapped existing preamp tubes between themselves, no difference

[otto]

I was thinking maybe it's really spiky feedback and it foxes the meter.

what could I check to verify the feedback you are thinking about?

[Alex R]

- Only a scope really. Even then it's famously hard to track down as such oscillation tends to circulate through the whole amp.

Still think it's worth having one new preamp valve to swap in.

My next suspect would be filter caps failing to give enough separation between stages.