N Cook wrote:

Cured the main problem but the owner seems to live with what to me seems an
excessive amount of stray mains interference, without any signal leads and
input contacts shorted to ground.
This amp is 110V running off a 240/110 V autotransformer with the chassis
having an earth to mains earth (UK)

Probably illegal and should almost certainly be linked out. You might want to
consult your liability insurance


Other than probably something to do with the awkward cross-linking of the 2
sections of 2 7025, of normal and vibrato channels . It is possible to
minimise the noise using one "normal" signal input and nothing in the
"vibrato" inputs but the vibrato volume set on about 3 of the scale, not 0.

You're just cancelling various sources of 'hum'. Some are in opposite phase so
you'll find various nulls like that.


Is there a recognised fix/amelioration of this effect. ?

First off, have you got a 'balanced' source of 110V ? Balanced mains tends to
have a useful effect on a lot of old kit.

Another thing .... the PSU reservoir caps may need replacement. If they're old,
their capacity has almost certainly degraded and as a result there'll be more HT
ripple than there should be. Also, imbalance of the bias current in the output
stage will cause ripple to appear in the output. In the absence of individual
bias pots, a new set of output valves might help. This hum will be there even
with the preamp pots down.

Graham