Microphonic valve...... only when guitar volume near full

[Papus]

Brothers, I have an issue with my Papus Signature 2061x.
Lately it goes into insane high-pitched microphonic squealing, but only when the guitar's volume knob gets above 8 or 9.
It's evident throughout the preamp gain range, but becomes worse and more easily triggered at higher gain settings.
This occurs on multiple different guitars, but seems worse with single coils.
Master volume level has no effect on the squeal, it occurs regardless of whether the MV is at 1 or 10.
This is NOT microphonic guitar pickups. It occurs with multple guitars, all of which do not exibit microphonic pickups except at insane volume and gain levels - and even then it's more like a low-mid howling sound, not high-pitched squealing.
I'd say the squealing frequency is around 8-12kHz.

Now I know a lot of people will immediately say "microphonic valves", but I don't understand the relationship to the guitar's volume knob.
Anyone care to explain the science behind this, thanks.

[Anitoli]

It's related to out put potential. Something is causing oscillation when the pup is up. Looking at the schematic if the oscillation is loud enough even though the pot may be all the way down it may still be able to get past it through the tone pot. Swap out the first tube and see what that gives you.

[surfnorthwest]

I may be full of shit here, but I would mess with your pickup height and see if that solves it.

[Dana,s blues]

If a valve has gone micro phonic then slightly tapping it with your finger will produce a tapping sound through the speakers, just like a real microphone would except it will be very faint! A good valve will not do this. Good luck
Dana's blues

[TM1]

I would try a piece of shrink tubing over the valve and see if that cures the offending valve of it's problem!