Microphonics

New owner of a 5 yr old FIVE

I noticed in Mark IV mode 90W , when Master is past 12 o'clock it starts to enter in microphonics.
This happens even unplugged.
2C+ and Extreme are pretty silent (well a small normal humm).

Question is what tube?
I have 2 12AX7 spares and I replaced one by one V4, V5 and V6 - still squeals (2C+ and Extreme modes still silent)

Has anyone encountered this before?
Thanks!

Hmmm... Mark iv mode is a little more saturated than the other two modes in that channel. So when you dial in a high gain tone for c+ mode or extreme and then switch to mark iv mode with those same settings the chances for microphonics are definitely higher. May as well roll a known good 12ax7 through v1, 2 and 3 too. What's your settings? And is the loop bypassed? High presence, treble and gain settings can exacerbate the issue. Mark iv mode is the one that has the most tendency to squeal so hopefully just a tube or some experimenting will get this resolved.

Of V1, V2, V3 only V1 has anything to do with channel 3 and I tried that too. No visible (audible) effect...
Haven't tried V2 or V3 - doesn't really makes sense to.

Gain is about 2 o'clock and only increasing the Master over noon puts it in microphonics.
Solo is about 8:30 and effects loop is on, but not-used.
Treble around 1:00, Presence Noon to 1:00

This happens in all modes - 10, 45 or 90W.

Thanks for the feedback

Maybe try a Mesa SPAX7 preamp in V1?

The Mesa SPAX7 will help, I use one in my Mark V in V1. Helps to calm the top end a bit.

One way to find out of a tube effects a particular channel, pull it. If you get no sound though the speaker, it is part of the signal chain.
[The following is based on the Mark V schematic] Regardless of the tube chart on the amp, V3 is used by all channels.

CH3 circuit path:
V1A -> CH3 tone stack -> V1B -> V5A -> V4B -> V3A -> [reverb, runs parallel to a 150K resisotr: V4A send, V5B return] -> V6A (CH3 Master volume) -> EQ ->V6B (if FXloop is used, solo and master volume, otherwise passes on to PI)->V7(A/B)PI section -> feed to Power tubes.

V3 is part of CH3. If you do not believe me, pull it and find out for yourself. Actaully all circuits pass though V3A.

CH2 circuit path:
V1A -> V2A -> CH2 tone stack -> V1B -> [reverb, complex filters: V4A send, V5B return] -> V3A (CH2 Master) -> EQ ->V6B (if FXloop is used, solo and master volume, otherwise passes on to PI)->V7(A/B) PI section -> feed to Power tubes.

CH2 circuit path:
V1A ->CH1 tone stack -> V1B -> [reverb, complex filters: V4A send, V5B return] -> V3A -> V3B (CH1 master) ->EQ ->V6B (if FXloop is used, solo and master volume, otherwise passes on to PI)->V7(A/B) PI section -> feed to Power tubes.

There is only one preamp tube that is used by one channel and not by the others, V2 is associated to CH2 only.