Sunn Model T master volume

[Sansteeth]

Hello everyone,
been a long since I've been on here, quite busy with work.
I'm working on this 2nd generation Sunn Model T, I'm noticing something strange with the output, past a certain point on the master volume, the signal starts clipping (so far so good) and then the signal gets weaker and cleans up.
I'm not familiar with the Master volume arrangement on that particular amp, is there some kind of negative feedback on there?

here are my observations so far
Normal Channel
Norm volume to the MAX
Treble & Bass pots to the MAX
Mids at 5
Mids switch to 750

Output goes up to around 27Vac @ 8ohm with MV at 4.5/10, then output clips between 4.5 and 8.5/10 and then cleans up again at 8.5/10 and goes down to 24Vac at 10/10 on Master volume pot.
If mids pot is maxed out like the other tonestack pots then the dip after 8.5/10 on the MV pot is not as dramatic

If the midswitch is on 1.5K, then the max is 17Vac with MV at 7/10 and then dips down to 10Vac from there
If Midswitch is on 3K, I'm seeing the same phenomenon.
Besides that, but maybe related to it, I'm surprise by how much signal I lose switching from 750 to 1.5k or 3k position on the Mids switch.

The Sunn Model T is an interesting amp for me, the mid control arrangement is wild to me, so is the Master volume, if y'all have any input on whether or not what I'm seeing is normal or if I should investigate further.

Thanks a ton!

[pdf64]

You’ll probably be seeing the effects of bias shift / excursion / onset of blocking distortion.
It’s normal for that kinda amp design.

[Sansteeth]

Hey Pdf64,
thanks a lot for your response!
I see how how bias shift could affect what I'm seeing, that makes sense
Another lead: after talking with another amptech friend, he mentionned that the MV arrangement might work as an attenuator too, possibly to keep noise down at lower volume and keep the amp clean at higher volumes.
This arrangement looks pretty wild to me!

Thanks a lot again for the pointers

[pdf64]

The master volume is strange; it may be that when set to max, the signal at the input the the V3 tone controls is attenuated by the first track.
So it might be there to prevent the total system gain becoming too high.