Hey Max, seems you have found a workable solution , now we need to work out why and, if applicable, come up with a way of building it into the unit (possibly as part of the MDRT mod).
Ciao Rich!
Well, yes...this is a very good workaround and can 100% get rid of the issue.
Now the best would be to understand it and fix it.
I assume that if this problem would be 100% related to mods, it should arise in the same way with any unit that has those mods.
More or less the same happens to my MP-1 and to the ones owned by MikeB and herby.
As soon as they will have time to run a simple test with a buffer or a buffered pedal (the Boss DS-1 or any other Boss pedal will do the job...they are all buffered) in front of their MP-1s, we'll know if this WA would work for everyone...but I assume that it will.
Then, as I discussed with Phil, I think that the only way to understand where in the signal path (input stage?output stage?) the issue has its origin would be that he, as soon as he will have time of course, would take a MP-1 with the same mods (SS+ Ultra with MDRT), connect it to a power amp and speakers with no other devices in between, connect a guitar and set a patch with my "extreme" settings:
OD1 : 7.5
OD2 : 7.5
Master : 5.5
Bass : 6
Mid : 6
Treble : 9
Presence : 12
With such patch and no buffer the MP-1 should squeal like a hot girl having an orgasm.
I assume it will be not so easy to come to a full understanding and find the fix...also because, as Phil explained, the MP-1 already has its FET buffer...maybe that buffer needs to be modified to lower impedance more?
It's still not so clear if the issue could come from an internal oscillation (but in that case the buffer should have no effect, I assume...) or from an external radio frequency captured by cable/guitar and sent into the MP-1 to be amplified by tubes.
We also do not know if environmental conditions could modify it (frequency peak due to different source of radio frequency/noise/disturb? different locations?).
It's a little messy, yes.
But we've a starting point, at least.