Hi folks! How are you guys? It's being a while, uh???
I'm back to revive this thread because...I found a possible solution to the squeal issue.
This evening I was talking to a dear friend who's a guitar player and a smart DIYer and electronic tech.
I don't remember how, we ended up talking about my modified MP-1 and I said that I still have to swap R17 to a 560K resistor as MJMP suggested a while ago (yes, I'm lazy...
).
The conversation took an interesting turn when I mentioned the MDRT mod and the new 190V (EDIT: it's 240V) voltage now going to the tubes.
Ricky made an interesting hypothesis about high radio frequencies coming from external environment, going into ADA through the guitar's pickups and being now amplified by 190V (EDIT: it's 240V) powered tubes (this could explain why there were no squeal before mods).
That made sense to me, because:
1) when no guitar is plugged: no squeal
2) when guitar is plugged with volume turned to 0: no squeal
3) when guitar is plugged and volume's fully open: squeal
So, Ricky suggested something from a video he watched: an input filter placed among cable's jack and MP-1 input.
A 68K resistor placed between signal wire and cable jack and 1M resistor placed among signal and ground wires.
I said "ok, I'll do an attempt ASAP".
Then, an idea came to my brain.
I just took my Boss DS-1 (powered and off) and placed it between guitar and MP-1's input: the squeal is gone!
I can now turn Output Volume to max with OD1 = OD2 = 10.0 and set my preferred eq and there's no squeal at all.
Simply no squeal anymore.
The Boss has some magic inside: a buffer.
Simply putting a buffer among guitar and MP-1 is enough to get rid of that squeal: signal is less sensitive to external noise.
No noise is going into ADA through pickups, no noise to be amplified.
I've done this with another pedal (a Klon Centaur clone) that has a good buffer and result is the same: the squeal is 100% gone.
When I remove the buffer, the squeal is there again unmodified.
So, I'll do another attempt with the filter Ricky suggested, I think this is the way.
I didn't change anything inside my MP-1 yet (R17, some caps...) and I'm not going to do that now: if it's external noise, a filter will kill it.
@MJMP: Phil, if this would be the point...do you think that MDRT mod would require to add an input filter (or a better one, if a filter is there already) inside MP-1 as a permanent mod?
Does all this make sense to you, mate?
@MikeB: can you do a similar test, man?
Of course, if that were the problem...then environment can be its origin and different environments can produce different effects...in some cases, I assume that the squeal could change (or disappear) just by moving the MP-1 to another place.
A filter like that should manage a good range of high radio frequencies, by the way.
I should take my MP-1 to another place, to be sure about that...but I'm happy enough with that buffer, so far!