Mesa 2:90 Adjustable Depth Mod?

Luca80 said:

Hi everyone,
I have a 2:90 power amp that pairs my Triaxis. While it's just great tone wise, it would be great having a global adjustable control for the deep mode, which is great with some Triaxis modes but not others. In fact, sometimes I found myself zeroing completely the bass setting on the Triaxis to reduce a particular preset's low end. On the other hand, if I disengage the deep mode, presets are too flat. Basically, it works at two extremes only.
Hello. I may be not much of assistance but I'm very interested in your thread cause I've owned one pretty much when they came out . tho I dont use mine with a triaxis. Though I'm no amp tech, I've done a good bit of repairs on mine.
Now I know that I can save the deep mode status in each Triaxis' preset, however point is to have a quick global control without having to edit the single sounds.

Idea would be having a dual concentric pot for presence and depth, since there is no room for an additional pair of pots/knobs on the front panel.
Do you mean just modding the presence pot value? A treble bleed circuit ? Or perhaps a push/pull pot?
Having a look at the 2:90 schematic (provided that it is correct), I can see the deep mode is engaged by RELAY 1, which is a DPDT type so engages it on both the channels at the same time.
I've spent many hours tracing the schematic. Its pretty accurate. The only thing that's not showing is a 100k resistor In series on the base of Q5 transistor in switching matrix, at least in my amp.

It looks like that, when deep mode is engaged, RELAY 1 closes and part of the signal flows through R272 (3300k resistor). If disengaged, all the signal goes through C4.
Maybe, but as mentioned, I'm no tech, changing value of the resistor? Can resistors change frequency curves? I know in preamp circuits caps usually do that. Or change C4 to a different value. Higher values pass less bass? But that may affect things down stream. Plus you would need to do it on both channels right? I don't know.
So questions are:
- is it possible to install a pot in parallel with R272 to have adjustable depth? And if yes, what value would be recommended? As you mentioned, where?
- is RELAY 1 normally open or closed? That is, when deep mode is on, is RELAY 1 open or closed?
It appears that its closed when engaged. Man, I really wish that I would have went to electronic school cause there's sooooo much I want to know. Good luck. I'll be following this thread to see if a real tech chimes in. Blessings
Thanks for help!!!

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