Mesa 2:90 Adjustable Depth Mod?

[Luca80]

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.

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.

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.

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.

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?
- is RELAY 1 normally open or closed? That is, when deep mode is on, is RELAY 1 open or closed?

Thanks for help!!!

 

[Henz]

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!!!

 

[Luca80]

- Do you mean just modding the presence pot value? A treble bleed circuit ? Or perhaps a push/pull pot?
No, CTS (or other brands) makes dual concentric pots, think of the one used for the main level out on the Triaxis panel to have an idea. The two pots are built one over the other so they're completely independent but they use a single hole on the panel. One would be used for presence (250k) and the other one for adjusting depth (value to be established yet).

- changing value of the resistor? Can resistors change frequency curves?
Usually resistor and cap work together and their values give the high or low pass filter's cutoff frequency. Since the goal is to achieve adjustable depth, my idea is that the resistor value has to change by using the additional pot, basically between no depth (when it's disengaged) and full depth (when it's engaged).

- But that may affect things down stream.
I don't think it will affect that. From the schematic, you can see in the negative feedback circuit there are in series: depth section, presence pot and modern section (which is basically another filter). No matter if depth is on or off, it doesn't really change the tone's top end.

If RELAY 1 closed means depth engaged, that means that the additional resistor/pot has to go from infinite resistance to simulate and open circuit (relay open) and R272's value when closed.