Effects of an unballanced ltp phase invertor when driven?

[leftu2]

I've been modding my peavey triumph 60 over the last few weeks.
I've changed a few cathode resistors and bypass caps on a few stages, added a 5h choke
and successfully moved the tone stack further down in the circuit (pre master vol) as
originally it was immediately after the 1st preamp stage and before the gain controls,
i hated that...lol (muddy distortion).
I also have a bypass cap on the 1st stage plate resistor to help tone down some of the
upper high end freqs. The amp is working perfectly, dead quiet at idle....

My issue is the more i turn the master volume up (post tone stack and pre phase inverter) ,
the more high freqs I'm loosing.....significantly.
I'm not talking at extreme volume levels, it's happening by a 1/4 the way up. It's like if someone
was turning the treble control down at the same rate the master volume is going up.

I've been reading up on the ltp phase inverters over the last few days.
During my search I've noticed that  the marshall plexi/jcm, slo100 ltp inverters are using
1M for both grid leak resistors and mismatched plate resistors (82k/100k).
My amp has is running 68k/470k (R77 & R75) for the grid leaks and
68k/68k (R78 & R83) for the plates. My tail is a 22k vs those amps'10K.

I have a plexi, jcm 800 and a slo and I don't have this issue with those so (and I might be way off base)
but I'm wondering if when the more I turn the volume up on this amp, is the fact that one side of the
phase inverter is being driven harder than the other, could be canceling highs?
Are the ltp values of those amps keeping my issue from happening in them?
I have to believe that the issue is after the master volume in the circuit, I assume the changes I
made have nothing to do with this issue but I could be wrong
 

[Merlin]

I don't think your problem is with the LTP per se; one side is not being 'driven harder' than the other. But the input resistance of an LTP is roughly twice the value of grid leak, or 2x68k = 136k in your case. That is unusually low, so as you turn up the volume pot, the loading on the tone stack becomes heavier, which may explain the change in tone (although normally I would expect that to cut bass rather than treable, hmm). You can always change the 68k to 470k or 1M if you wish, and see what happens.
 

[pdf64]

From the text description in post 1, it's unclear to me how those changes have been implemented in relation to the original design, particularly in regard of the signal paths of the different channel modes.
Schematics are a thing because they are the best means of communicating this info.
Without the schematic it's not really feasible (for me) to hypothesise what's causing the issue.
Unfortunately it's not trivial to redraw the whole thing, but it may be feasible to amend the original, eg by hand, MS Paint.

[leftu2]

From the text description in post 1, it's unclear to me how those changes have been implemented in relation to the original design, particularly in regard of the signal paths of the different channel modes.
Schematics are a thing because they are the best means of communicating this info.
Without the schematic it's not really feasible (for me) to hypothesise what's causing the issue.
Unfortunately it's not trivial to redraw the whole thing, but it may be feasible to amend to original, eg by hand, MS Paint.

That's fair, let me see what I can do. I know you can draw and make notes in pdf.
I just assumed since everything was working right before I turned up the master volume that the problem had to be after that. I'll get that done today

[leftu2]

Hey guys, thanks for your help.
Lol...... while modding the schematic so you guys could see my preamp mods, I believe found the issue.
It was a stupid oversight. Also I didn't account that I'm playing through a tiny 1x12 (70/80 speaker) marshall cab
 (the one that came with my 1 watt JVM) and that the bass response in the cab happens pretty quick due to the size of the cab.

Oh, this is the 1st amp I've had with a plate driven tone stack. I've read that the tone controls are more responsive with a plate driven tone stack and they ain't kidding, man this thing is aggressive. Now I need to get more asymmetrical clipping out of it.
Thanks again.....