Peavey Rockmaster Preamp mods

[Bob-I]

I have 2 Peavey Rockmaster Preamps that's I've had hanging around for like 20 years. For a simple setup direct into the PA these work great, but there are some things I think can be improved.

The gain channels have a funny middy tone, almost like a cocked wah. I upped some of the coupling caps and bypass caps on the first few gainstages and it's a tad better. I'm still getting harshness in the highs so I'm working on taming that with a low pass filter.

My reason for posting is that the tone stack on the crunch/gain has me baffled. It's not like anything I've seen before, meaning not the typical FMV where these a slope resistor and the treble acts as a voltage divider, mid and bass drop to ground.

The schematic is attached. I'm ok with the bass and mid controls (bottom and body) but the edge is not what I'd like. I'd like to get more shimmer and less snot. I'm not following how C22 C23 and R31 interact.

There's also some confusing stuff going on with the gain stages. V3b seems to be driving the tone stack by way of C19 and the post volume controls (relay switched). Then the signal exits at J400 to the grid of V4A. The plate of V4A then goes through C24 and back into the tone stack

I'm just looking to get the edge/treble control to give more shimmer. Typically I'd use a FMV tone stack with somewhere between 250pF and 500pF but I'm not understanding the 2-100pF caps.

Any help is appreciated.

[Bob-I]

Wow, 32 views and no reply. I guess I'm not the only one confused by this tone stack.

[tictac]

Seems like the signal path from the output of the 2nd triode to the output of the 3rd triode might cause some over-trebliness... Have you considered perhaps raising the value of C14 (.005?) C17 (1uF?) and jumpering the diodes CR1,2?

That tone stack you were mentioning does look a little wacky and part of the problem could be the way it's drawn. Maybe you could redraw that tonestack to a more familiar looking configuration and then it might be more apparent whats going on there...

just a thought...

TT

[Bob-I]

Seems like the signal path from the output of the 2nd triode to the output of the 3rd triode might cause some over-trebliness... Have you considered perhaps raising the value of C14 (.005?) C17 (1uF?) and jumpering the diodes CR1,2?

Yes, C14 is now a .0047uF, C17 is 1uF, huge improvement especially on the clean tone but the OD is still very middy.

That tone stack you were mentioning does look a little wacky and part of the problem could be the way it's drawn. Maybe you could redraw that tonestack to a more familiar looking configuration and then it might be more apparent whats going on there...

just a thought...

TT

Good idea.

[Bob-I]

I think what I figured out on the tone stack with help from another forum, is that it's an active EQ. After the RC filter one side of the pot goes to ground, the other side goes to a gain stage.

I adjusted the 2-100pF caps on the edge control, brought them down to 47pF. It now controls higher frequencies, maybe around 8K. It's a tad too much buzzyness but I can tame that with a low pass filter.

Next I'll look into getting a solid low end thump from it.