Peavey Rockmaster pre mods

[fuzznut]

Yes, the preamp. Yesterday I did a lot of work and there was a lot of discovery happening. I liked the sound so much, I duplicated the circuit in my "test bed" amplifier chassis. What I found was:

1- the rockmaster is, save for a few minor changes, the same circuit that is in the XXX and the JSX.
2- It already has a low pass filter (C500 and R600). This is why the bass response is so great. But it's slope is not enough to get rid of that annoying midrange.
3- The JSX has the extra low pass cap, like what I did, except that they put them on the wiper of the volume pots.
4- The XXX and JSX both have "C14" at .0022, instead of .001, and this is the chosen component on those amps in which to add more "meat". In the JSX, you can double it to .004, in the XXX you can bypass it all together, which must sound like hell if you use too much gain.
5- all these amps, the last gain stage is moderately loaded down, an example would be the 56k resistor to ground in a trainwreck express. It is my understanding that this has the effect of giving compression and sustain, which might be why this preamp "sings" so well. I don't know, I'm just a bum on the street.

[silentrage]

A bum?
No way, you're like my new god!
I can't wait to try the mod!!!
Man I wish I knew enough electronics to discover some on my own, so fast the only thing I found while prodding around in an amp is what a shock feels like.

[garyhb]

Fuzznut - thanks. Nearly bought the pre a few years ago but didn't because of the midrange. Wish I had now...

Silentrage, shock? if you were like me you probably weren't silent... Glad you lived to tell the tale.

[silentrage]

Yeah, got shocked twice in my mark iii, about 3 times in the rockmaster, I wonder how I lived too. @_@

[rockon1]

Been a while and I pulled out my Rockmaster. Still too harsh. Anybody try fuzznuts low pass mod? Also is there possibly an easier spot to place this mod? Thanks Bob

[rockon1]

So I found a suitable spot, across R400 with a .0047 cap, and... WOW!!! What a difference!! That annoying peak is gone, and the sound is much more even, and fuller. Between the edge control an the presence control on the power amp, the highs are much nicer now and not fizzy.
So whoever has a rockmaster, I suggest trying this. The .0047 cap is not written in stone, it depends on your gear, but I found that for me the annoying midrange starts leaving at .0022, and above .001 I have to compensate the HF rolloff too much with the edge control. Try it!!

Tried this today. Really changed things but a bit to dull/dark. I'll have to order some .0022 caps and try them. Bob

[rockon1]

Tried this today. Really changed things but a bit to dull/dark. I'll have to order some .0022 caps and try them. Bob

OK so I put an .0022 across R400 and I really like it- as long as I dont pull the gain boost on either the crunch or lead channels. Toned down the edgy upper mids and treble nicely. It becomes too upper mid/trebly aggressive again with either gain boost engaged. If anybody is following this thread perhaps they might explain why it works so well until I engage the gain boosts? Heres a schematic. Thanks Bob

[rockon1]

Been a while since I revisited my Rockmaster. No need for big rigs! Anyhow just for curiosity I hooked it back up to my Classic 50/50. Still have the .0022 cap across R400. Seemed a bit harsh still. Hmmmm. Opened it up and looked at the glass. Bunch of old Sylvania short plates and an RCA blackplate. Nice tubes but I wanted to try the darkest tubes I know of -JJecc83s's. Put all four JJ's in it and either something else happened along the way(connection-wise)or the JJ's really smoothed it out. If I had the dough for a single rack space power amp I'd definitely make a cool 2 rack space head with it. Bob