Peavey Rockmaster Preamp Mod Q's

[runaway!]

Hello

I am in the planning stages for doing some of the common capacitor mods to my PV Rockmaster preamp. While I expect that to go without a hitch, I have a couple of Q's about further mods I cannot find info on - plus I'm not too good at reading schematics yet.

I've read about cathode followers a bit, apparently adds some 2nd order harmonics that the high gain modern amps lack - like many Peavey products. Is it possible to add a cathode follower to this preamp? Is it simply about adding a resistor somewhere or does it involve some serious plumbing? I should also clarify that I'd like to do this by converting half a 12ax7 stage instead of adding an additional tube. Unless adding a tube is less complicated.

Looking at the schematics (attached to the thread) for the RM and my power amp (Peavey Classic Series 60), it appears both have a SS rectifier. Am I seeing this correct? I've seen tube rectifier conversion kits available, which one should I replace to get some sag? Can I do both?

There also is an active eq on the crunch and ultra channels, is there a way to bypass this entirely? I'd rather use a passive rack eq for tone shaping, and I suspect an active eq won't do well with a cathode follower.

Rockmaster Schematic
Download peavey_rockmaster_preamp.pdf from Sendspace.com - send big files the easy way

Classic Series 60 Schematic
Download Classic 60 schematic.pdf from Sendspace.com - send big files the easy way

Thanks for your support!

[Enzo]

There are no unused triodes in the Rockmaster preamp. If you wanted to use one for a cathode follower, which function would you like to give up to get it?

A cathode follower is not a feature like reverb - you don;t generally just add one like you add a pedal to your floor board. It is usually in a circuit to change impedances. Like driving the tone stack.

A CF is a whole tube stage itself, so it would require some resistors being different, but it would also mean reconfiguring the circuit.

Adding a tube? Open up your RM and tell me where another tube would even fit?

Sag. The RM preamp, like almost every preamp you will see, is all class A circuits, which means the power draw is basically constant, so there is no sag to be had. You could add a tube rectifier to the power amp, but really, why not just try a little compressor pedal?

I suspect an active eq won't do well with a cathode follower.

Well I guess that all depends upon where you put that cathode follower.


You could replace the active EQ with a passive stack, but that EQ is a gain stage, and a passive tone stack is a big gain loser, so your gain channels would have a lot less of it, the gain. But I suppose that would free up a triode for you.

[runaway!]

Where to put a tube? If I had to, I'd make it on a separate board and wire it in if it was relatively easy to tie into the circuit.

I'm not the biggest fan of an active eq, and if it could free up a triode for a CF - then it is right up my alley. I've got way more gain on the RM than I care to utilize.

I have a rack mount EQ I'd use for shaping, I'd rather bypass the active eq entirely.

When you speak of reconfiguring the circuit, do you mean the entire tube chain? Or just that specific tube? I was under the impression that just the tube gets reconfigured, as the CF circuit is different.

However, I get the idea - you don't think it is reasonable to hack into it, considering your experience - I will drop the idea.

One last thing, my Classic Series 60 PA - You don't by chance have a link to the zip file with the mods on it? At least the bias mod? All the links i've found are dead. Also, wtf does the three 12ax7s do in it? A preamp of sorts?

[Enzo]

My question about where to put a tube was because I don;t remember there being physical room for another tube. Is there actually space where a 12AX7 and a socket would fit?

REconfiguring meaning you would be building a different circuit wihtin the confines of the existing product. For example cutting traces and rewiring the sockets.I am not necessarily telling you not to do this, but I want you aware it is not a simple matter of moving a wire or two. One needs to get on the schematic and decide what the specifics are.

PV Classic 60 -
I only know of two small tubes in that amp, plus the two 6L6s. One is the phase inverter, and the other is the input buffer. The line level inout has to be boosted up to tube signal levels for the PI and beyond.

I am not aware of any zip files, I don;t usually look anything up for a bias adjust conversion. What other mods do you mean? Have you looked over at The Blue Guitar

[runaway!]

My question about where to put a tube was because I don;t remember there being physical room for another tube. Is there actually space where a 12AX7 and a socket would fit?

I'm pretty sure there isn't, maybe with some real creative thinking. I was going along the lines of making it a 2u rack unit (not too smart with electronics, but I can make stuff and bend sheet metal pretty well). Then make a breadboard wired up with my new circuit(s) and tubes for placing in the new upper half with wires running to the appropriate spots on the main board.

REconfiguring meaning you would be building a different circuit wihtin the confines of the existing product. For example cutting traces and rewiring the sockets.I am not necessarily telling you not to do this, but I want you aware it is not a simple matter of moving a wire or two. One needs to get on the schematic and decide what the specifics are.

I'm not at all shy of getting in there and getting it done, I am pretty darn shy on reading schematics and designing circuits however.

PV Classic 60 -
I only know of two small tubes in that amp, plus the two 6L6s. One is the phase inverter, and the other is the input buffer. The line level inout has to be boosted up to tube signal levels for the PI and beyond.

You're correct, only two - my bad.

I am not aware of any zip files, I don;t usually look anything up for a bias adjust conversion. What other mods do you mean? Have you looked over at The Blue Guitar

I'm not entirely sure, I've seen the dead links to the file a few places - and it's file name was always plural. So I am kind of assuming it had more than a bias mod. Is there anything else that could be done to it in theory? And I didn't see anything in BG, not even in the archives.

[Enzo]

Don't forget Peavey also made the Classic 60/60, which is a stereo version of your amp. That might explain some plurals.

If people are linking to the same place, then they all will get the same result.

Bias adjustable mods are pretty straightforward, and can be done without a lot of sweat. I couldn;t tell you if someone has written up specific instructions for the C60.


Other than make the bias adjustable, I am not sure what other mods one might want to do. This is simply a power amp. I suppose someone could have engineered a presence control into it.

Bias mod would be essentially increasing R22 by a factor of 6-8, and making R20 variable.

[runaway!]

Bias adjustable mods are pretty straightforward, and can be done without a lot of sweat. I couldn;t tell you if someone has written up specific instructions for the C60.

Bias mod would be essentially increasing R22 by a factor of 6-8, and making R20 variable.

So, a 5k pot? Or maybe a smaller value pot and some resistors in series?

[Enzo]

Make R22 about 3k. 2.7k and 3.3k are standard values.

R20 is 47k now. Replace it with something like a 27k resistor and 20k pot in series. or anything remotely similar.

[runaway!]

Great, thanks for your help!

[kbhe]

Tube v4 is cathode follower in Rockmaster Preamp! Check it out! Common send,common return.