Graft tube pre into existing tube guitar amp?

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jjai

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This may be territory that has already been covered, but I haven't been able to find anything in the archives--

I recently purchased a Roland Bolt 60 guitar amp (which is the only tube guitar amp that Roland ever made). It's a 60W 2x6L6GC power amp with a 12AT7 phase splitter. It sounds decent, but the preamp, which is solid state, sounds HORRIBLE, especially when overdriven.

I had the crazy idea of grafting a 2 channel tube pre onto the power amp, and eliminating the existing SS preamp completely.

I wanted to check to see if you all think this is worth the effort/possible/how difficult it'd be? (I'm not a total newbie but I'm not a guru. I've built a couple stompboxes, mics, and various little gizmos, but have very little knowledge about tubes.)

Specific questions are:
- could/should the preamp tube(s) share the same transformer as the power amp/where would the pre get its power from?
- are there any plans for a suitable 2 channel tube pre that I could use as a model?
- I don't have a schematic for the amp, and can't locate one. Is that reason alone to abandon this project?
- What about the idea of grafting in a solid state pre, just one that sounds better?

I fully understand that the smart thing to do would be to just plug an off- the shelf pre into the power amp in on the back of the chassis.... but that wouldn't be any fun!!

-jerry
 
It sounds do-able. One or two 12AX7s set up in the usual way shouldn't tax the power supply enough to cause problems. Step #1, of course, is finding a schematic for the amp. An experienced tech could trace it out well enough to do what you're proposing, without the schematic, but I don't know your level of skill/comfort.
 
These amps were always nice, one of the things I liked was all the ins & outs.

Bearing this in mind, you could build a separate pre & go straight into the power amp input for e.g. I know you want to integrate it into the main amp, but before you do any major surgery, this might be an option.

Here is something to whet the appetite :

http://www.tubefreak.com/tubefreak.html

Peter
 
before you change the preamp to a tube one, post what ICs are used in the preamp. I have changed preamp ICs and some caps in some solid state amps like the marshall lead series and they did sound alot better IMO.

I am not saying not to tube the amp but tremember Music Man amps were solid state pres to phase invereter.

The tubes used sound like maybe a fender style tube pre could be a good first try
 
Maybe one of these would be ok .... the preamp section for a hi-octane for instance.

http://www.ax84.com/projects.htm

This seems lke a good idea for a project to me. It could probably be done pretty easily. I built a hi-octane it is a nice high gain preamp section.

Kiira
 
Or you could do a F**der circuit -- lots of those in the Tube Amp Book. Mostly two stages of 12AX7 with a tone/volume control circuit in between. You'd power it via a dropping resistor and filter cap from the main B+ supply, or more likely two dropping resistors and filter caps. The schematics give you enough information to calculate the tubes' current draws, after which calculating the dropping resistor(s) is a piece of cake.

Find a schematic, someplace (advertise on a guitar newsgroup?) and mod away.

Peace,
Paul
 
Sounds reasonable to me...I'd just suggest start simple... get it going with one channel then build up. Remember you're still limited by the quality of the output transformer........ while you're doing your homework, find a mod to convert it to run on EL34's ...it's a simple adjustment.....you might prefer the tone....you might not.

Actually, consider doing this first. After playing the amp with EL34's running class A (high current, running hot) ....... you may never bother with the preamp distortion again. :cool:

Should find something useful here: www.blueguitar.org
 
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