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RCA pa amp to guitar amp conversion


 
4/24/2003 5:50 PM
hasserl
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RCA pa amp to guitar amp conversion
I recently picked up an old RCA pa amp that I am considering converting into a guitar amp. The amp uses 1 x 7025 & 1 x 6AU6 in the pre-amp, a 6SL7GT twin triode for the PI and two 6L6's for power. I am considering using the Matchless Clubman as a basis for a guitar amp design, since this was the one amp I found that uses a pentode in the pre-amp (though it is a 6SH7 not a 6AU6). And the number of pots available matches what I would need for a Clubman style design. I also see that Mark Hammer used this pentode in a couple of his homebrews.  
 
Anyway, can anyone offer some other suggestions for topologies using this tube arrangement? Or other words of advice before I start out on this? One of the thngs I am concerned with is the Clubman parrallels the first pair of triodes thru a Bass and Treble pot to the Pentode for two gain stages before the PI. Will this be enough for decent gain? Will I need to add a pedal in front of the amp for good classic rock type tones? Would it be better to cascade the triode stages instead of running them parrallel to get up to three gain stages?  
 
Mark, any sugestions on biasing the 6AU6?  
 
Thanks for any help offered.  
 
hasse
 
4/24/2003 8:51 PM
Jason A
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hasserl,  
 
Oh, that sounds like a pretty sweet find. Check out Mark Lavelle's site for using 6AU6's at http://www.harmonicappliances.com/  
 
Jason
 
4/24/2003 10:58 PM
hasserl
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Thanks Jason. This thing is so clean it looks like it was installed in a cabinet somewhere many years ago and never touched. Here's some poor quality pics: http://www.webphotos.com/list_photos.asp?mi=3&smi=1&a=106388
 
4/24/2003 10:13 PM
Mark Lavelle
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quote:
"One of the thngs I am concerned with is the Clubman parrallels the first pair of triodes thru a Bass and Treble pot to the Pentode for two gain stages before the PI. Will this be enough for decent gain?"
The 6AU6 can get you a gain of 100, so you should be feeding your PI plenty of signal (assuming no problems with the effects loop – it's pretty high impedance).  
 
As Jason mentioned, I've got a little experience with 6AU6s. See the Decimator & Pentotron schematics on my site: http://www.harmonicappliances.com
 
4/24/2003 10:54 PM
hasserl
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Hey Mark, thanks for the reply. I knew it was you that built a couple of amps with the 6AU6, I don't know why I posted the wrong last name. Brain fart I guess. Sorry about that.  
 
I'll probably skip the effects loop and go straight from the volume pot thru the caps and resistor listed to the PI. I'm just a little concerned that when it's done I'll need to push the front end with a pedal to get it to break up with a good bluesy sound. Maybe my concern is unfounded.  
 
I wonder how this design will work with the 6L6's instead of El34's. I'm not sure if the PT is up to supplying the added heater current draw of those tubes.
 
4/25/2003 12:23 AM
Mark Lavelle
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Wow! That thing is in great shape!  
 
I think you're right on target with the Clubman topology minus the loop. If you've got any signal at all after that tone stack (and it doesn't look like it'll pull it down all that much), the 6AU6 will be able to boost it higher than the 6SL7 can handle without breaking up. If you insist on worrying about it (;)), I'd worry whether the 6SL7 will be pushing the 6L6s hard enough. You might end up wanting a 12AX7 for the PI...  
 
Can't comment on 6L6s vs. EL34s (I'm a low-wattage guy), but with the bit more gain in the PI that you can get from a 12AX7 I'm pretty sure you'll push the 6L6s as hard as (or even harder than) the EL84s in the Clubman. If it were mine, I'd stick with the load and impedance that the trannys were designed for.
 
4/25/2003 6:35 AM
Regan
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Isn't the 6sh7 the octal equivalent of the 6au6?  
Sounds like the clubman is right on target.  
Regan
 
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