Newbietube wrote:Hi all I've just aquired a working but rather old peavey deuce amp chassis as the basis of a bass amp project. My plan is to completely gut the entire amp and start over. This will mean removing the pre amp pcb which is all transistor and probably replacing the power tube sockets and psu also. I've found a pre amp circuit I want to copy but the main issue is the driver circuit which is transistorised. I need a circuit which will work with the existing power amp as i do not wish to change that too radically as I'm not sure if the O/P transformer will handle other circuits too well. I have a schematic of the original peavey circuit and it appears to be similar to a fender bassman but uses no screen resistors and has diodes going to ground from the O/P tranformer primarys.
Any idea on this will be gratefull recieved.
Cheers Just
This amp really isn't anything to write home about. I service Peavey amps in my shop and I got one of these things in right now. I would say that your idea of a complete gutting, outside of the output stage is a wise choice.
Most of the expense and work is already done for you in lieu of the power and output transformers and the physical connection of the output tubes. So that is the most expensive part done right off the bat.
What I will say is to look over this site:
www.ax84.com
Click on the "building blocks" tab and from there you can go to various pre-amp and power amp designs.
They do have a 50 watt EL34 amp listed there I believe. You could also copy the output driver/splitter section for a Fender Twin. These amps used 6L6 output tubes.
One thing you will have to add is a filament transformer for the pre-amp tubes. Reason being is that if you are going to add 4 or 5 preamp tubes, the currents do add up and it can tax the existing power transformer. So you can get an extra 2-3 amp 6.3 volt transformer and bolt it to the chassis. Given that the average 12AX7 draws 8ma, you might be ok. But with 4 tubes that would be 32ma extra on the B+ supply. The existing transformer MAY handle that. If in doubt or you want to have a 5 or 6 pre-amp tube design then you are going to have to with a second full transformer that will supply both the filament AND the B+ to the output drive. This is realistically better because you can totally isolate the pre-amp supply from the power section.
What you also might want to do is put a bias selector in, in case you want to try out EL34 tubes in the amp. The EL34 has a similar loading to the 6L6, but you have to bias it differently. This will expand more options in terms of the types of output tubes. You probably can use KT-66 and KT-77 tubes too.
You will also need to get a nice Greenlee punch to punch out perfect holes in the chassis so this way the job looks 'pro'.
I believe the end result will be vastly superior to the amp's original design. But if you follow the AX84 site, you can taylor the pre-amp stage to your musical tastes/playing preference. Combine that with any special mods you made to your guitar and you can created your own nice signature sound.
Hope that helps.
Geo
Exit Light, Enter Night, Take My Hand...We're off to Never Never Land.