I've got an old Peavey Heritage VTX 212. It's 120 watts powered by 4 6L6 GC tubes, but it has a solid state preamp. There are three separate PCBs; preamp, tube, power, and the amp has a nice roomy chassis to work in.
I think a fun project would be to replace the solid state preamp with tubes -- maybe something Reeves-inspired. I'm sure the power requirements of a tube preamp are different than a solid state preamp, so I'd have to figure that out.
Does this sound doable?
Replacing a Preamp
Moderators: pompeiisneaks, Colossal
Re: Replacing a Preamp
There are only a few things to worry about here.
1) Heater current and heater voltage - each 12Ax7 needs 300mA at 6.3V or 150mA at 12.6V, and each has 2 triodes. If your PV doesn't have a tube as a phase inverter, that's one whole 12AX7 right there (for most 4-tube power amps). Fender-style (non-OD) preamps need one more tube for the preamp section and one or two more for reverb and tremolo if you want them. Marshall-style preamps need 2 tubes. Most OD type preamps - Mesa, Dumble, etc use two or more tubes. The upshot - your power transformer's heater winding needs to be able to source the extra heater current, or you add a filament transformer to feed them.
2) Holes in your chassis - each tube needs a socket and a hole in the chassis to stick it in - roughly 1" diameter hole + 2 mounting screw holes.
The high-voltage supply to the preamp shouldn't be a big problem with a big power amp like that - a couple of mA per triode.
Hope this helps!
1) Heater current and heater voltage - each 12Ax7 needs 300mA at 6.3V or 150mA at 12.6V, and each has 2 triodes. If your PV doesn't have a tube as a phase inverter, that's one whole 12AX7 right there (for most 4-tube power amps). Fender-style (non-OD) preamps need one more tube for the preamp section and one or two more for reverb and tremolo if you want them. Marshall-style preamps need 2 tubes. Most OD type preamps - Mesa, Dumble, etc use two or more tubes. The upshot - your power transformer's heater winding needs to be able to source the extra heater current, or you add a filament transformer to feed them.
2) Holes in your chassis - each tube needs a socket and a hole in the chassis to stick it in - roughly 1" diameter hole + 2 mounting screw holes.
The high-voltage supply to the preamp shouldn't be a big problem with a big power amp like that - a couple of mA per triode.
Hope this helps!
Re: Replacing a Preamp
Thanks, this is great!
Re: Replacing a Preamp
Very doable project.
I recently picked up a solid state Randall just so I could have the head cabinet and chassis the rest is going. I tried to amp out and it obviously has serious PCB problems and every control was bad so out the PCB goes. That will be this winter.
Mark
I recently picked up a solid state Randall just so I could have the head cabinet and chassis the rest is going. I tried to amp out and it obviously has serious PCB problems and every control was bad so out the PCB goes. That will be this winter.
Mark
Re: Replacing a Preamp
I've actually converted 3 of these amps, 2 - 65watt models and one 120 watt model. The PT handles the additional heater current with no trouble at all. All 3 amps are Dumble style amps but each a little different.
On the first one I used 3 12AX7s, V1 for clean, V2 for overdrive stages and V3 for PI, then the 2 6L6's for output. The second one I added an ECL86 for reverb, on the 3rd I used a 4th 12AX7 for an active effects loop.
#2 is my main gigging amp in the 2x12 combo cabinet with Eminence Tonker/Swamp Thang speakers. Sound amazing and solid as a rock. Only problem, it weighs about 100 lbs.
IMHO it's best to just gut it and make your own boards. I used 3 boards, power supply, main board and relays for channel switching.
On the first one I used 3 12AX7s, V1 for clean, V2 for overdrive stages and V3 for PI, then the 2 6L6's for output. The second one I added an ECL86 for reverb, on the 3rd I used a 4th 12AX7 for an active effects loop.
#2 is my main gigging amp in the 2x12 combo cabinet with Eminence Tonker/Swamp Thang speakers. Sound amazing and solid as a rock. Only problem, it weighs about 100 lbs.
IMHO it's best to just gut it and make your own boards. I used 3 boards, power supply, main board and relays for channel switching.