I thought I'd share some of my past PA amp conversions. I have not kept very good records or notes of this stuff. So in most cases I am just writing from memory. Most of these were PA amps that drive speakers, but a few of them were mixers designed to be used with a separate power amp. Most of the mods were based on keeping the stock power and output transformers in place and using the power sections mostly stock. I nearly always change out the weaker driver/inverter stages for my favorite long-tail pair type setup. The bulk of the changes are getting rid of stupid grid-leak bias preamps and tone stacks designed for program material instead of instruments. Even when I do keep stock bits of circuits, I always rewire anyway just so I am sure everything is put together correctly.
Bell Pacemaker 20:
This was an early mod. It wound up needing a replacement power transformer. I think I put some kind of deluxe hybrid circuit in there. I think it had two 6AV6 preamp tubes, one 12AX7 driver/phase inverter tube, and two 6V6 power tubes. There are volume, treble and bass controls. Mode switch that did something cool, I can't remember exactly what right now.
I've done two of these. They make insanely great guitar amps. It has all octal socketed tubes, so I took a Bassman/Plexi type circuit and adapted it for 6SF5 preamp, 6SL7 driver, and coke bottle 6L6G tubes. One of my first paint experiments as well.
Precision Electronics (Grommes) Mixer:
This was originally a line level tube mixer so this one doesn't have a power section. It is essentially the preamp and tone stack from my version of the Bassman/Plexi with a two-band EQ. It also has an extra drive circuit. Stick it in front of even the cheesiest amp and have full tone!
This amp was ready for a champ-like treatment. At first I set up the 6AV6 and single ended 6L6 as a mostly stock power section. The interesting bit comes with the preamplifier stage. Originally I had the 6AU6 set up as a pentode, but the gain was just WAY too high, so I triode strapped it and all was well. Single volume control (microphone) and the tone control (phono) is just my favorite simple high end boost or cut. The tone switch is a mode control that implements or removes a negative feedback loop, taking the amp from tame to wild.
Newcomb Pathfinder 10:
This amp had a very odd stock circuit. It had a 6AU6 pentode and half of a 12AX7 acting together as a phase inverter. I got rid of all that weird stuff and installed a deluxe type circuit in here. Single input, gain control, and treble/bass controls. '10' watts of sweetness
Magnasync Moviola Squawk Box:
I installed essentially the Jekyll & Hyde amp circuit in this lovely little box. It was a beeitch trying to get the internal speaker to keep from rattling, but once I did this was a solid combo amp that had a surprising amount of power. I hooked it up to a full stack of 8x12" speakers once and it was plenty loud enough to keep up with a whole band. Not bad for a 6AQ5 single-ended power section!
Another donor with the Jekyll & Hyde circuit. Though this one has a solid-state power supply and a 7868 power tube so it has a very different vibe. I've made 4 of these and they have all turned out amazing. So simple I think I could mod one from start to finish in less time than it takes to watch the original Star Wars trilogy. (That includes teardown, drilling, total rewire and recap, and basic testing!)
Another very early project for me. I think this was the last time I kept the original can caps installed. They worked great for a while, until one day when they failed and fried up the power transformer and nearly started a fire. I reworked it with a new transformer and all new caps. Lesson learned. The circuit was still pretty much stock. It had four mic inputs and one transformer balanced line output. Great tone with way too much gain! EQ section was subtle but cool.
None of these beauties is still in my collection. They were all sold long ago to further the cause. I just thought I'd share some of the past to help show the evolution to the present. I have several similar amps awaiting mods for the station, so stay tuned.