Having taken some voltage measurements on Friday night (more of that in a moment) I dedicated yesterday afternoon and evening to stripping the old amp. I tried to preserve as much that is directly usable as possible. This includes:
- The power transformer and its wiring
- The rectifier tube socket and its wiring
- The amplifier tube sockets (I did destroy the brown noval, which is a pity but I’m intending to use it right now, so I’m not devastated)
- The heater wiring to the sockets
-The output transformer and its connections to the power tube sockets
I’ve left some terminals attached to what will ultimately be the front face of the amp but I’m not sure if I’ll end up using them or stripping them at a later stage (they are riveted in place so stripping is a pain)
I’ve also left the expression pedal housing (silver box on the top) because it’s the same height as the power transformer and allows the amp to stand nicely on its head. I may even leave it there to protect the tubes until I build a proper cabinet for the amp.
Back to the voltage. It looks like the secondary of this transformer is +-375V, which is quite a lot higher than the +-335V on the schematic. This, coupled with the fact that it was originally meant to run through a tube rectifier and I’m intending to use diodes, means that my B+ is going to be high… up around 490VDC. Even putting this through a stoutish resistor will put it at the upper end of what the 6V6S can handle.
So… I have a couple of options. Either I can look at dropping the voltage… or I could upgrade the power tubes
Dropping the voltage could be as simple as putting an extra filter stage ahead of the power tubes but I might also look at just using the tube rectifier since I have it and it’s already wired.
Changing the power tubes to 6L6s is very tempting, but there are a few draw backs. Firstly, I want to play this at home and I’m worried they might just be too loud (time for that l-pad) secondly, I doubt that the poor output transformer would be able to take it. It was specced for 6V6s and I doubt that they would have included enough fat to move to their bigger brothers, even if I could sort out the impedances.
I’m going to toy with the options and see what I come up with. I'm hoping to finalise my design and start ordering parts in the next few days. With a little luck I might even be able to start the build proper next weekend.