I luckily picked up one of these at the local Ham Radio swap meet. I'm going to turn it into a guitar amp / experimentation platform, to try out some ideas I've had. It will not be a "get a fender schematic and go for it" type build. An additional bit of luck, the amp came up on the Variac with no red plating of the 7868 output tubes and connecting a speaker, I've confirmed a signal makes it through.
Here's what the amp looks like with the cover off;
I gutted the whole thing, leaving only the output stage intact. Bought two new sockets, salvaged one from the mic transformer position. I was hoping to use at least some of the original circuits, but after wrestling several crimped wire leads off socket pins, I decided on a clean sweep.
Even testing my filament connections - smoke! How could this be? Turns out one wire at the upper left corner of the black socket possibly was touching a ground tab provided by that socket. Dont know why that would short circuit the heater winding...looking at the terminals where the PT heater winding attaches, neither are grounded. I ended up replacing the TWP from the power tube sockets and now all 5 tubes light. The original TWP was in bad shape; the short may have been there in a breached jacket in the first place. Nice way to beat up my PT, right out of the chute. Hopefully, this incident isnt foretelling of how things are going to go with my project.
I'm going to start completely **s backward, by making the phase splitter be the very first tube. Dual pots for all controls isnt economical, but this isnt a production design. From what I'm reading, using a 12AX7 as this first tube, LTP with a current sink, I need a small negative voltage somehow at a small current. I realize I cant steal it from the existing -20V output tube bias supply, because, there's current and that would make that voltage not be -20 anymore.
Hopefully, my next post will show a solution and the first stage built from input jack to two, 180 deg out of phase signals, coming from a dual pot which I'd label "Pre". Thanks for watching, be sure to click "subscribe" ;')
Here's what the amp looks like with the cover off;
I gutted the whole thing, leaving only the output stage intact. Bought two new sockets, salvaged one from the mic transformer position. I was hoping to use at least some of the original circuits, but after wrestling several crimped wire leads off socket pins, I decided on a clean sweep.
Even testing my filament connections - smoke! How could this be? Turns out one wire at the upper left corner of the black socket possibly was touching a ground tab provided by that socket. Dont know why that would short circuit the heater winding...looking at the terminals where the PT heater winding attaches, neither are grounded. I ended up replacing the TWP from the power tube sockets and now all 5 tubes light. The original TWP was in bad shape; the short may have been there in a breached jacket in the first place. Nice way to beat up my PT, right out of the chute. Hopefully, this incident isnt foretelling of how things are going to go with my project.
I'm going to start completely **s backward, by making the phase splitter be the very first tube. Dual pots for all controls isnt economical, but this isnt a production design. From what I'm reading, using a 12AX7 as this first tube, LTP with a current sink, I need a small negative voltage somehow at a small current. I realize I cant steal it from the existing -20V output tube bias supply, because, there's current and that would make that voltage not be -20 anymore.
Hopefully, my next post will show a solution and the first stage built from input jack to two, 180 deg out of phase signals, coming from a dual pot which I'd label "Pre". Thanks for watching, be sure to click "subscribe" ;')