First, thanks everyone for reviving this thread and chipping in a lot of helpful information. I'd left the Ampeg for a few weeks to work on other projects, but seeing all the feedback inspired me to put the R-12Rb back on the bench the past couple of days.
So, the amp is now completely rewired and works (including reverb), except the output is very low. I made a number of changes to the wiring/ schematic I posted earlier in this thread... after finding another copy of an original schematic (not frayed and missing bits like the one in my amp!), and being able to test voltages to identify where the remaining problems are.
One big problem I realized on power up: Those original 7591s are toast, and so is one of the 6SL7s -- the one that was in the phase inverter position.
- Was getting very low voltages in the P.I. (each plate should be around 210-215 VDC but both were reading around 100 VDC. Swapping out the 6SL7 for another one for the reverb brought the voltages up to where they should be, but now the reverb barely work. Obviously, I need another 6SL7.
- Another of the 6SL7 and the single 6SN7 are very microphonic... lots of dinging and ringing when you tap on them or the chassis. I've read microphony is typical in these tubes. Probably I'll just hunt up a set of 6SL7/ 6SN7 and replace the bunch.
- The 7591s were both bad. Very intermittend output and the familiar spits and crackles of bad power tubes. I wanted to keep going, so I installed a new set of 6V6s (as suggested by a member above), and swapped the cathode and grid connections to the pins, to accommodate the different pinout. Probably I'll hunt up some 7591s and can always convert back, but adding the 6V6s eliminated the noise issues.
- As mentioned to Nick, above, I installed an artificial centre tap -- 2 X 100 ohm resistors to ground from the indicator light -- which eliminated the hum.
The low output is still a concern. I'm worried about the output transformer at this point (plate voltages, B+, rectifier, etc are at, or close to, voltage on the schematic), but am going to check over all my wiring once again in case there is still something I overlooked. Working inside these old Ampegs,with their cramped little chassis, is like building a ship in a bottle -- so it gets tough to see what he hell is going on inside there and get the DMM and scope leads clipped where they need to go, without shorting against neighboring components.
Here's a pic of the amp in voltage test mode...
View attachment 609080
Octal preamp tubes are notorious for being microphonic - you may have to go through a few to find some non-microphonic ones.
Tip - look on ebay at lots of old stock 6SN7 and 6SL7s from a good seller - if you can get them cheaply enough, its a good way to get a bunch at one time.
I have a M-15 that came to me with a computer fan cut into the back and the B+ caps detached and hanging - I am afraid that the power xf in that one will be toast too.