Working on a '79 112 RP 65, all parts date to late '78/early '79, came with EL-34s installed and in need of some TLC. It has the black background badge logos with silver lettering/characters.
Cleaned it up cosmetically and electronically, cleaned all pots, jack, sockets, fully recapped and biased it up correctly.
It is a very sweet sounding amp, great Phasor. Came with a clearly non-original reverb tank, replaced it with a recommended 4FB3A1B reverb tank that souded better, but still not particularly strong. Reverb is much better with a 8BB2A1B tank (like the one in my RD 50).
The big issue is that this amp doesn't get loud. My 210 65 (12AX7 PI) is considerably louder on Low power with Volume at 3 than the RP is at High power with Volume at 5 (Master Volume at 10 for both). Tried all combinations of Gain and Master Volume settings, in every case the RP 65 is significantly less loud. It will break up nicely with Gain at 8 and Master turned down, so I don't think the output transformer is bad. And voltage pretty much doubles at the power tubes when flipped from Low to High, so I think the power transformer is ok.
There was no tube chart in the cabinet, but it came with E34s and it has the large white 1.5K resistor on pin 4 of one of the power tubes. According to the schematics, EL34 is the right power tube for this amp. My understanding is that MM went to 6L6 power tubes in 1980.
Replaced all the opamps, didn't seem to make a difference
On a hunch, I installed a pair of 6L6 power tubes, rebiased, and both the volume and reverb improved dramatically. And the tone of the amp is as good as before.
Do I have some kind of transitional amp that requires 6L6 power tubes? Or are my driver transistors shot, and need to be replaced to sound right with EL34s? Would anyone have the proper voltages to check?