|Power||2x 6L6 or 2x 5881|
|Preamp||2x 12AX7 2x 12AU7|
The M-212-8B Maestro is a rare 2x12" amplifier that produced around 1952-1953. Not many survive today.
The circuit was divided between two chassis units.
The top chassis had controls, inputs, and pre-amp tubes, while the bottom chassis had transformers, rectifier and power tubes.
This amps had a lot of old school radio tricks that are bound to confuse modern tube amp technicians.
The filaments for all tubes are strung together in series and placed in series with a 75Ω resistor between the 6L6 power tube cathodes and ground. This results in the following: (1) slow startup for the amplifier, (2) 6L6 cathodes at at +51VDC relative to ground, and (3) DC supply for filaments. More complexity ahead: instead of connecting to ground, the grid leak resistors for the 6L6's connect mid-string in the heater series so the grids at at +29VDC relative to ground, so the 6L6 cathodes are around +30VDC relative to the grids.
It is possible to undo all of this and attempt a more traditional cathode bias setup, but it is a lot of rewiring, and it won't be easy. If the amp is working, I'm not sure it's worth the effort to try to fix it.
There were eight tubes in the M-212-8B (hence the "8" in "-8B") plus a additional ballast tube that looks like a metal can tube of some type. This ballast tube was in the output speaker circuit and was an attempt to match the output transformer to the two 12" speakers. The amp can be modified to not use the ballast tube. Matthew Miller sent in transformer readings of a M-212-8B he was servicing. The impedence ratio is 313Ω:1Ω.
The original speakers were two 8Ω speakers in parallel, which if connected directly to the output transformer would have put 1.2KΩ load on the 6L6s (not good). Instead, Magna put this ballast load in series with the speaker. If the 8Ω speakers are wired in series and directly connected to the output transformer, the load on the 6L6's is about 5KΩ, which is better for 6L6's.
The ballast tube incorporates a 3KΩ resistor to supply the amp's NFB circuit. If the amp is modified to remove the ballast tube, a resistor will need to be added in place. (the NFB yellow wire actually connects to the TRS plug that goes to the upper chassis and NFB is applied to a triode there.