Magnatone Guitars!

New for Fall 2013, a complete guide to Magnatone guitars and the stories behind them!

1938-1960 Steel Guitars

1956-1963 Bigsby/Barth Era

1964-1966 Starstream Era

 

Magnatone Maestro M-212-8B

Years 1949-1951?
Power 2x 6L6 or 2x 5881
Power Bias cathode
Preamp 2x 12AX7 2x 12AU7
Speakers 2x 12"
Rectifier 2x 5Y3

 

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.

Schematic

 

DC Heaters

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.

92-105-A ballast tube

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.

NFB

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.

DA/MagnatoneAmps.com

 

 

Except where otherwise noted, text/written content on this site is licensed under a Creative Commons Attribution 3.0 License.
Creative Commons License

web mechanics: text-to-html with Markdown, css handled with Blueprint, glued together with PHP