Music Man RD-50

[lowell]

I recently got ahold of one of these and have been using it as my main rig lately. It kicks ass! I can't believe that the clean channel, which is all I use, is all solid state pre and only tube in PA.

What's up w/ the voltage gain transistors on the power tube cathodes? Also are the power tubes in a "common grid" arrangement?

+30v on the grids and signal entering through the cathode?

Also the screens are insanely low compared to a typical Fender/Marshall etc... why is this?

[loudthud]

Those transistors should be described as providing current gain. They probably drive the 6L6's into class AB2 territory where you don't need high screen voltage. In short, the design is completely different than the typical guitar amp. I wonder what the primary impedance of the OT is.

[Steve Conner]

Yes, you can think of it as a "common grid" arrangement.

The tube is activated by the voltage between grid and cathode. It doesn't care if the cathode stays in one place and the grid moves around (conventional amp) or the grid stays in one place while the cathode moves around (Music Man)

The tube might not care, but the driver circuit does. The cathode is harder to drive than the grid, because it collects all of the plate and screen current, giving it an effective impedance of only a few hundred ohms. But that's no problem for power transistors. They love high current, low voltage, whereas tubes love high voltage and low current.

So, another way of seeing the Music Man circuit is to say that the transistors provide the current gain, and the tubes provide the voltage gain.

[lowell]

ok interesting. the only thing I don't get is that the transistors are in a voltage gain arrangement aren't they? It'd make more sense to me if the transistors had the signal leaving the emitter instead of the collector.

[cjlectronics]

I recently repaired one of these amps and I'm in total agreement. That amp kicks ass!!

[RudeBoy]

ok interesting. the only thing I don't get is that the transistors are in a voltage gain arrangement aren't they? It'd make more sense to me if the transistors had the signal leaving the emitter instead of the collector.

They look more like current sources to me, with the current set by the voltage developed across emitter resistors R51 and R54.

[Old Tele man]

...they're CATHODE MODULATED push-pull.