MusicMan RD-50 what?

[Randall]

Got a RD-50 in for a re-cap and a cleaning, etc. Now that it is running, and sounding about right, I am flummoxed about the current draw and the output circuit. I get 607 volts on the plates with 5 -6 mA as measured with a cathode current socket probe. But then, I don't see how signal is getting to the grids on pin 5 here, or what the transistor circuit is for on the cathodes. Is this a uncommon 6L6 circuit that has evaded me up til now? The amp doesn't sound like it is only drawing 5 mA, is there another way to look at this?

http://el34world.com/charts/Schemati...cman_rd50b.pdf

PS, screen, grid and plate voltages all look about right.

[Leo_Gnardo]

Yup, that's how they do. Similar to some Peaveys and also other MM where a pair of medium size power transistors that drive the cathodes not the grids. This had me flummoxed too when I encountered it. The lack of bias current seems ominous too but no worries. You'll see a little crossover pip on your scope if you test the amp with a sine wave, and a cleanly played guitar may sound just a tad gritty on account of that. Believe me I've spent many an hour trying to overcome that problem, and long ago threw in the towel. That's the way they are, it's the 'nature of the beast' so to say. If your RD 50 sounds otherwise OK just smile, button it up & get on to the next thing.

[The Dude]

Yep. Like Leo says the cathodes are being driven instead of the grids, which requires more current- hence the transistors. I think of it as the transistors providing current gain and the tubes voltage gain.

[Randall]

Thanks guys, I wondered if it was something along this line, but have never heard of this. Sort of makes it a bit hard to tell if the tubes are matched or operating well, does it not?

[J M Fahey]

That circuit is very clever and *forces* the tube to put out whatever current is sent to it, as long as cathode can supply *some* emission.
Power tubes work linearly, supply current as ordered, and only stop when actually *dead* .
So matching is not really necessary, brand is not important (except that it can stand the high voltage or not) and tubes last surprisingly long.

[g1]

5 to 6mA idle current is normal for these amps. They are considered self-biasing and self-matching.
The tube current can be calculated by measuring the DC voltage across the 6.8ohm resistors on the emitters of the drive transistors.
(35 to 40mV across each 6.8ohm resistor)