Magnatone 213 bias question

[Bob Opera]

This is a cathode-biased amp powered by two 6V6s. In troubleshooting a fuse-popping issue (seemingly caused by a bad rectifier), I noticed that the cathode current on the power tubes was about 48 ma, with the voltage across the tube at about 350v. Isn't that current extremely high? There's a 250ohm (actual resistance: 270ohm) cathode resistor between them. I measured 26 volts across it. According to my calculations this yields a plate dissipation of about 17w. The two grid resistors are 470k. I had access to another perfectly working 213 and got roughly the same figures. Are these figures normal and/or my calculation wrong? Thanks.

http://www.magnatoneamps.com/schemat...tone_213-a.jpg

[Enzo]

SOunds like class A numbers. The book says 14 watts, and 17 watts is not all that much higher.

[trobbins]

From your measurements, I get 31.2W for anode plus screen dissipation. Anode dissipation by itself may be down near 14W.

[Enzo]

How did you get 31.2W? And 17W of screen dissipation???

26v across 270 ohms gives me 96ma, 48ma per tube. 350v across the tube by 48ma gives me 16.8W. Since it is cathode measurements, that includes screens. My numbers agree with his. Did we miss something?

[g1]

I think he (trobbins) took the 350V as plate volts and subtracted 26V off it, and he was giving the total for 2 tubes.

[Enzo]

Ah. OK.

[Bob Opera]

Thanks guys. This is indeed a Class A amp.
The 350V is the voltage across the tube (Vplate - Vcathode)

[trobbins]

Perhaps try get 6V6GTA type for the next tube swap, and keep the rear nice and spaced so that airflow is always good :-)

I could only find one photo of a 213 rear in the Maggie forum photo albums.

Oh, and don't burn up those varistors! Geez they make for a nice vibrato.