Posted by Damir [ 213.202.114.79 ] on March 17, 2006 at 09:30:07:

In Reply to: Tube overheat problems... posted by Perplexied on March 16, 2006 at 18:43:08:

You probably have "bias voltage" problem on (at least) one output tube.
I think it`s a "Sovtek" 5881 based amp, with high anode voltage, about 500V - correct me if I`m wrong.
Pull out both output tubes and measure voltages on both 5881 sockets:

-between pins 2&7 (heater) - 6,3V AC
-pin 3 (anode) - about 500V DC (carefull!)
-pin 4 (g2) - about 500V DC (carefull!)
-pin 5 (g1) - about -50V DC (negative to ground)
-pin 8 is the ground (check it)

If something isn`t like above, probably -50V bias voltage on the pin 5, then you must check where you "lose" it.
Also, check the screen grid resistor, connected to pin 4. Mostly, it is wired between the pins 4 (g2) and pin 6 (empty, used as a soldering point). Be carefull - high voltages!

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