My bias as measured on my 300b goes wildly up and down. The negative voltage to the tube is steady. The B+ voltage is also changing wildly up and down. This is a mono block and the other works properly as wired. I am using DC heating and do not use a hum pot. The 10 ohm resistor is soldered directly on the leg of the 300b. Any ideas.
It's oscillating at low frequencies. The instability could be due to an open/too small power supply capacitor,
a too large coupling capacitor, or a bad/wrong connection that causes either of these conditions.
Check the feedback loop if any, also.
a too large coupling capacitor, or a bad/wrong connection that causes either of these conditions.
Check the feedback loop if any, also.
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That is the weird thing. As I said these are mono blocks. The other amp works perfectly and is an exact copy of the one I am having trouble with.
It could a be a case of motorboating. Have experienced it myself in the past. Try isolating the power stage to see if it goes away.
An accurate schematic, is worth more than shooting off 10 shotgun shells in 10 different directions, in an attempt to guess the cause of the problem.
+1 on the schematic.
Does the amp do this with just the 300B installed and no driver tube(s)? What about just the driver tube(s) installed? (you'd have to look at the AC output of the driver stage with a scope to look for an oscillation)
Does the amp do this with just the 300B installed and no driver tube(s)? What about just the driver tube(s) installed? (you'd have to look at the AC output of the driver stage with a scope to look for an oscillation)
Swap the tubes over and see whether the problem goes with the tubes. I had the same problem once with a PP amp and it turned out to be a bad connection on one of the tube pins.