I have a Fender Deluxe Blackface from ‘68. It had a bunch of issues, but the final one is that it has low output. I get 30-40 volts pk-pk at the phase inverter, but the volatge at the speaker only goes to about 3-4 volts before it starts to flattop. The power tubes have -43 volts bias on them to get the bias current around 23mA, which is too high. I have known good tubes in the 6v6’s.
I am running out of things as to the cause. I am starting to think I have a bad output transformer. There is also an imbalance between the power tubes idle currents, even though they have the same gate voltage, and are matched tubes.
The schematic says I should have -35 volts bias voltage, and I need -45 volts.
Any ideas.
The client is calling me every couple of days to ask if it’s done yet, so not a good amp to ge,t stuck on.
I am running out of things as to the cause. I am starting to think I have a bad output transformer. There is also an imbalance between the power tubes idle currents, even though they have the same gate voltage, and are matched tubes.
The schematic says I should have -35 volts bias voltage, and I need -45 volts.
Any ideas.
The client is calling me every couple of days to ask if it’s done yet, so not a good amp to ge,t stuck on.
Could be leaky coupling capacitors going to the output tube grids.
Try lifting one end of each capacitor, and if it affects the grid bias and
output tube plate current, that's your problem. If any leak, replace them all.
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No, the voltage on the grids is -45. No leaky coupling caps.
I will check again to be sure, and may lift the caps. But 90% sure that isn’t it.
I will check again to be sure, and may lift the caps. But 90% sure that isn’t it.
If you have a high impedance meter, you can measure directly across the grid resistor.
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