Bassman AB165 wrong voltages

[nabokov]

Hi anyone can help? wrong voltages in red

[sluckey]

I don't see anything alarming.

[bepone]

Hi anyone can help? wrong voltages in red

voltage after rectifier diodes? looks like phase inverter tube is higher , maybe too high input/wall voltage

[nabokov]

Hi anyone can help? wrong voltages in red

voltage after rectifier diodes? looks like phase inverter tube is higher , maybe too high input/wall voltage

the voltage after the rectifier diodes is 329v, I only highlighted the voltages that seemed too different from those in the schematic

[martin manning]

Check the value of the 22k PI tail resistor by measuring from where you have marked 149V to ground. It may have drifted up.

[Stevem]

With the amp in play mode what do you have for voltage at the input to the choke, and as noted on the schematic what do you have for negative bias voltage ?

[nabokov]

Check the value of the 22k PI tail resistor by measuring from where you have marked 149V to ground. It may have drifted up.

you're absolutely right, I took it off the circuit and it measured 54k, it was the only one in that area that I hadn't replaced. I have now put 10k + 15k in series and the voltages are much better

[nabokov]

With the amp in play mode what do you have for voltage at the input to the choke, and as noted on the schematic what do you have for negative bias voltage ?

input to the choke: 438v, negative bias: -57,3v

[martin manning]

Check the value of the 22k PI tail resistor by measuring from where you have marked 149V to ground. It may have drifted up.

...I took it off the circuit and it measured 54k, it was the only one in that area that I hadn't replaced. I have now put 10k + 15k in series and the voltages are much better.

Great!

[Stevem]

I am assuming that your reported -57 is what you had before you replaced that 10K resistor in the bias circuit, if not then the bias voltage is still too high!

What was going on before you replaced that resistor was that the output tubes where not idling and pulling current, in that state the power supply voltage was not getting pulled down and such was the reason for your high voltage readings through out the amp.

[nabokov]

I am assuming that your reported -57 is what you had before you replaced that 10K resistor in the bias circuit, if not then the bias voltage is still too high!

What was going on before you replaced that resistor was that the output tubes where not idling and pulling current, in that state the power supply voltage was not getting pulled down and such was the reason for your high voltage readings through out the amp.

Yes it's true, it was before I replaced the 22k resistor. In addition, the 2 10K resistors and the 15K on the bias potentiometer were also gone.
All this research began to solve a hum promblem.
Now it's almost gone. The voltages are ok but I still have 329v AC (instead of 320v AC) coming out from the secondary of the PT. Anyone know a way to fix?

[martin manning]

I still have 329v AC (instead of 320v AC) coming out from the secondary of the PT. Anyone know a way to fix?

That's not anything to worry about... it's less than 3%. Your line voltage will vary more than and affect the secondary voltage accordingly.