‘81 Princeton Reverb Bias Question

[Perkinsman]

This is circuit AB1270. I’d like to use the Robinette biasing method but I’ll not sure where the output transformer center tap is located. The pics show a red, blue & orange-brown wire coming from the output transformer, the blue & orange-brown going to the plates of the 2 power tubes. Is the red the center tap?

[52 Bill]

This is circuit AB1270. I’d like to use the Robinette biasing method but I’ll not sure where the output transformer center tap is located. The pics show a red, blue & orange-brown wire coming from the output transformer, the blue & orange-brown going to the plates of the 2 power tubes. Is the red the center tap?

Yes, the red wire is the center tap.

[Perkinsman]

Thanks, I checked voltage there and it's about 100v lower than the 420v on the schematic, thats 25% off. I took voltages on the power tube plates & a few of the preamp tubes and they're low too...when I checked the bias, one 6v6 was 61ma and the other was 80ma, I believe 34ma is recommended. Can someone please point me in the direction I need to be going to find the source of the problem here. Thanks.

[Enzo]

What is the bias voltage at the tubes and how clean is it? meaning how free of ripple?

Your B+ is 100v low? How free of ripple is that?

I attached the 1981 schematic.

[Perkinsman]

Thanks for doing that. I subtracted each of the PT plate voltages from OT center voltage. Is that the bias voltage? I don't know how to check a signal on the scope yet. The initial complaint was that the amp distorted after being played for a couple hours, after cooling down it was fine for a couple more hours.

[Enzo]

The bias voltage is the negative voltage on the control grids of the power tubes - pin 5 of each. It controls the current we measure when adjusting the tubes. If the voltage is too low, the tubes work harder and drag your B+ down. It also needs to be smooth DC - no ripple.

Your 420v measures really low? Could be the filter cap not doing the job. The resulting ripple will cause a low reading.

Problems with either power supply COULD cause distortions.

Please explain in detail exactly how you measured the tube current.

[alexradium]

Thanks for doing that. I subtracted each of the PT plate voltages from OT center voltage. Is that the bias voltage? I don't know how to check a signal on the scope yet. The initial complaint was that the amp distorted after being played for a couple hours, after cooling down it was fine for a couple more hours.

filter caps if original,its time to go,also check the rectifier tube and any resistor on the power supply string

[Perkinsman]

The bias voltage is the negative voltage on the control grids of the power tubes - pin 5 of each. It controls the current we measure when adjusting the tubes. If the voltage is too low, the tubes work harder and drag your B+ down. It also needs to be smooth DC - no ripple.

Your 420v measures really low? Could be the filter cap not doing the job. The resulting ripple will cause a low reading.

Problems with either power supply COULD cause distortions.

Please explain in detail exactly how you measured the tube current.

My bias voltage is -12.3. The filter caps are the original, 1981’ish can. They (20/20/20@450) measured from 20-24uf at full voltage on my Sprague cap tester and charged-discharged properly. I calc’d bias current by measuring & subtracting each plate voltage from the OT center tap voltage & dividing by the resistance from each plate to the center tap.

[Helmholtz]

My bias voltage is -12.3.

That's the reason for your excessive idle currents and will cause low B+. Grid bias should be around -33V.
What's the voltage across the bias filter cap?

[Perkinsman]

The bias voltage is the negative voltage on the control grids of the power tubes - pin 5 of each. It controls the current we measure when adjusting the tubes. If the voltage is too low, the tubes work harder and drag your B+ down. It also needs to be smooth DC - no ripple.

Your 420v measures really low? Could be the filter cap not doing the job. The resulting ripple will cause a low reading.

Problems with either power supply COULD cause distortions.

Please explain in detail exactly how you measured the tube current.

Voltage across the bias cap is -12.5

[Perkinsman]

Voltage across the bias cap is -12.5

[Helmholtz]

Voltage across the bias cap is -12.5

I guess the bias cap is leaking. But also check the 100k and 22K voltage divider resistors.

[Perkinsman]

Yep, the cap was bad. All voltages seem correct now. I’ll measure everything & report back. Thank you guys.

[Perkinsman]

The bias voltage is the negative voltage on the control grids of the power tubes - pin 5 of each. It controls the current we measure when adjusting the tubes. If the voltage is too low, the tubes work harder and drag your B+ down. It also needs to be smooth DC - no ripple.

Your 420v measures really low? Could be the filter cap not doing the job. The resulting ripple will cause a low reading.

Problems with either power supply COULD cause distortions.

Please explain in detail exactly how you measured the tube current.

Bias voltage now at 37.5vdv. Brought B+ up & calc’d 18.9ma for bias current, which is about 70% of max dissipation so I believe I’m good now. Thank you.

[olddawg]

I would still check for ripple on 40 year old filter caps. Do you have an o’scope? Do you have any hum?