‘81 Princeton Reverb Bias Question

[Helmholtz]

You're moving the target while (trouble)shooting!

I understand the amp once worked fine (probably for almost 40 years). So there shouldn't be a reason to change component values.

Modifying the circuit involves the risk of obscuring the root cause of the problem (which hasn't been identified yet!) - also it's irritating and frustrating for those trying to assist with troubleshooting.

BTW, I think a bias voltage of 31V is too low and recommend rechecking idle currents.

[g1]

BTW, I think a bias voltage of 31V is too low and recommend rechecking idle currents.

Agree if you mean 'too hot'.
'65 re-issue shows approx. -40VDC with similar B+.
I do not expect voltages near spec. of old schematic, but rather higher due to modern line voltage.
I think hot bias may be pulling down B+, and also causing early distortion.

An idle current number would be very helpful.

[Perkinsman]

Let me try to put this schematic issue to rest. I have been using the schematic Enzo posted once I realized it was different than the schematic I was using and is the circuit I have. I am not mixing them. The one I had been using was not my circuit but confused me because of so so many similarities with very few exceptions. Lesson learned! I have returned the bias cap to the original (80uf160v) values & now it is reading -31, which is apparently low. I don’t see a bias msmnt on the schematic. Idle current now is 27ma

[Helmholtz]

Agree if you mean 'too hot'.

I do mean too low regarding absolute value or "magnitude".

To me, being a scientist, there is no doubt that -31V is a (6V) higher voltage than -37V (because -31 = -37 +6), just as -31°C is a higher temperature than -37°C. (Higher generally meaning more positive or less negative.)

But from a discussion I had with Enzo and other members 2 years ago I understand that the scientific view would lead to confusion. So I adapt to forum convention (believe me it's not easy for me as it seems illogical). Meaning that I treat bias as absolute value of the grid-to-cathode voltage at idle. In this context a higher bias means a more negative voltage and vice versa.

My 80s Super Champ (B+ = 400V) is fine with a grid bias of -33V. But it's idle current and not bias voltage that counts and determines dissipation. And different 6V6s will require different bias voltages for the same idle current. So it doesn't make much sense to recommend a specific bias voltage.

[Perkinsman]

Isn’t 27ma too hot for this amp? I still have no voltages on V4a plate & cathode.

[Helmholtz]

Isn’t 27ma too hot for this amp?

Plate current of 27mA with a plate voltage of 420V is hot but not unusual with vintage Fender 6V6 amps. Any signs of redplating?

I still have no voltages on V4a plate & cathode.

V4 b plate & cathode which are still 0.

Which of the above statements is correct? 0V at pin 1 or pin 6?

[Perkinsman]

My plate voltages are 394 on both 6v6, so it's running at 95% of plate dissipation..I think that's way too hot, bias on this amp should be around 19ma. It was actually 19ma with the lower value cap that was on previously.

Regardless, I still have no voltage on V4a, pins 6&8, ie, plate & cathode. I do have voltage at V4b, pins 1&3, ie, plate 188v & cathode 57v.

[Enzo]

Well, every plate gets its voltage from a power supply. Presumably the supply itself is working or you'd have no high voltage on any of the tubes. So start with the plate resistor to pin 6. What is on the other end of it? Got high voltage there? then the resistor is likely open.

[Helmholtz]

My plate voltages are 394 on both 6v6, so it's running at 95% of plate dissipation..I think that's way too hot, bias on this amp should be around 19ma. It was actually 19ma with the lower value cap that was on previously.

According to the Design Max tube rating system (used since early 60s) the 6V6GTA is a 14W tube. So you have PD = 394V times 0.027A = 10.6W. 10.6W/14W = 76%.

For lower dissipation increase the bias shunt resistor (22k or 27k).

I do have voltage at V4b, pins 1&3, ie, plate 188v & cathode 57v.

These voltages would make sense with V4A (pins 6/8), not V4B. Are you sure you are counting tube pins correctly? Do you get sound?

[Perkinsman]

6v6’s are not GT so the max power is 12W, correct? That would make it 87%. You are correct, I posted late last night & switched the pins. I have nothing at 1& 3, just 6&8

[Helmholtz]

6v6’s are not GT so the max power is 12W, correct?

I dunno what tubes you are using. Both Fender schematics specify 6V6GTA. For lower plate dissipation use a higher value bias shunt resistor than 22k.

You are correct, I posted late last night & switched the pins. I have nothing at 1& 3, just 6&8

Means V4B (the trem oscillator) has no supply. Check the 220k plate resistor and its connections.

Again: Are you getting sound and do you still have the early breakup issue?

[Perkinsman]

I put a 27k back in & brought the bias current to 20ma. I do have sound but no tremolo due to V4b. The 220k plate resistor is good. I’m not sure whether I have early breakup yet because I haven’t put it back into the cab until I get the trem working.

[Helmholtz]

The 220k plate resistor is good.

Measure voltages at both ends of the 220k resistor. If both ends measure 0V, post a picture of the area concerned.

I’m not sure whether I have early breakup yet because I haven’t put it back into the cab until I get the trem working.

Don't forget to never power on the amp without a (speaker) load.

[Perkinsman]

Measure voltages at both ends of the 220k resistor. If both ends measure 0V, post a picture of the area concerned.



Don't forget to never power on the amp without a (speaker) load.

Absolutely. I use a free standing 8ohm speaker cabinet. I'll follow up on that resistor tomorrow.

[Perkinsman]

Absolutely. I use a free standing 8ohm speaker cabinet. I'll follow up on that resistor tomorrow.

Voltages on both sides of V4b 220k plate resistor is 0. Here's some pics