64 Princeton Amp Power Tube Dissipation

[caesparza]

Hi guys,

I recently acquired a 64 Princeton Amp. I like a lot the sound of the thing, it needs a volume replacement, since it doesn't clean anymore. Anyway I checked the voltages to determine the BIAS for the tubes, it came equipped with a couple of Electro Harmonix 6V6GT the readings for these power tube were:

6V6 (1) Resistance: 162.7 Voltage Drop: 1.76 Plate Voltage: 399 Plate Current: 10.81mA Dissipation: 4.31watts
6V6 (2) Resistance: 163.4 Voltage Drop: 3.28 Plate Voltage: 396 Plate Current: 20.07mA Dissipation: 7.94watts

Clearly there is a mismatch between the tubes.

Then I replaced with a couple of vintage Sylvanias 6V6GTA and got the next readings:

6V6 (1) Resistance: 160.7 Voltage Drop: 3.2 Plate Voltage: 374 Plate Current: 19.91mA Dissipation: 7.44watts 62%
6V6 (2) Resistance: 160.7 Voltage Drop: 3.9 Plate Voltage: 374 Plate Current: 24.26mA Dissipation: 9.07watts 75%

Would that 1.63 watt difference be critical in the operation of the amp? I understand that in order to make the bias adjustable for each tube I would probably have to make the power tubes cathode biased.

Thanks for your help.

Have a good day..
Carlos

[Chuck H]

Just because you got the higher current reading from 6v6 (2) both times I'm suspecting the OT may be a little hinky. But it could still be a coincidence.?. You could swap the tubes and see if the current levels follow the tubes or stay with the sockets.

[caesparza]

Hi Chuck,

Thanks for your answer. I swapped the tubes and the readings stayed with the tubes not the sockets. What do you think of the numbers with the second set. Should I need to balance the dissipation?

Thanks again for your help.
CE

[Chuck H]

It's complicated. That's a pretty significant mismatch, but if you're not pushing the amp all the time it won't matter much and it may be contributing to the amps tone. If you ARE planning to run the amp dimed a lot then you MAY be stressing the one tube carrying most of the current and inviting failure.

But consider also that those are idle current measurements. We don't know if the tubes are more balanced when conducting signal. Which is often the case.

[Enzo]

And it is just a guitar amp, not a hifi or precision lab gear. SO what if it is a little imbalanced? It surely won't hurt the amp, and such things are part of what goes into the tone of an amp.

[caesparza]

One of the blue molded coupling caps (the .022, third one from the input) the DVM is reading almost 18.8 volts, is the DCV supposed to be 0 after a coupling cap? that is what I understand, I checked it closely and it has a crack, I guess I would have to change it.

To check this caps for leaks its necesary to lift one of the legs (the negative one) and check for DCV, isn's it?

Thanks guys.
CE

[Chuck H]

One of the blue molded coupling caps (the .022, third one from the input) the DVM is reading almost 18.8 volts, is the DCV supposed to be 0 after a coupling cap? that is what I understand, I checked it closely and it has a crack, I guess I would have to change it.

To check this caps for leaks its necesary to lift one of the legs (the negative one) and check for DCV, isn's it?

Thanks guys.
CE

Ideally, yes. To get a finite measure of leakage you would need to lift and load the downstream end of the cap in question. But that's not really necessary here because the additional current is following the tube, right? So it's the power tubes that are behaving differently. Just to be sure you could measure grid voltage on both tubes, then swap the tubes and measure again. A tad easier than lifting the caps and testing I suppose.

[g1]

Unless someone is very familiar with the layout of this exact version of the amp, we don't know which cap you mean.
Where is it located on the schematic?
Not all caps have 0 volts at one end. In some circuits, your 18.8V at one end of a cap could be correct.
"3rd blue cap from the input" is not much to go on without a picture.

[caesparza]

Ok, got the parts to tune up this little guy.

I've atteched the corresponding schematic, you are absolutely right g1.




Ok, my doubt is this one: I'm getting a DC voltage reading of 20.15 on the .022 cap that goes to the X point on the layout, that ends up on Pin 7 of the 12AX7 (grid). Should that point read something close to 0 VCD?

Thanks.
CE

[eschertron]

Short answer, no. The grid on pin 7 of the second 12AX7 (TP 4) is above ground by the 56k resistor on the cathode. It's the phase splitter, and the voltage at the grid needs to swing wide from idle. The schem shows a DC offset of closer to 65v at that point.

[caesparza]

You are absolutely right. I was looking at that 1M resistor connected to the other end of the cap, and that 63.8 volts suggest otherwise. So that 20.15 VCD seems like a resonable number at that point?

Thanks eschertron.

[pdf64]

...Would that 1.63 watt difference be critical in the operation of the amp?...

Just to note that plate dissipation is immaterial to the operation of the amp.
Plate dissipation is just a limit.
The important variable with regard to the operating point is the plate current.
The only relevance of calculating idle plate dissipation (and setting bias according to that) is to try and ensure that when signal is applied, the plate dissipation limit won't be exceeded.

[Enzo]

20v probably fine, but also meaningless. You cannot measure voltage at TP4, pin 7, with a meter, the meter impedance is too low. Note the 65v on pin 8, pretty close to the 63v at bottom of 1k. You need to measure the voltage at TP 4 with respect to the cathode, NOT ground. Then if you get some voltage, subtract the cathode voltage to know the bias.

[caesparza]

Ok, here is the relation of the actual amp readings (left) with the ones proposed on the schematic (right):

Tube1Pin1: 200 190
Tube1Pin3: 1.46 1.5
Tube1Pin6: 204.8 190
Tube1Pin8: 1.42 1.5

Tube2Pin1: 243.6 260
Tube2Pin3: 2.11 2.4
Tube2Pin6: 237.2 220
Tube2Pin8: 62.7 65

Tube3Pin3: 411 420
Tube3Pin4: 409 415
Tube3Pin5: -36.9 -34

Tube4Pin3: 411 420
Tube4Pin4: 410 415
Tube4Pin5: -36.9 -34

Voltage Drop Primary-Tube3: -3.92
Voltage Drop Primary-Tube4: -5.12

Resistance Primary-Tube3: 166.9
Resistance Primary-Tube4: 166.9

Plate Current Tube3: 23.49mA
Plate Current Tube4: 30.68mA

Voltage first filter stage (square): 416 420
Voltage second filter stage (oval): 409 415
Voltage third filter stage (triangle): 349 370

What would be the option to try to balance the Plate current on both tubes?

The amp doesn't have any noises, filter caps haven't been replaced, I know that because of the age, that would be the recommended thing to do. But could it be possible that those caps are still working properly?

Right now I'll install a 3 prong cord and remove the "death cap" from the circuit.

[eschertron]

After a quick skim, I don't see anything wrong with the voltages you posted.

So the output tubes are a mite unbalanced, so what? That's exactly what lends to the charm of the vintage sound. Not sterile, not hi-fi, but warm and (almost) fuzzy. Do what you need to make the amp safe, and replace components that look like they need replacing. Tell us how it sounds. Crackles? Buzz? Squeal? If not, play it for a while.