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Thread: 64 Princeton Amp Power Tube Dissipation

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    64 Princeton Amp Power Tube Dissipation

    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

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    Bent Member Chuck H's Avatar
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    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.

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    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

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    Bent Member Chuck H's Avatar
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    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.

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    Lifetime Member Enzo's Avatar
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    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.

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    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

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by caesparza View Post
    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.

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    don't forget the joker g1's Avatar
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    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.

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