Preamp Tube with Two Different Plate Voltages

Discussion in 'Amp Tech Center' started by Dan_Pomykalski, Sep 3, 2020.

  1. schmee

    schmee Doctor of Teleocity Silver Supporter

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    Isn't this common? I thought more often it had to do with the amp itself changing the voltage you see not the tube.? For instance, in a Fender amp pin 1 and pin 6 may be shown on the schem as 280V vs 380V.
     
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  2. tubegeek

    tubegeek Friend of Leo's

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    There are just a few factors.

    What is the voltage at the supply end of the plate load? What is the value of the plate load? What is the current through the tube?

    Then V=I*R.

    That's all you need.

    If the supply voltage and the plate load are the same between sections, then you can draw conclusions based on the resulting voltages at the plates. If not, then there's no real expectation of the same voltage - if they are the same, it's a coincidence, not a match.
     
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  3. Paul G.

    Paul G. Friend of Leo's

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    Ooops! I've got you now.

    I get a bit preachy sometimes, ask my wife.
     
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  4. Dan_Pomykalski

    Dan_Pomykalski Tele-Meister

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    Except for one of my tubes was 180v on both halves, and the other tube was 190v on one half and 200something on the other in the same location (V2), so in this case it wouldn't have been the amp changing the voltage. I was asking if it was common to have two different voltages in a single tube with identical plate load resistors.
     
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  5. PCollen

    PCollen Friend of Leo's Silver Supporter

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    Need to see your schematic .....
     
  6. tubegeek

    tubegeek Friend of Leo's

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    Pedantic? Moi?

    I feel ya bro. No harm no foul.
     
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  7. Dan_Pomykalski

    Dan_Pomykalski Tele-Meister

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    Why?
     
  8. Wally

    Wally Telefied Ad Free Member

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    I would suppose that the differences in the two tubes results in that position would be attributable to the difference in the individual
    triodes. This is why audiophiles pay premium prices for matched triodes....even going so far as to wanting four matched triodes in two tubes for certain positions in stereo amplification.


    as for why P Colleen wanted to see the schematic, I would posit that he wants to see the functions of the two triodes in that circuit.
    I suppose that in a certain situation the tube with the two different voltages could actually be a tube with triodes that are more closely matched than the other tube and that it is the circuit that creates the difference in voltages. Or....it could be the other way around. The circuit might yield those identical voltages with a tube that has matched triodes. Without the schematic, one might not have much of a way to make an informed analysis.
     
  9. Dan_Pomykalski

    Dan_Pomykalski Tele-Meister

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    I certainly understand how a schematic could help in many situations, but based on the information I gave and my question, it didn’t really seem all that important. Especially since others were able to answer my question without it.
     
  10. tubegeek

    tubegeek Friend of Leo's

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    We made some assumptions and we could've been off base there.
     
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  11. Wally

    Wally Telefied Ad Free Member

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    Dan, I reread the thread. I am going to think that the question was answered with as tubegeek notes some assumptions. Imho, without knowing the exact application of those two triodes one cannot assume that the tube that yielded the same plate voltage in both triodes is the tube that had triodes that were more closely matched just as one could not accurately deduce that the other t7be was the tube with unbalanced triodes. If one had a 12AX7 that was guaranteed to have closely matched Triodes, then one could make some informed assumptions depending on the results. Ex: if one were working on a stereo circuit and had a balanced tube in a position that should yield balanced results, one could assume that all was correct if the voltages were as expected. If the voltages showed to be different, then one might assume that other components in those two ‘mirror image’ circuits were out of spec.
    There are more variables in your question than were accounted for, as tubegeek observes with his observation about assumptions.
    PCollen could elaborate in a more technical manner than can I as his technical knowledge surpasses mine, ime. But...without a schematic there is little that can be done except to assume and guess.
    Here is an example. If one were working on certain 6G Fender amps in which the triodes in V1 are shared between the two channels and the two triodes share a. Common cathode bypass cap, then one could make certain observations as tubegeek did on page 1 and those observations of the difference between your two tubes would be valid. In an AB763 preamp where V1’s triodes are dedicated to a single channel, one cannot makes the same assumptions and observations because the plate supply voltage and the cathode voltage supply might not be identical, and therefore one cannot assume that the balance or imbalance is in the tube or in the circuit.

    So, imho, your question may or may not have been accurately answered.no big deal from my perspective, but it is an interesting situations....and that is why I asked what amp circuit this was. Imho, PCollen’s question was more to the point than mine....but he and I were both on the same track, I think.
     
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  12. Dan_Pomykalski

    Dan_Pomykalski Tele-Meister

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    I feel like my initial question made it pretty clear that I expected both voltages to be the same. I think one would be able to make some safe assumptions based off that information alone.

    Like I said, I understand how a schematic can be helpful in a lot of situations, but a general question about differing plate voltages when they’re expected to be the same doesn’t really seem like one... I mean would me uploading a schematic of an AB763 change any of the information that people have posted so far? Had the circuit indicated that differing voltages were normal, I wouldn’t have had the question in the first place. Saying a schematic is needed to answer my question seems more like a reactionary catch phrase like “only you know what sounds good” than something that’s actually, truly necessary.
     
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  13. Dan_Pomykalski

    Dan_Pomykalski Tele-Meister

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    Could’ve.
     
  14. Wally

    Wally Telefied Ad Free Member

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    Dan, I am just musing here; but I had no idea what you were working with nor upon what you had based your assumption that the voltages should be the same. If we had known that you were talking about V1 or V2 in an AB763 cased circuit, then we would have known what you had assumed. However, if the tube were V3 in that circuit, then we would have other input upon which to base some assumptions.
    I will also observe that my thoughts in my post you quoted on the first two gain stages in the AB763 are valid. Just because you got 180 volts on both plates does not mean that those triodes are closely matched. They could be slightly unmatched but working in two separate circuits that have different values for the plate resistors and/or the cathode resistances that yield a balanced result as to plate voltages despite the imbalance in the triodes. Fwiw, one of those triodes is operating with a stated cathode resistance of 1500 ohms while the effective cathode resistance in the second triode is a stated 1640 ohms. Small effect, probably..,but a difference.
    At any rate, the triodes in that second tube you tried are undoubtedly better matched. Now, what is the difference in operation?
    What does that higher plate voltage cause to happen in the circuit? That might be a more telling question/answer since we are not dealing with a stereo circuit that demands close matching.
     
  15. Enzoamps

    Enzoamps TDPRI Member

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    Worth keeping in mind that what the tube is doing matters. AS someone mentioned, a phase inverter might prefer to have the sides similar. But if your 12AX7 is say shared between two input channels, about the only difference might be on one channel, the volume needs to be at 3.4 instead of 3.3, and since I set the volume by ear anyway...
     
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