Princeton Clone bias issues

Discussion in 'Amp Tech Center' started by schmee, Nov 20, 2019.

  1. schmee

    schmee Poster Extraordinaire

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    Having a helluva time with this. I thought the bias was changing every time I tried to finalize this build, but what I discovered is rotating the intensity pot is changing the bias from 20ma to 100+ ma! What's really weird is even if the vibrato circuit is disabled, (so not working) it still does the same thing.
    I tried various bias schemes, even eliminated the adjusting pot and wired it like the original schem. No help.
    What the heck am I missing here? I have tested the bias pot and the intensity pot and find no issues with the pots.
    What in the world would make the bias measured ma change that much?
    [​IMG]
     
  2. corliss1

    corliss1 Friend of Leo's

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    Post some voltages at various points in the bias network?
     
  3. schmee

    schmee Poster Extraordinaire

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    It's been a while since I measured, but the plate voltage is high for sure, (~450-470) it also varies though with the intensity pot being turned! I guess I need to go back and measure things again. But right now I'm trying to solve the biasing issue and move on from there. It's a vintage PR power transformer so I expected the plate voltage to be pretty high.
    Any thoughts on how the intensity pot could make that big difference, or even effect it when off? My other Princeton builds I just turned down the intensity or turned the Vibrato off to bias... which is better. That's what's kind of stumping me here. Something has to be a bad cap or wiring issue.... but man I've been through the wiring a few times now.
    The .1 uf cap on the board at the brown intensity pot wire end... what if that was bad?
     
  4. D'tar

    D'tar Friend of Leo's

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    If that cap was leaking dc you could be adding positive voltage into the bias circuit so yes that would be an issue.
     
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  5. D'tar

    D'tar Friend of Leo's

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    If youd like some extra eyes on things.... post pics.
     
  6. peteb

    peteb Friend of Leo's

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    Take the power tubes out and focus on the bias volatage. If the current went from 20 to 100 mA, you must have passed over the -34 bias Volts called out on the schematic.
     
  7. D'tar

    D'tar Friend of Leo's

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    For the intensity pot to change the bias without the tremolo active... the pot would have to have a voltage differential on either side. Healthy coupling caps isolate the bias dc voltage to the grids of the power tubes because it has no where else to go. It is basically an open circuit like powering on an amp with no tubes installed and having full B+ voltage at the pre amp plates. The position of the intensity pot is irrelevant to the dc bias supply voltage when the circuit is healthy and wired properly. I would pull the tubes, disconnect the negative bias supply to the intensity pot and check for positive voltage on the brown wire at the pot. You may have to reconfigure your bias resistors once you figure this out.
     
  8. schmee

    schmee Poster Extraordinaire

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    I would, but this is the one I'm putting in a very tight Mono 15 chassis. Almost impossible to see anything in there. This thing is so damn simple it's hard for me to believe I can't find this issue. It makes some sound with a guitar plugged in, but garbled and a bit thin and noisy. With nothing plugged in it's dead quiet. I may try switching the OT primaries to the power tubes, maybe it's wrong, and just not a type that squeals with those wrong. I'm still using the Mono 15 OT and I assume the wire colors are normal on those. Just trying to build a little amp for Jazzy stuff in the Mono cab and chassis that's reliable.
    I'll get some voltages today.
     
  9. schmee

    schmee Poster Extraordinaire

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    delete
     
  10. schmee

    schmee Poster Extraordinaire

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    Well, low and behold the bias voltage is about 42 VDC, but POSITIVE! I just cannot figure out why though. I double checked connections again today for probably the 4th or 5th time. All are connected per the layout.
    What can make it positive? Maybe that .1 cap is the culprit.
     
  11. D'tar

    D'tar Friend of Leo's

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    Ensure you have the black lead to chassis and red lead to test point. I am sure you know that schmee but thats the first thing I look at when I see reverse polarity.

    A backward diode will put positive voltage on the grids. A leaking coupling cap would also. 42 positive volts is huge discrepancy.
     
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  12. D'tar

    D'tar Friend of Leo's

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    Did you disconnect the bias feed and check for voltage from the bias pot and also voltage on the intensity pot/grids of the powertubes? This will let you know if the bias is positive or a coupling cap is leaking.
     
  13. schmee

    schmee Poster Extraordinaire

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    It's now without any bias pot like the original amp until I find the issue. It's near the same voltage on pin 3 of power tubes as at the intensity pot center tab. Still positive both locations.
    I need to check the diode I guess, went through that when I did that section and it's under wraps but I'll open it up. But would it read DCV at all with the diode wrong?
    Yeah I made sure black lead to chassis, thought of that too. :>)
    The voltages are all high in the power section but seem to be the right relation to each other. Plate voltage is 475! I'll solve that when I get beyond the bias issue...
     
  14. D'tar

    D'tar Friend of Leo's

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    Yes. The diode will rectify half of the AC wave. One direction would be positive. Oposite direction negative. Both DC
     
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  15. schmee

    schmee Poster Extraordinaire

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    OK, The Diode was wrong way. I had it right at one time and forgot I bonehead switched it in diagnosis. When I put it right I have -40 VDC.
    I biased at 17 ma or so then went back a few minutes later and everything was wacky. Now had -29 VDC.
    I changed out that .1 uf cap that goes to the intensity pot (brown wire) After that I realized that I can still adjust bias with the intensity pot! I can adjust from 0 to 65 ma or more. ... and it also effects the bias voltage.
    One minute I'm 385 plate volts, next I'm 470 plate volts. Due to intensity pot adjustment I think.
    UGH, why the heck is the intensity pot working like that...? The .1 coupling caps are new Mallory 150's and I really doubt it's them.
    SO I guess my real problem I'm back to is solving the Intensity pot related issue.
     
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  16. D'tar

    D'tar Friend of Leo's

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    The pot must have reference to ground somehow which it should not.
     
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  17. D'tar

    D'tar Friend of Leo's

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    One terminal to bias supply

    Wiper to power tube grid leaks

    Other terminal to trem driver coupling cap
     
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  18. schmee

    schmee Poster Extraordinaire

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    Yeah, that's exactly how it is. Makes me wonder if there is an under-the-board issue. There are no under board routed wires on this.
    I guess I will desolder the wires to the pot and check each solder tab for ref to ground.
     
  19. D'tar

    D'tar Friend of Leo's

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    Is Bias supply to the intensity pot coming from the wiper of the bias pot?

    Edit... no more bias pot, right
     
    Last edited: Nov 23, 2019
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  20. D'tar

    D'tar Friend of Leo's

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    I would try removing the bias supply to the intensity pot then ohm power tube grids to ground rotating the pot. You should not have any continuity there.
     
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