Bias way high and ramping up....

Discussion in 'Amp Tech Center' started by schmee, Oct 23, 2019.

  1. schmee

    schmee Poster Extraordinaire

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    I posted in my old thread yesterday, but that thread is a bit off topic at this point really. I'm building a non Reverb BF Princeton in my old MONO 15 chassis. It's been done for days except one major issue:
    When I turn it on, the bias ma reading just climbs out of sight. So I turn the amp off when it reaches 65-90 ma!
    I am kinda stumped right now. This thing is so damn simple it's ridiculous.

    -Double, triple checked the wiring and board.
    -Adjusted the bias pot (was added in lieu of stock layout)
    -Removed the whole bias pot and made it stock Princeton. Same problem.
    -Voltages in the whole amp are pretty high, but that's not too abnormal IME.
    -Known good power tubes.
    -Plug in SS rectifier.
    -I'm using an actual PR power transformer.

    I'm starting to wonder if I have another bad CE Dist cap can. Would filter caps do this though?

    So... lead me to things that would cause very high bias readings.. with a stock layout...
    [​IMG]
     
  2. dan40

    dan40 Tele-Afflicted

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    Without power tubes installed, do you have steady negative DC bias voltage at pin 5 of each socket? If not, we will need to figure out why before installing the power tubes again. Another option could be a gassy tube if you do have good bias voltage on pin 5. Be sure the grid leak resistors are grounded also.
     
  3. schmee

    schmee Poster Extraordinaire

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    I'll check pin 5 voltage. Did a quick check the other day but not enough to see if steady. I think it was -35 or so... but that was early on in this fiasco!
     
  4. dan40

    dan40 Tele-Afflicted

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    Monitor the voltage for several minutes on both sockets to be sure that it stays steady. If it drops off, there is a problem in the bias circuit. Also be sure that the pot sweeps the voltage up and down when adjusting it.
     
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  5. schmee

    schmee Poster Extraordinaire

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    "Be sure the grid leak resistors are grounded also."
    You mean the 220K's? Off pin 5? This amp has no resistors on the sockets...
     
  6. dan40

    dan40 Tele-Afflicted

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    Should be a pair of 220k resistors mounted on the board that the bias circuit feeds. They would be the grid leak/bias splitters.
     
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  7. D'tar

    D'tar Friend of Leo's

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    Bad bias cap

    Leaking coupling capacitors from PI
     
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  8. schmee

    schmee Poster Extraordinaire

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    OK, that's what I thought you meant, I think of them as splitters. But what do you mean "be sure they are grounded" They dont really go directly to ground anywhere...
     
  9. schmee

    schmee Poster Extraordinaire

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    OK, tested the voltage at pin 5. It ramps up slowly, maybe taking 5 minutes to -103.2 VDC on one socket and stays there. Then when I test the other pin 5 it's immediately at -103.2
     
  10. slider313

    slider313 Tele-Afflicted

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    Make sure the 100k resistor off the bias diode isn't open. Check all components on the bias board; cap, diode and resistor(s).

    Did you add the jumper wire from the bias cap negative to the diode negative? It's under the board on the original Princeton.
     
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  11. peteb

    peteb Friend of Leo's

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    the 220K resistors do connect to ground, thru the intensity pot, the 27R and ground.




    I am under the impression that if there is no grid leak to ground, a negative charge will build up on the grid, like grid leak bias, but if the path to ground is not there the negative charge would probably continue to build until the tube is biased in cut off.






    princeton_aa964_schem.gif
     
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  12. D'tar

    D'tar Friend of Leo's

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    How about a pic of the bias board? Cap orientation...

    To collect that much negative voltage you must be losing the bias circuit ground connection through the 27kr or bias pot/resistor paralleled by the bias cap.

    The tube are going into cut off and should be the safest way to fail;)

    Edit to say I just read what pete said and wrote pretty much the same thing.lol
     
  13. schmee

    schmee Poster Extraordinaire

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    Thanks guys. A pic is kinda tough as this is the "princeton into a mono 15 chassis" project which is hard to even see anything in person!
    A couple things:
    -There is no bias board per se. Instead there is the cap, diode and 100K resistor just strung together off the Intensity>Bias pot > to the Rect Tube socket. It's only like 2" distance in this build and hangs out in space. I thought about changing that but it should work fine.
    However THERE IS NO 27K RESISTOR across the cap, instead it's like this (below). Maybe that is my issue, I had this layout laying around in my archives by someone else, so just went with it. Maybe it's BS, but I don't see where it shouldn't work. (I usually do it a bit different ;putting a pot in series with the normal 27k resistor, or 22k, across the cap)
    [​IMG]
     
  14. D'tar

    D'tar Friend of Leo's

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    If you're wired exactly like that it should be good. The cap is + side to ground right?

    Monitor where the neg voltage returns to zero with respect to the chassis....

    Should be at the 27k on the back of the bias pot. Should have -ve vdc on the pot terminal side and zero on the ground side of the resistor. If not zero there, then the pot casing is not grounded well or bad joint etc. If no voltage on the pot side of the 27k, you have bad Potentiometer
     
  15. schmee

    schmee Poster Extraordinaire

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    I actually have a ground wire to the bias pot case just to ensure ground. Not sure it's a new pot though. Maybe I'll try another.
     
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