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67 SF Princeton Reverb low plate voltage and red plating power tubes

Discussion in 'Amp Tech Center' started by dbspyder, Jan 23, 2018.

  1. dbspyder

    dbspyder TDPRI Member

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    with new 1n4007 diode, intensity pot wire disconnected I get -20VDC, so it's in the bias circuit correct?
     
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  2. peteb

    peteb Friend of Leo's

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    That seems correct that it is in the bias circuit.



    The bias voltage should be double that at Max.
     
  3. dbspyder

    dbspyder TDPRI Member

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    the only other component in the bias circuit is the 100k resistor correct? I've replaced everything else...

    If so, is the resistor wattage an issue(1/4w, 1/2w, etc)
     
    Last edited: Jan 24, 2018
  4. peteb

    peteb Friend of Leo's

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    It was considered getting off track, the diode and the negative DC, but I think it is important to understand how this bias circuit is wired and functions.


    I didn't initially see the diode making the DC negative. Diodes pass current in one direction, and I didn't see how it could invert, but it's not inverting, it is filtering like outcaster said.





    Outcaster is right, in the strictest sense, using the true definition of current direction, but I see it a little differently.




    The direction of electron flow is opposite to the direction of current. In the bias circuit of the PR, the diode is a little triangle that looks similar to an arrow pointing right. In diodes this is the direction of electron flow not the direction of true current flow. The electrons are traveling left to right, attracted to the pulsing high voltage of the PT. unlike filter caps, the bias filter cap is reversed and electrons are entering the cap from the chassis and flowing to the PT thru the diode. The diode (is forward biased?) and allows the foward traveling, positive part of the AC wave form to pass while blocking the negative, leaving everything left of the diode negative.
     
  5. robrob

    robrob Poster Extraordinaire Ad Free Member

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    If the 100k bias resistor drifts low to around 80k then your bias voltage would actually be higher than spec so I'm leaning toward a short circuit somewhere between the bias components and the power tube grids.

    With the amp off verify there's no residual voltage at the bias filter cap--the resistor in parallel with it should bleed it down.

    Measure resistance from the power tube grid pins (even with the tubes in place the grid is a dead end) to ground. With the tremolo circuit disconnected it should equal the 220k power tube grid leak + the resistance parallel to the bias cap (bias pot + resistor).

    Turn the intensity pot for minimum resistance as you measure.

    If the resistance is quite a bit lower than what's expected then something is shorting the circuit to ground.
     
    Last edited: Jan 24, 2018
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  6. peteb

    peteb Friend of Leo's

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    Good idea Rob



    I believe you have.

    100k R not yet
    Diode check
    Cap check
    22k R is a 50k pot


    You've confirmed the diode has 50K resistance to the chassis?




    To understand, 140 VAC is on one side of the diode and - 20 VDC is on the other end with the pot maxed?



    To further Robs suggestion that there is a short over near the tubes, it would be a good idea to disconnect the bias circuit right there at the intensity pot to isolate it until you can get -35 volts up.




    The bias board looks correct.


    The resistance wattage, I am not sure which is best.
     
    Last edited: Jan 25, 2018
  7. dbspyder

    dbspyder TDPRI Member

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    I'm reading 52VDC at the filter cap with the amp off

    with the tremolo disconnected at the intensity pot i get no measured resistance from pins 5, with it connected I got 289
     
    Last edited: Jan 25, 2018
  8. Outcaster

    Outcaster Tele-Holic

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    To be clear, you're seeing -52VDC at the bias cap with the amp off? If so, the bias board is working fine and I'm leaning toward the coupling caps after the phase inverter. If it's +52VDC at the cap can, then it just hasn't bled down yet...

    Let Rob or Wally double check me here but if you disconnect the wire at the D node of the cap can (the one closest to the board and with one end of that 18K resistor on it) and have the rest of the amp wired normally you remove any DC voltage from those two coupling caps without having to lift them both. Pull the PI tube, power it up and check the bias voltage at the diode and pin 5 of each power tube. If the bias voltage is good, one or both of those caps are bad. Have the bias pot at it's max resistance when you take that measurement.
     
  9. dbspyder

    dbspyder TDPRI Member

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    +52VDC on each terminal of the filter cap, it started going down slowly with the DMM clipped to it, so I'm guessing it's working fine. There are 2 slightly melted plastic spots near the grounded tabs, but I'm assuming the heat from whatever gun to original owner used caused that.
     
  10. dbspyder

    dbspyder TDPRI Member

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    I misread a previous post, let me check the bias filter cap resistance and get back to you. I read filter cap and missed the rest...lol
     
  11. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    Common way of doing it (and fwiw I have successfully done it this way). Make sure the values are correct, polarities are correct, and the wiring under the board is correct.

    upload_2018-1-25_9-5-54.png
     

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  12. Wally

    Wally Telefied Ad Free Member

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  13. dbspyder

    dbspyder TDPRI Member

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    I have a GZ34 in there, would that make any difference using a 5U4GB?

    the coupling caps you are referring to, the .1mfd blue caps on the board closest to the filter cap?
     
    Last edited: Jan 25, 2018
  14. dbspyder

    dbspyder TDPRI Member

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    well i swapped out the resistor and BAM -60VDC at the other end of the diode. so -34 at pins 5? reason I ask is because the negative voltage goes down some from the intensity pot to pins 5.

    At the intensity pot its -44VDC( after I've dialed in the correct bias with the 50k pot) and at the pins its -34VDC. I'm assuming this is normal considering that voltage is running through resistors on its way to the power tubes
     
  15. peteb

    peteb Friend of Leo's

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    All right!

    No mystery, just a bad part.

    -34 on the grids, pin 5, for starters, then you can bias it where ever you want to
     
  16. Outcaster

    Outcaster Tele-Holic

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    Thats good. As Rob said, the 88K or so you measured there should have made the bias voltage more negative so prrhaps a bad solder joint.
    That should be at least a 1 watt resistor (the 100K on the bias board). Hoffman recommends a 3 watt. I recall using a 2 watt on mine.
    Also, not critical but its a good practice to put a 10K resistor between the bias pot and ground, where your white wire is coming off that pot. That prevents any possibility of shorting you bias voltage directly to ground. Obviously not required for the circuit to work though.
     
  17. dbspyder

    dbspyder TDPRI Member

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    good deal, I left it at -34 and I've been playing through it with no issues so far. its a bit phlubby as in the lows at around vol 3, but i plan on hooking up a 12" cab and seeing if that makes a difference.
    the trem works perfectly although the reverb seems weak, but that's a battle for another day!
     
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  18. dbspyder

    dbspyder TDPRI Member

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    I'm not sure on the wattage I used(1/2 or 1/4w perhaps based on the size), but I have plans to swap in a larger wattage tomorrow.

    I'll go ahead and add the 10k.

    Thanks to everyone for all the help! I'm honestly thankful for the problems that pop up, as I end up learning a great deal from them and the people who graciously take time out of their day to lend a hand. :)
     
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