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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. Outcaster

    Outcaster Tele-Holic

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    The other thing to check for is DC leakage from the coupling caps after the phase inverter. That could drive the bias voltage more positive than it should be if all of the bias circuit seems in spec
     
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  2. peteb

    peteb Friend of Leo's

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    it looks like the voltage is lost after the resistor.

    the 100K resistor reduces the AC voltage, but I wonder if it should reduce it more.


    but you have high voltage AC after the 100K resistor and then it drops way down. maybe the cap is leaking your DC to ground.

    The pot should not leak the DC to ground, at least not very much if the pot offers a large resistance between that part of the circuit and ground. I think you've confirmed this.





    I don't know how the diode could make the DC negative, it only smooths out the AC into pulsing DC.

    I said the AC should be negative, I will have to put a hold on that comment, I don't think AC can be negative. It only is the difference between the peaks
     
  3. peteb

    peteb Friend of Leo's

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    the mystery is where is the high voltage disappearing to.


    Like Wally said check the voltage at every point along the way, from the rectifier tube to the bias resistor to the grids.



    at some place along there the voltage has to drop from over 100 V to less than 15 volts. you should be able to find where that happens.




    good luck
     
  4. dbspyder

    dbspyder TDPRI Member

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    Is it possible that when I installed the cap I may have pushed it down far enough that the tabs are grounding out at the chassis? I soldered it with the board in place when I'm betting I should have unscrewed it from the chassis first...
     
  5. peteb

    peteb Friend of Leo's

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    i found this on how a bias circuit works. it says the diode produces negative DC from AC

    it says it but I'm not sure how it does it?






    Here we have a typical Fender bias circuit.
    The AC voltage comes from the Red/Blue wire on the power transformer.
    The center tap, Red/Yellow wire, is the other end of the transformer winding.
    The Red/Yellow wire is grounded to the chassis to complete the bias circuit.
    The AC voltage from the power transformer Red/Blue wire then goes to the 470 ohm bias range resistor which knocks the AC voltage down a bit.

    >>>> The AC voltage then enters the bias diode and gets rectified into a fluctuating negative DC voltage. <<<<<<

    The negative DC voltage is then smoothed out by the 47uf at 100 volt bias capacitor.
    The smoothed negative DC bias voltage then enters the bias pot.
    The other end of the bias pot has a 27K 1/2 watt resistor that is soldered to ground.
    The total resistance for this circuit to ground is 10K + 27K or 37K.
    The bias pot and 27K resistor form a voltage divider.
    The middle wiper of the bias pot sweeps across the 10K resistance of the bias pot and selects the voltage that gets sent to the power tubes.
    The negative voltage then leaves the middle wiper of the bias pot and goes to a junction where two 220K 1/2 watt resistors are soldered together at one end.
    The bias voltage goes through each of the 220K resistors and appears at the two junctions.
    Each of these two junctions have a wire that leads to each power tube input grid, pin 5.
    Usually the bias voltage goes through a 1.5K grid stopper resistor first and then on to pin 5 of the power tube.
    That is the complete bias Circuit complete from transformer to power tube.
     
  6. peteb

    peteb Friend of Leo's

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    i think the answer is that the diode puts out positive DC, but because the bias cap is reversed, positive to ground, this (somehow) causes the DC to become negative
     
  7. Outcaster

    Outcaster Tele-Holic

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    The diode cuts the AC wave in half allowing either the + or the - portion of the AC wave to pass. In the bias circuit, the negative pulses pass. The cap smoothes those into non pulsing negative DC. Non of which has anything to do with the problem
     
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  8. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The orientation of the diode dictates positive or negative DC.
     
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  9. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    Diode Polarity:



    upload_2018-1-24_15-32-50.png
     
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  10. dbspyder

    dbspyder TDPRI Member

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    the 100k resistor reads 88.6

    also, if it needs replaced I have several either 1/4 or 1/2w resistors with that value, and 56k 1w resistors, but nothing higher in wattage. can I use the lower wattage resistors?
     
    Last edited: Jan 24, 2018
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  11. robrob

    robrob Poster Extraordinaire Ad Free Member

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    88.6k isn't off enough to cause your problem. I'd leave it alone for now.

    I would replace the bias cap and if that didn't do it I'd replace the diode. One of those should fix it unless there's a short somewhere like you speculate with the new cap lead. Make sure it's + to ground and - to the diode and watch the diode's polarity too if you replace it.
     
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  12. dbspyder

    dbspyder TDPRI Member

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    i bought a 2 pack of 100uf/100v caps but no other sizes, so i suppose I'll order a few other sizes. The only other one I have is the old Mallory I pulled out 70uf/100v and I wouldn't think the jump from 70uf to 100uf would cause such a huge loss in negative voltages, but I could be wrong!

    If I'm looking down into the chassis from the faceplate side, the positive is on the right, is that correct? the original was wired that way

    I checked the leads on the cap and they're not grounded

    I have the 1N4007 diode, should i go ahead and replace it?
     
  13. Outcaster

    Outcaster Tele-Holic

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    The positive end of the cap should go to the ground lead that comes out from under the board and screws to the chassis. A good close up pic of the bias board may help. From one of the earlier pictures it looks like you're missing a resistor after the bias pot although that wouldn't cause the problem you're having
     
  14. zook

    zook Friend of Leo's

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    There is confusion regarding the + and -. On a schematic, particularly the Fender Bassman, the terminal markings are reversed.

    upload_2018-1-24_19-14-5.png

    I have found that the diagram as pictured above is the common reference, but the AC goes into the Anode and exits as DC from the Cathode

    Also, in the Bias board, the diode is reversed to allow for negative DC Voltage.
     
  15. dbspyder

    dbspyder TDPRI Member

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    the + is going to the chassis

    I'm getting negative voltage though, just not nearly enough so I'm fairly certain everything is oriented the way it should be
     
  16. dbspyder

    dbspyder TDPRI Member

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    I replaced the diode and now I get -15 at pin #5 and -19 at the wire leaving the diode to the intensity pot.

    a change, but just not quite enough!
     
  17. Outcaster

    Outcaster Tele-Holic

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    Hook the meter up and leave it connected for a minute. Do you see the voltage get more negative slowly over that time?
     
  18. dbspyder

    dbspyder TDPRI Member

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    so far she rose to -16 on pin 5 and has been holding steady there
     
  19. Outcaster

    Outcaster Tele-Holic

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    Sounds like the cap is okay then. I'm out of ideas. I'd still like to see a better picture of the bias board.
     
  20. dbspyder

    dbspyder TDPRI Member

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    1516851208809.jpg

    There you go
     
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