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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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    I have a diode mode on my DMM and while still attached to the board(amp off) i probed the leads and read 599 and flipped the leads and got nothing, so I'm assuming that means it's working correctly.

    I'm currently working on adding the 50k pot for bias adjustment but where do I test to check the bias voltage on the PT?
     
  2. Outcaster

    Outcaster Tele-Holic

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    I just went through this. Check the resistor on the bias board. I'll bet it's drifted quite a bit.

    Edit: the 100K resistor ahead of the diode
     
  3. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Measure all the bias circuit voltages:

    The AC bias voltage from the power transformer

    The DC voltage coming out of the diode

    The DC voltage at each component junction between the diode and the power tube grid pins

    What's really important is the negative voltage at the power tube grid pins
     
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  4. dbspyder

    dbspyder TDPRI Member

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    negative voltage on pin #5 is 5.0 on both tubes

    358VAC on pin #4 and #6 on rect. tube

    -6.2VDC coming out of the diode

    working on the rest and ill post as i go
     
    Last edited: Jan 23, 2018
  5. dbspyder

    dbspyder TDPRI Member

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    its AC current running from the rect. tube to the first bias resistor correct? if so its at 356VAC at the beginning of the resistor and drops to 140VAC after the resistor.
     
  6. peteb

    peteb Friend of Leo's

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    -5 volts here is way too low.

    It should be more like -35.





    A low power tube bias will cause extra current to flow and could cause red plate.
     
  7. dbspyder

    dbspyder TDPRI Member

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    1516771926545.jpg

    Alpha B50K pot installed for bias, everything look ok?

    At Max clockwise I read -9.0 on pins #5, as I turn counter clockwise the negative bias goes down( @ 8 o clock I read -7.0)
     
  8. Wally

    Wally Telefied Ad Free Member

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    Ime, it is best to follow the voltage from the source through to the power tube pin 5.
    So, you have good voltage coming off of that rectifier for the bias feed. You have 140vac after the 100K resistor. On the other side of the diode, you have -6.2vac. I would suggest lifting the Connection from the bias circuit to the intensity pot and checking the voltage there again. If you still see -6.2vac, then your problems are likely to be found in the bias circuit....that 22k resistor or the bias cap. If, however, that voltage looks good with the pot disconnected, your problem is in that pot or something downstream of the bias circuit board.
    I would suggest foregoing the installation of a bias pot until you understand why the bias voltage is not anywhere near what it should be. After establish a good bias circuit, then apply a potentiometer.
     
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  9. zook

    zook Friend of Leo's

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    My next guess would be that the 100K resistor that the bias tap is connected to has drifted. It might also be that the bias voltage of your new PT is not the same as the original. If the 100K resistor checks out okay, try a 22K in its place and see how the bias is then
     
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  10. peteb

    peteb Friend of Leo's

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    I agree entirely with both posts above.


    I agree disconnect the tremolo circuit from the bias circuit.

    then make sure you have max resistance to ground across your added pot. 50K.





    with out the power tubes in, turn it on, whats the voltage on either side of your 100K bias resistor?
     
  11. dbspyder

    dbspyder TDPRI Member

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    I took measurements on the intensity pot and measured the same negative voltage.
    I will try it disconnected and see what that yields.

    Should I just go ahead and replace the 100k resistor? If so what wattage resistor do I need? I have a box full of 1/4w resistors but I'm guessing those aren't going to work..
     
  12. dbspyder

    dbspyder TDPRI Member

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    We can calculate whether or not the resistor is working correctly based on the voltage drop correct?
     
  13. peteb

    peteb Friend of Leo's

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    whats the resistance on either side of the 100K? the answer to the problem should be apparent then.
     
  14. dbspyder

    dbspyder TDPRI Member

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    356VAC going in and 140VAC coming out
     
  15. peteb

    peteb Friend of Leo's

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    thanks

    maybe I was overly optimistic


    I thought that there would be low voltage, instead it is higher than expected.


    still @-10 V on the grids? pin 5?




    and shouldn't the volts be negative in the bias circuit?




    princeton_reverb_aa1164_schem.gif
     
  16. Outcaster

    Outcaster Tele-Holic

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    That's voltage, what's the resistance measure? If it's near 100K, then the cap is likely the culprit. I tried a 100uf cap and had issues. Used a 70uf and it worked fine. Had the same issue you're having
     
  17. dbspyder

    dbspyder TDPRI Member

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    Isn't the voltage positive until I reaches the diode?

    With the 50k bias pot I got it up to -9.0.

    I'll pull the resistor and check it when I get home. I couldn't get it to give me a resistance probing the ends with it in the board.
     
  18. dbspyder

    dbspyder TDPRI Member

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    Hrmm I've read others used a 100uf and did just fine. Was your negative bias as low as mine?
     
  19. Outcaster

    Outcaster Tele-Holic

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    Yup, it just wouldn't build up a bias voltage. Over several minutes it would make it to maybe -20v or so
     
  20. dbspyder

    dbspyder TDPRI Member

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    Man I wish I had a place to purchase components around here. I'm all for trying out a different cap but that just means I'll have to order one and patiently wait on it's arrival...[emoji35]
     
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