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'78 Princeton(non-reverb) is humming

Discussion in 'Amp Tech Center' started by cyborg_stew, Feb 20, 2020.

  1. cyborg_stew

    cyborg_stew TDPRI Member

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    My Princeton has recently developed a 60hz hum when powering up. The hum "surges" shortly after the amp is turned on, then it quiets down to being barely audible. It isn't affected by the volume or tone controls; however, the hum is pulsing and can be altered by turning the intensity knob. Knowing the little that I do about the tremolo circuit in these amps, I'm wondering if there is a problem in the bias circuit or possibly a coupling capacitor. The tremolo seems a bit weak as well. Any help would be greatly appreciated. I'm new to the tube amp world, and trying to learn as much as I can about them.

    Also, I've measured the bias a few times over the last 2 years and noticed a trend. The resistance of the OT has been dropping evenly on both halves, and the plate voltage of the 6v6's is slowly on the rise. Not sure if this has anything to do with the recent hum or not.
     
  2. boredguy6060

    boredguy6060 Poster Extraordinaire

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    Do you have any extra tubes that are known to be good?
    If so try swapping out tubes. That’s the first place I’d start looking.
     
  3. cyborg_stew

    cyborg_stew TDPRI Member

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    I swapped both preamp tubes to no avail, and I only have one 6v6 that is good. I switched out each of the power tubes with it, and it made the hum worse/louder. I expect that's because swapping only one tube, rather than a pair, caused a mismatch.
     
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  4. boredguy6060

    boredguy6060 Poster Extraordinaire

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    I suspect you’re right about the mis-match. It’s hard to track down a hum without having the circuit right in front of you and some chop sticks and a meter.
    Good luck,
     
  5. Jon Snell

    Jon Snell Tele-Holic

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    An unbalanced pair of output valves is a classic cause of 100/120HZ hum. Not mains frequency hum.
    I would try a new matched pair and maybe check the value and condition of the main smoothing components.
     
  6. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Have a look at youtube...

    Two Types of Tube Amplifier Hum and How to Determine the Source
    by Uncle Doug.
     
  7. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The trem on a princeton has a relationship with the bias of the 6v6s. Typically the hotter the bias the weaker the trem.
     
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  8. Wally

    Wally Telefied Ad Free Member

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    If one is not familiar with working with live circuits...taking voltage measurements, measuring current draw, etc...imho, it is tech time in a situation like this. Even if a pair of power tubes resolves this hum problem; imho, a tech review is of value.
     
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  9. cyborg_stew

    cyborg_stew TDPRI Member

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    Thanks to everyone for replying.

    I'll take the chassis out this evening and get some readings on the filter caps and report back. I'm starting to suspect that's the issue, cuz I recently discovered that it may be an original cap can. There's a new electrolytic soldered to it inside the chassis though, so that kinda confused me.

    I'm fairly new to working in amps, but I can safely use a meter without killing myself or causing damage to the amp. I only do the things that I know how to. And for the other stuff I'll leave it to the pros.
     
  10. cyborg_stew

    cyborg_stew TDPRI Member

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    DSC09125.JPG
     
  11. cyborg_stew

    cyborg_stew TDPRI Member

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    Uncle Doug is great! I've seen this video before. And if I remember correctly, he explains that hum associated with filter caps is 120hz as opposed to 60hz. That's why I was thinking it wasn't the cap can. But now I'm not so sure it is a 60hz hum. lol. I'll have to double check.
     
  12. cyborg_stew

    cyborg_stew TDPRI Member

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    The tubes are biased close to 70%. Is that considered hot for a bias vary tremolo?
     
  13. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Yes.
     
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  14. cyborg_stew

    cyborg_stew TDPRI Member

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    Thanks. I don't use the tremolo very often, so it's not really an issue for me. I just thought it may be related to some other problem in the amp.
     
  15. cyborg_stew

    cyborg_stew TDPRI Member

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    The 12th week of '77?

    DSC09224.JPG
     
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  16. cyborg_stew

    cyborg_stew TDPRI Member

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    I did some basic testing on the cap can with a meter monitoring voltage, and it appears to be operating just fine. I know if it's original, it will need to be replaced anyway.
     
  17. cyborg_stew

    cyborg_stew TDPRI Member

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    One interesting thing I found in my probing is this: on pin 7 of V2 there is 29VDC. This doesn't seem right, as this is the grid, and the schematic doesn't have any voltage listed. I'm thinking the .022 coupling cap must be leaking from the plate of V1.

    Another thing is I can't get a steady reading on pin 1 of V2. Is that normal since it's part of the tremolo circuit? When I disengage the trem via the footswitch jack, the reading is a steady 261VDC.

    Also, should there be 83VDC on pin 8 of V2?
     
  18. Jon Snell

    Jon Snell Tele-Holic

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    The grids of the 6V6s should be around -33 ish. If one is higher than the other the valve with less negative voltage on the grid is faulty. They should be almost identical.
    I have had these amplifiers in for repair and the filter or main smoothing capacitor has been open circuit and they didn't hum, just sounded cr*p at power!
     
  19. elpico

    elpico Tele-Afflicted

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    Is it the same circuit as the earlier (aa964) non-reverb princetons? If so V2b is the phase inverter and pin 7 is actually within 2 volts of pin 8 BUT most meters can't measure the voltage at this point correctly. It's a well known gotcha about split load phase inverters. And 83v is reasonable for pin 8 yes.
     
  20. cyborg_stew

    cyborg_stew TDPRI Member

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    Yeah, the circuit looks identical to the aa964. After looking at the schematic further, I now see how 83v make sense on pin 8. I don't really understand your explanation of the voltage reading on pin 7. You're saying that my meter won't measure it correctly? I'm new to this stuff, so please forgive my confusion. lol
     
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