Princeton Hum

Discussion in 'Amp Central Station' started by Kozmo, Jun 17, 2019.

  1. Kozmo

    Kozmo TDPRI Member

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    Just replaced all the electrolytics in my '66 BFPR and although it sounds great, there's what sounds like a 60 cycle hum that is louder than I remember hearing with the original caps.

    It's not overbearing and I don't notice it when I'm playing, but I feel like it could be quieter at idle.

    The hum is present even with nothing plugged in and the volume at zero. The only thing that effects the hum is the tremolo intensity which has me thinking it has something to do with the bias filter cap.

    The original cap was a 50uf 50v and I replaced it with the same values- positive to ground.

    Any ideas?
     
  2. Dacious

    Dacious Poster Extraordinaire

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    Does it hum with everything turned to zero?
     
  3. Kozmo

    Kozmo TDPRI Member

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    Yes
     
  4. Dacious

    Dacious Poster Extraordinaire

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    Then it's definitely your filter caps. If you oriented your bias cap incorrectly it would pop, but also redplate your tubes.

    I would look at your soldering on the cap can tabs if you used one. Did you check the values of the dropping resistors between each leg? Usually they don't give trouble but old B&A resistors can crack.
     
  5. Kozmo

    Kozmo TDPRI Member

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    Thanks Dacious. What's B&A?
    I did replace the cap can and all the soldering looks good. I reused the original 18k 1W resistor on the cap can tabs- it reads about 20k in place. I should probably replace it, but again, it was there before I changed the caps and the hum wasn't. I'll double check everything and hopefully figure it out. Thanks again for steering me in the right direction.
     
  6. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Cap can! Genius, Professor Dacious, sheer genius.

    A picture of the cap can lugs, wires, and dropping resistors may help the eagle eyes here but this has to be the top suspect.
     
  7. Dacious

    Dacious Poster Extraordinaire

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    20k is not too much - Fender has 20% tolerances on values, they're not that critical unless the component is actually damaged. A resistor in my VC was sitting there pushed together by it's leads. Unsoldered it fell into two big pieces.

    B&A=Bradley and Allen, makers of the brown carbon comp resistors Fender used. You can't measure them.in situ, have to pop one end.

    Alternatively, measure voltage with them in place at the test points, especially at the preamp tube plates. If they're being powered with too much voltage you can also get a hum, especially phase inverter.

    If it's 15-20volts high no problem. If it's more than that, might have to check others. Also check your bias voltage - if you're trying to pull too much bias it can cause humming. Changing the cap can may cause alterations throughout the amp.

    The Princeton power trans is the main limiting factor in these amps. Try to pull too much power and you'll notice B+ will drop and the transformer may hum.
     
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  8. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Sometimes called Allen-Bradley... :D
     
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  9. Dacious

    Dacious Poster Extraordinaire

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    Yep, them too ....:D
     
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  10. Kozmo

    Kozmo TDPRI Member

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    Wow, thanks guys.
    Dacious, you've given me some good homework. I'll get on it and report back what I find. Thanks again for all the info and tips!
     
  11. Kozmo

    Kozmo TDPRI Member

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    Okay, finally got a chance to get the hummy Princeton on the bench. All voltages read around 10% over what's on the schematic- so well within spec/tolerance.

    I removed the new CE cap can and reinstalled the original- hum was still there so I switched 'em back. Installed yet another (same value) bias cap- still hum.

    When I first turn on the amp, I can hear a faint hum (OT?) that then gets amplified through the speaker when the tubes warm up.

    Could the OT be on it's way out?

    I think my next step is to order a TO20 and see if that quiets the hum. Plus I wouldn't mind a little tighter low-end.
     
  12. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    I have multiple PRs, and highly recommend that TO20 OT. However I don't think your hum problem root-cause is the OT.

    A cheap and worthy next step imo should be to sheet-metal insulate the interior top of your cab, and make sure that cab insulation metal makes a solid connection to the mounted chassis. You can buy thin sheet steel at True Value, Ace, HD etc and it doesn't cost much, and you can staple it in if you keep firm pressure. Doing this has resulted in a *totally noticeable* reduction in hum for me in multiple venuesituations (all of which have less-than-modern wiring, and dimmed lights).


    Worst case, if my suggestion doesn't help, you'll have Faraday-caged the circuitry and can chase lead-dress or something other than blasting localized RF.
     
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  13. galaxiex

    galaxiex Tele-Holic Silver Supporter

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    Lead dress and heater balance...
     
  14. Kozmo

    Kozmo TDPRI Member

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    Thanks for the suggestions. If all else fails...
     
  15. monkeybanana

    monkeybanana Tele-Meister

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    I recently had a hum on a new to me old amp. I don't think it hummed at zero but it was present anywhere else. Turned out the input jacks were corroded where the ground and tip leafs touch so the jacks were not grounding. A little DeOxit solved it. I could hear my DVM set on continuity slowly come to a steady beeeep, as if going, Yay! Or clouds parting or angels going, Ohhhhhh. Haha, I'll stop now.
     
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  16. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Last edited: Jun 22, 2019
  17. schmee

    schmee Poster Extraordinaire

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    The "hum when warming up" sounds like the rectifier tube. I have one that does that, but it's ok after it gets warmed up. Pretty loud while warming up too.
    Are you sure it hums more now than before the cap job?
     
  18. peteb

    peteb Friend of Leo's

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    I would monitor the AC voltage (60 Hz) on the filtercaps/B+ nodes as I did these changes.


    I have replaced two original cap cans with new CE cap cans. The original cap cans taken out showed no signs of running low. In one case I noticed that the new cap can actually increased the small amount of AC on the B+ By a small amount. That’s the job of the filter cap, to keep the DC high and the AC low. AC voltage here at the wall voltage frequency will definitely put a hum on your amp.
     
  19. Kozmo

    Kozmo TDPRI Member

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    Thanks for the reply's and helpful information.

    I found a thread on "some other gear-related site" where someone described the exact same situation and symptoms on a '71 PR: Increased hum after replacing filter caps. Another symptom they described is that the hum would decrease about midway on the volume and then increase again as the volume was turned up. Sure enough, mine's doing the same thing. They finally determined it was a faulty "new" cap can (CE Mfg. from AES- same as mine) and ordered a new one, but never posted the final results.

    I've read that there were some faulty cans from CE years ago but that they had gotten better and more consistent.

    I suppose I could order another one (expensive little boogers) and keep my fingers crossed.
     
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  20. schmee

    schmee Poster Extraordinaire

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    I had a bad CE cap can for a Princeton. You could temporarily put in individual caps and see if that cures it. If you dont have that may 20uf I have used 33, and 40 somethings with no obvious effect in the past.
     
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