Princeton Reverb periodic crackle when overdriven - tremolo-related(?)

Discussion in 'Amp Tech Center' started by CheesyCucumber, Dec 14, 2019.

  1. CheesyCucumber

    CheesyCucumber TDPRI Member

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    Hi!

    I'm a little stumped debugging an issue with a self built AA1164 PR, everything is fine with the amp except when I set the volume anywhere above 9.5. I get a cyclical crackle that fades pretty quickly after initial attack. It appears the periodicity of the crackle matches the tremolo. Is this behavior expected at these volume levels? I'm assuming its some kind of crosstalk but I can't pinpoint it.

    Things I'd done before reaching the conclusion above:
    • Chopstick-ing and close-up inspection to check for loose / broken / cold solder joints, leads or sensitive components
      • none found except some sensitivity in signal wires in close proximity; changed these to shielded wires and sensitivity is now gone
    • Plugged into an external cab (in another room) to ensure it's not vibration related
      • not vibration related
    • Swapped out each tube with multiple different spares
      • no diff
    • Timed the crackle because I had a nagging suspicion the speed matches the trem beat
      • yes it does
    • I then moved every single lead around the tremolo circuit and tested the behavior after every move but to no avail.

    Circuit mods:
    • Screen and grid stop resistors added to power tubes
    • Grid stop added to PI
    • Adjustable bias pot added
     
  2. CheesyCucumber

    CheesyCucumber TDPRI Member

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    Further information

    The issue only manifests at a certain range of input levels. i.e. it does not appear if the signal input level is lower or higher than a small range.


    spikey.jpeg
    ^ 440Hz sine wave at volume 10 taken at the output / speakers, note the 4 spikes.

    I'll take some gut shots and readings across each stage tomorrow
     
  3. darkwaters

    darkwaters Tele-Afflicted

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    When you replaced the tubes, did you spray the tube sockets and tube pins with contact cleaner ? I had a mysterious intermittent crackle in my PRRI and that fixed it.
     
  4. CheesyCucumber

    CheesyCucumber TDPRI Member

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    The tubes (including the multitude of spares) are either new and corrosion free or have had their pins cleaned / sanded prior to testing. The sockets are new and were cleaned during the build process (circa May 2019) to remove any flux / etc. residue.

    I can give it a try but I doubt it'll change anything :)
     
  5. darkwaters

    darkwaters Tele-Afflicted

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    I'm guessing that Singapore is humid like here. Humidity, in my experience, is the enemy of electronics. I'd spray the inputs and everything else for good measure.
     
  6. schmee

    schmee Poster Extraordinaire Silver Supporter

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    That's easy, don't play over 9 on the dial. :lol: What are you doing up there anyway?:eek:
     
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  7. CheesyCucumber

    CheesyCucumber TDPRI Member

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    Stability testing ;) was putting the amp through all its paces and noticed the issue

    If you're hinting that the behavior is expected, I'll take it and just leave well enough alone :lol:
     
  8. King Fan

    King Fan Friend of Leo's

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    I may be missing something, but if it varies with the trem I might wonder about the trem pots, or a leaky cap in that circuit, or something in the bias circuit. What's your bias voltage and dissipation?

    Also the RCA jack and footswitch leads are always suspect. Does it persist when you kill the trem with the footswitch? What happens if you pull the footswitch?
     
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  9. schmee

    schmee Poster Extraordinaire Silver Supporter

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    I'll have to crank mine up there and see if it does the same thing I guess. I just have never had one past maybe 5... They just seem to get nastier past 4...
    The trem is turned "off" with a switch right? (or grounded plug jack) I do know on a PR type you have to switch it off, not just turn the intensity off... if you are biasing etc..
     
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  10. King Fan

    King Fan Friend of Leo's

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    Interesting, schmee. I forget, did you super-size your OT, PT, speaker, etc.? I can get my PR with stock iron and vintage 10 alnico up to at least 7. (I agree no one except Jim C. has ever played a PR dimed; he's using some super-secret special-mild sauce.)

    FWIW, one thing that can take the 'nasty' out of PR OD is a 470K PI grid stopper. If you haven't tried it, it's the easiest mod in the world...

    FEE06B75-3ED2-417C-98C7-4450AC9D3E01.jpeg
     
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  11. schmee

    schmee Poster Extraordinaire Silver Supporter

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    I'll check that out! So that 470k is just between pin 7 and the 1 meg I guess? Yes, I have built stock and supersized ones. The ones with big PT and Deluxe OT dont suffer much from getting too nasty. I was talking of a stock PR regarding that.
    I guess I can't really try mine at 9.5 for comparison because the one I have right now has a big PT in it. Although it might be interesting to compare, as it could be a circuit anomalie in the design I suppose....
     
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  12. tubelectron

    tubelectron Tele-Holic

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    I would suspect something there... o_O
    All of the other mods you mention concurs to stability, so...
    This is a curious phenomenon, though.:confused:

    Did you tested the normal unadjustable bias circuit to see if the issue still continues ?

    Another idea, maybe better :

    I you have not tested it, I suggest you to wire some 500p to 1n directly at the socket from grid to cathode and see if the trouble continues. My SF PR had 1.2n. I removed them and the amp had dirty overdiving when cranked, due to RF oscillations. I put a pair of 680p (470p were not enough) that eradicated the RF overdrive issue.

    You can see them on the picture below :

    [​IMG]

    -tbln
     
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  13. CheesyCucumber

    CheesyCucumber TDPRI Member

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    I'll check the bias cap for leakage :) what should I look at for the pots?

    Bias dissipation is at 19.3 - 19.8 mA for about 60% on JJs. Plates sit at a really toasty 424V. I didn't record the bias voltage, it should be around -29V or -31V (I'll confirm this soon).

    I'd initially thought it was a bias issue but the fact that the oscillation / instability goes away at higher input signal levels didn't really jive with that (?)

    The issue persists when I kill the trem with the switch or by grounding it out elsewhere, I feel like it's less prominent but that's likely confirmation bias on my part.

    There doesn't appear to be a difference with or without the footswitch plugged in. I'll try swapping out the jack and re-wiring the footswitch lead. :D
     
    Last edited: Dec 15, 2019
  14. CheesyCucumber

    CheesyCucumber TDPRI Member

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    Yup! Trem turned off / grounded in order to bias, otherwise it'd throw off the readings
     
  15. CheesyCucumber

    CheesyCucumber TDPRI Member

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    I'll try without the bias pot in circuit and see if there's a change. If that doesn't work I'll try adding the stability caps :lol:
     
  16. tubelectron

    tubelectron Tele-Holic

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    Well, IMHO, test the stability caps first, as it is an easier mod and I'm finally more inclined to think that it maybe the issue...

    But it's your amp, OK ? :D

    -tbln
     
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