Princeton Reverb - weird buzz during OD

Discussion in 'Shock Brother's DIY Amps' started by BigDaddy23, Oct 16, 2019.

  1. BigDaddy23

    BigDaddy23 Tele-Meister

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    Hey all - Hoping u guys can help me out here!

    I have this strange buzz type sound that happens when I push this amp I built when I have it cranked up to distort readily. It is a definite buzz (kinda like the old torn speaker sound) at the beginning of a chord strum. Tends to be in the lower strings/register. It clears after 1 or so seconds to normal OD decay. The amp has a grid stopper of the PI for blocking distortion. I’m not sure what this is a symptom of though and was hoping that you learned lot might be able to help!

    Cheers,

    BD
     
  2. Dacious

    Dacious Poster Extraordinaire

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    What value resistor is on the grid stopper? May not be high enough.

    Where are your volume and bass knob set to?
     
  3. BigDaddy23

    BigDaddy23 Tele-Meister

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    Hey mate - 470K grid stopper. Bass about 2, treble 4
     
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  4. King Fan

    King Fan Friend of Leo's

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    Lower frequencies do use more power, so it *could* be a purely electronic buzz, like blocking distortion, but more often, especially on a home build, mechanical vibration can propagate into the apparent signal, and louder vibrates more. The speaker vibrates the cab and the chassis and the wires, sockets, tubes, and board. Is it more on some notes than others? Sympathetic vibrations tend to favor some frequency.

    Does it happen if you plug this chassis into another cab/speaker? If absolutely not, you've got a cab issue. Double check all cab, baffle, chassis, and speaker screws and bolts. Play the amp upside down and on its front and back. Press on the baffle and other bits (or have someone else do it). Sit on the cab. Etc.

    Position may indicate cab noise etc, but may also point to what I'll call electro-mechanical buzz anywhere in the amp. Speaker wires or jack? If not, try (careful here) playing the amp with the chassis sitting on the cab but not connected, or sitting on something solid next to the cab.

    If it seems the speaker/cab are vibrating something inside the chassis, you can forcefully tap on everything in the chassis (with something small, hard, heavy, like a heavy pen), in a *powered-down / fully discharged amp.* You're trying to find anything that vibrates. Caps on board? Tube sockets? etc. etc.

    If it persists and you haven't found it yet, you can try tapping hard (with something *non-conductive* this time) while the amp is live.

    Mine sounded a lot like yours, varied some by note and some with amp position, but gave variable results on a lot of these tests. Turned out to be the board or one its standoffs hadn't fully seated on the chassis (cuz I hadn't lined up the holes just right).
     
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  5. tubelectron

    tubelectron Tele-Holic

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    Some rattling somewhere ? I mean : mechanical noise vibration. Is it heard through the speaker or not ?

    It may be also a self oscillation of the circuit occuring at high volume pot setting, due to unwanted coupling reaction gain.

    This problem occurs on some SF models. Fender put 1200pF accross the 6V6 grids to GND, but 680pF is enough and doesn't produce a very noticeable treble loss. It's what I did on my late 70s SF PR.

    You can also try to increase the serial grid resistor of the 6V6 from 1500R to 4700R instead. A 10 to 22K serial resistor in the input grid of the phase-shifter can help too.

    Hope this helps... ;)

    -tbln
     
  6. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    What Output Transformer are you using? (I was getting a maybe-?-similar "woof buzz" when using humbuckers+flatwounds. Upgrading the Output Transformer was a huge improvement and that problem disappeared)
     
  7. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Double check the connections at the speaker tabs and the speaker jack. All the jacks really. I've had loose spade connectors that only acted up at high volume and vibration. Also, brand new switchcraft jack that didn't clear the ground tab properly and made intermittent noise. I always run amps cranked for a while after building or working on it because it seems like a lot of issues need the extra stress/vibration to present themselves.

    Sent from my moto g(6) using Tapatalk
     
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  8. BigDaddy23

    BigDaddy23 Tele-Meister

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    Thanks so much for the replies fellas - I will check these things out when I am home tonight.

    It’s certainly an ‘electric’ buzz coming from the speaker though with an abrupt stop after a second or thereabouts. I checked all the speaker and baffle mounting screws and they are all tight. No apparent note attributed to it but definitely on the E A D strings. Seems like it is almost cutting off as the large signal comes through and it returns to normal after it drops below a threshold.

    I did have a problem with a run away oscillation in this amp when I first built it. With the amp idling, it would begin to oscillate at about 2 o’clock on the volume. On the advice of you good folk, that turned out to be lead dress going into the PI. Perhaps this is happening again with some sort of sympathetic cross talk?

    I’ll post some gut shots of the finished amp for reference.
     
  9. Nickfl

    Nickfl Tele-Afflicted

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    How tight are those mounting screws? Too tight can warp the speaker frame and cause voice coil rub, which can sound exactly like what your are describing. It wasn't clear from your reply if you had actually tried playing the amp through a different speaker in a different cab but, if you haven't you should.

    I had a build once that I was convinced had either a mechanical rattle inside the chassis or an electrical problem causing a buzz exactly like what you are talking about and in the end I tracked it down to the speaker which had apparently been damaged in shipping. It was subtle too, it only really occurred with overdrive at a fairly high-volume and even when I moved the speaker cone gently by hand there was no rub to be felt but that's definitely what it was. it was just barely rubbing at the more extreme extent of its excursion and it made a buzzing note decay sound on overdrive.

    The only way to eliminate that possibility is to play the amp through a known good speaker and play your problem speaker with a good known amp to see if the problem follows the amp or the speaker.
     
  10. BigDaddy23

    BigDaddy23 Tele-Meister

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    Thanks @Nickfl - They aren't silly tight to have caused the basket to warp. I have spring washers under the screw heads to keep them in place and I could probably give them about another 1/4 turn.

    I haven't tried another speaker cab yet, but will give that a go first tonight.

    I checked that all the tubes were properly seated and that the speaker - cable connection is good .... al good there.

    I have read that it can be a bad reverb or PI tube too so I can swap out the 12ax7's one by one as a test.

    Thanks again for all your help folks!
     
  11. BigDaddy23

    BigDaddy23 Tele-Meister

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    Righto - I tried the following with no change:

    1. Another cab. Positioned alongside the PR cabinet. No vibration of components noted.
    2. Swapping out all the preamp tubes one by one.
    2. Bias was at 74% PD (plate -K = 433v with 20.5mA at each 6v6). So I dropped that to 17.5mA (63% PD) with no real noticable change.

    I noted that it does appear to be affected more by the treble control - With it sitting at more than 2 on the treb, the noise starts to kick in on the pick attack. Bass doesn't seem to make any difference.
     
  12. BigDaddy23

    BigDaddy23 Tele-Meister

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    Here’s a few gut shots.

    A4B6CDE6-65EC-4552-93C6-D25B3AB2B115.jpeg

    DE62D9AE-4BEF-493F-8791-7351773BEF28.jpeg

    DFA43033-F44C-43F3-9C88-B174B8F8FEB8.jpeg
     
  13. tubelectron

    tubelectron Tele-Holic

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    Yes, IMHO it's that "triggered" oscillation issue, if it is not a speaker / speaker mounting / adverse vibration problem. It's exactly what I had on my SF PR, that I cured with 2x 680pF at the 6V6 grids.

    [​IMG]

    [​IMG]

    -tbln
     
  14. BigDaddy23

    BigDaddy23 Tele-Meister

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    Hammond 1760E. I doesn't appear to be there in the lower freq.....its an intermittent, raspy kinda buzz, almost like static that cuts out about 1sec as the chord decays.

    @tubelectron - thanks for this. I reckon I have a couple 680pf 500V black mica caps in the parts bin. What's the acceptable tolerance given the 430 odd volts on the plates? Do they need to be 1kV?

    I have a scope and tone generator so I'll hook them up ans see if I can elicit the problem and see it on the screen.
     
  15. D'tar

    D'tar Friend of Leo's

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    If the red here is capacitor blood leaking from a mortal wound such as the Hot Iron Kiss of Death.... I would replace them..... YMMV


    upload_2019-10-17_8-42-22.png
     
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  16. tubelectron

    tubelectron Tele-Holic

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    Well, these caps are to be installed at the input grid (pin 5) of the 6V6, not the plate (pin 3), so yes : a pair of 680pF/500V mica caps is perfect here.

    Wire them on the tube socket directly, like I did, for maximum efficiency.

    Make the test and tell us if the adverse noise is eradicated.

    -tbln
     
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  17. Nickfl

    Nickfl Tele-Afflicted

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    Have you gotten in there with a chopstick to poke around and see if there are any microphonic tubes, components or wires?

    A couple of weeks ago I did some mods on what started out as a 5e3 build and after the fact I was getting a scratchy, static like buzz on some notes. Mostly on the low e and a strings and much more with humbuckers than single coils. I though it sounded like a cold solder joint, but couldn't find and, but when I poked around I found that at the inputs, the leads between the board mounted grid stoppers and the v1 plates were very microphonic (but notably not the input leads between the grid stoppers and the jacks). I ended up moving the grid stoppers so they were directly mounted to the tube socket and that cleared things right up.

    I notice that none of your grid stoppers are directly mounted to the tube sockets, so that is something to consider as it is considered to be a best practice.
     
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  18. tubelectron

    tubelectron Tele-Holic

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    Reading this @BigDaddy23 , I'm unquizically sure that your amp has an unwanted coupling somewhere in the wiring, due to sensitive wires proximity, or by lack of shielding on one of them, or the like...

    This unwanted coupling provokes HF oscillation at high volume and high treble setting, the pick attack and the signal are triggering it. You should be able to see it on your scope.

    Let us know... ;)

    -tbln
     
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  19. King Fan

    King Fan Friend of Leo's

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    Good ideas. Microphonic wiring *or* unwanted proximity both seem possible with that Hoffman-style build and its long flying leads. Doug does that for sure; here's his build for those who haven't seen it.

    Img_8416.jpg

    Long, yes, but tidy, orderly, direct. Note only a few wires are long and flying. Most of the rest lie down on the chassis, key crossings are made at right angles, etc.

    Some careful chopstick wire-wiggling and repositioning seems like a good idea.
     
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  20. tubelectron

    tubelectron Tele-Holic

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    Flying wires, right angles : well yes, that's very decorative, but IMHO not always so efficient o_O as the Ham Radio wiring technique of the tube era, despite it's AF signal here.

    In a vintage tube RF receiver or transceiver, you never see that kind of decorative wiring : it would never work ! All signal wires are minimalist, straight or curved, almost never at right angles nor flying... :eek:

    As far as possible, I try to stick to that "vintage" Ham-Radio wiring technique, that I find much better in AF circuits with high gain and hum sensivity - OK : too bad for the decorative look ;) :

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    But it's me, OK ? :D

    -tbln
     
    Last edited: Oct 17, 2019
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