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Debugging Princeton Reverb - specific notes occasionally crackle

Discussion in 'Shock Brother's DIY Amps' started by itsGiusto, Aug 30, 2020.

  1. itsGiusto

    itsGiusto Tele-Holic

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    Got a strange problem on my 1972 Princeton Reverb. After I turn the amp on for the first time, if I play the 11th or 12th fret of the G string on my Les Paul, especially if I play it loudly, I get a weird loud crackle coinciding with the pick attack of the note. It really only seems to happen for those notes for some reason. If I keep playing that note again and again, it keeps crackling for a bit, but then the crackle seems to go away, until I turn the amp off, and let it sit for a bit. Then the next time I turn the amp on, the crackle will be back.

    I keep the amp volume at 3-4, treble at 3, bass at 3, reverb usually on at 2-3, but the problem seems to still happen even if the reverb is off.

    Anyone ever encountered anything like this?
     
  2. schmee

    schmee Doctor of Teleocity Silver Supporter

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    I've encountered similar. It's probably just the harmonic vibration of that particular note setting off a bad solder joint inside.

    BUT first:
    -have you tried a different speaker? A speaker voice coil rub will do that with a specific note or two, but it's almost always a much lower note.
    -You are sure it's not a cord, or a cheap jack in the guitar?
    -Try wiggling the tubes a bit with the amp on, does that set it off?

    -Inside the amp it can be a cold solder joint/ old solder joint. With the amp on, tap various spots on the circuit board, ESPECIALLY the green heater wires and other wires on the tube sockets. These can come loose in the solder glob and do exactly that.

    Read about precautions against getting shocked first, HIGH VOLTAGE!
     
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  3. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    @schmee pretty well has you covered.

    I would make sure the tube pins/sockets are clean. If you have some deoxit put it on the tube pins and work the tubes in and out of their sockets a few times.

    Tap on the tubes while the amp is on. Try to find a tube that is sensitive or microphonic.

    Hopefully it is a noisy tube or poor electrical connection rather than a solder joint problem.
     
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  4. E5RSY

    E5RSY Doctor of Teleocity

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    Had the same issue (twice) when I've had power tubes that were on the way out in a Pro Junior. Easy fix, if that turns out to be the problem.
     
  5. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    What @schmee said - just isolate. Different cable/speaker/tubes. That's the user-serviceable stuff that's relatively easy to knock out.
     
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  6. itsGiusto

    itsGiusto Tele-Holic

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    I was trying to see if I should do that earlier, and from googling, it sounds like this is kinda controversial. Some people seem to say that deoxit will corrode the sockets, causing failure soon afterwards, even if it fixes the problem short-term
     
  7. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Never had that problem. Metal will oxidize. It is even quicker when two dissimilar metals are in constant contact such as tube pins/sockets. General maintenance requires periodic cleaning. I assume Deoxit has data available for their product(s).

    Now, I have heard some people do not use a correct product for cleaning electrical contacts but that is a different subject. (WD-40)
     
  8. Wally

    Wally Telefied Ad Free Member

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    Massachusetts...humid environment? If you have been scared away from contact cleaners, which have never caused any problems as far as I know, the you could simply pull and reinstall the tubes a few times each. Sometimes that will clear of some oxidation and do away with odd problems.
    Now, I am going to put on that old broken record.... Has this amp ever been recapped and well-serviced? If not, that needs to be done. If it has fresh caps, then I am a big believer in checking all nuts and screws on jacks, switches, transformers, etc. I heard a smalL trailing noise yesterday when playing on a Super Champ. This noise has been com8ng and going the last two times I have played through the amp. I reached over and tried the nut on the input jack...sure enough it was loose. I finger-tightened it, and the noise went away.
     
  9. PolB-Qc.

    PolB-Qc. TDPRI Member

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    Shot in the dark, and my first login in a very long time....
    I had something like that with a new amp (for Me). Stock it had 12AX7's, I swapped Valve#1 for a 12AU7 and had 12AT7 V#3 in my reverb and Phase inverter V#4. I put back the 12AX7 in my preamp and cleared it up. Have you swapped tubes recently to lower the output?
     
  10. Wally

    Wally Telefied Ad Free Member

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    If I understand things correctly, you pulled a 12AX7 from V1 and then later reinstalled the same 12AX7, correct? If that cleared the problem, then there was some oxidation and/or dirt in that socket op and/or on the tube pins. The pulling and reinstalling cured the problem.
     
  11. PolB-Qc.

    PolB-Qc. TDPRI Member

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    Initially, I got it with the 12au7 in the preamp, the previous owner found it had too much gain. I was trying out new tubes to push the reverb more, it was very flat with the 12ax7 and is VERY SPLASHY with the 12AT7's. While trying it out, I had a very similar issue to the initial post. I replaced the 12au7 with a 12AX7 and have been playing surf tunes all afternoon without the crackle and fizz.
    You may be right, I did not Pro-Gold the contacts.
     
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  12. KATT

    KATT Tele-Meister Silver Supporter

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    I had a similar thing happen today with my Fender '57 Deluxe reissue. On certain notes it would crackle when I played certain strings.

    It was simply the volume knob was the issue (slightly dirty and or oxidised?). I turned it and the tone knob from 0 to full multiple times which cleared the issue and it was fine thereafter.

    It may not be the same cause as your amp's problem, but by far the easiest thing to try first.
     
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  13. TwoBear

    TwoBear Tele-Meister

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    Have you also completely disconnected the reverb tank. Do you live in a moist climate or by the beach, sometimes humidity can play a part. So as the amp heats up the problem subsides? Maybe something making a bad contact until it expands with heat... It seems I had that exact problem with a super reverb but I'm getting to where I can't remember that far back. The suggestions you gotten so far are top-notch... It probably wouldn't hurt to hear the sound, one man's crackle is another man's Buzz ha. Replacing each tube 1at a time, disconnecting or substituting the plugs on the reverb, even tightening the input Jacks and checking the second lower input to see it's making good contact, possibly plugging another cable all the way or partly in to the second Jack? Is that the one with the Mallory cap can? If so maybe give it a twist n tug while you're playing, as the Caps all ground there from the chassis to the can. You can also turn the amp on its side and upside down and see if that does anything. As was said earlier the speaker maybe sagging and rubbing inside the voice coil.
     
    Last edited: Aug 30, 2020
  14. schmee

    schmee Doctor of Teleocity Silver Supporter

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    Good point, that happened on my old 65 BFDR and I had to replace the volume pot eventually. Went completely dead at about 4.
     
  15. dan40

    dan40 Tele-Afflicted

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    Don't overlook the connections at the speaker also. I had some of those crimp on connectors become loose on an amp years ago and it would crackle when certain notes were played. A solder connection at the speaker is much more reliable but many amplifiers do use those crimp connectors on the speaker wires.
     
  16. Cruisin Home

    Cruisin Home Tele-Meister

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    itsGiusto

    I just finished refurbishing a 1973 Deluxe Reverb (silverface), pretty similar. To my horror i found 9 defects, 7 of which I know were factory defects! One of the most outstanding was no solder at all on one of the V2 filament connections. i also had to replace EVERY capacitor. It never operated correctly when it was brand new (family member bought it in 1973 when he was 15 years old). Just horrible workmanship during these dark years of Fender. He kept the amp for all these years despite it not working, although in dirty basement. The amp now sounds awesome! i am thinking of posting a thread on this refurb to show everybody what you can run into with these vintage amps of this period.

    I live in seacoast of NH. I see you are Massachusetts. If you are not far, you are welcome to come to my engineering company sometime on what was Pease Air force Base. I have a couple of test benches setup for amps. Be glad to give it a once over. I do believe in deoxit. dirty pots, jacks, sockets are a tone-kill. I also strongly recommend replacing all electrolytics.
     
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  17. itsGiusto

    itsGiusto Tele-Holic

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    Awesome, yeah, I'm about an hour away. I'd love to get together with a fellow amp-builder!
    Also, this amp is causing me significant troubles, if you have experience with Plexi amps: https://www.tdpri.com/threads/plexi-clone-build-cannot-figure-out-why-harsh-distortion.1044918/

    Hah, well, I did recently replace all electrolytics. I have a feeling that my work doing that might have caused this problem, unfortunately. In particular, my replacement of the can cap was probabaly not the best, in part because I don't have a strong enough soldering iron to do that sort of work.
     
  18. awasson

    awasson Poster Extraordinaire Gold Supporter

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    As @schmee suggested, don’t discount a voice coil that’s on its way out. I found that out recently.

    One of the 12” V30’s in my Mesa cab was on its way out and would crackle on some but not all notes or chords. I built the cables so I checked them first. I also built the amp and it has NOS tubes so that was my second area to explore. It wasn’t until I exhausted all of the DIY stuff a few hours into it that I checked the speakers and found the culprit. One of the V30’s had a voicecoil that was rubbing. I was relieved to know all of my work was solid more than anything else.
     
  19. Cruisin Home

    Cruisin Home Tele-Meister

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    good, i will PM you and we will try and make an amp repair date. You can bring the plexi-clone too. i looked at your thread and I'm sure others may have already told you that you have a lot going on in there. Please try and get me a marked up schematic of what you think the build should be at this moment.

    As for Marshall knowledge, I just brought a 2204 back to life and sounding great. You will see when you come to my place that my guys and i design medical brain imaging machines, with maybe 1 million components, not too many things have ever compare to that complexity. Not to say sometimes even these little amps can be hard to make behave. good to have spares of things. DIYs with lot of off the reservation mods/layout changes are certainly more problematic because we can be chasing a design/layout issue besides your typical component issues.
     
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