Static, bacon sizzle culprits?

Discussion in 'Amp Tech Center' started by Whatizitman, Jun 8, 2019.

  1. Whatizitman

    Whatizitman Tele-Afflicted

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    The amp in my avatar is making a somewhat intermittent frying bacon or static noise. Google suggests sizzle is typically pre tube related. I’ve swapped tubes, but the noise still shows up.

    It’s a simple grid-leak bias input design, if that matters in this situation. The sizzle first occurred when I tried a wah with it. It was very loud and sudden. Scary enough that I just turned it off and put away the wah. It did it again with a drive pedal, even at low to mid volume.

    I swapped the original tube for an original RCA from my SF Champ. Since then I’ve noticed it only occurs at higher volumes without any pedal. And it’s not loud, at first. I purposely have not used any pedals since then, though. The original tube from this amp is in my Champ, and appears to be ok.

    With my MIM tele with stock pups, and no pedals, the noise typically begins to show when amp volume is loud enough for output tube crunch, and only when playing. The amp has otherwise no excessive hum, with or without guitar and/or cable plugged in. Hum is imperceptible at low to mid volume (kinda cool that way, actually). The sizzle/static noise is unrelated. Once the sizzle starts it gets louder, worse and worse, until there is only loud sizzle. By that point it is consistent and occurring without me playing. I have to shut it off then.

    I’m wondering if it’s heat related? If I wait a while to turn the amp back on it seems back to normal. I can play it low volume. But if I crank it up, and play it, it’ll eventually start to sizzle. Just a tiny bit at first, and only in reaction to input from me. Like I’ll play a note, and there will be a hint of static with the note. If I keep playing, it just gets worse from then on.

    So, sizzle or static sound that appears to be related to input, affected by volume, but maybe not tube per se(?). What all should I be checking for?
     
  2. schmee

    schmee Poster Extraordinaire

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    Those are tough to find, but plate resistors for the preamp area (often 100k) can a problem. Cold solder joints in the preamp area (reflow the solder everywhere), cold solder joints on the heater wires on the tube cockets, or loose/dirty tube sockets MAY be the culprit.
    I have an amp right now that has a crackle or pop when it warms up. It's intermittent and not continuous. Haven't found it in 3 attempts over the last year.
    Some notes:
    "I have a PR build that did something similar. After 2-20 minutes of being on it would pop and crackle and make a constant "rustling paper" noise. It turned out that a cap in the tone stack leaked DC when it got hot. It took a while to track that down especially since every fix attempt that took long enough to let the amp cool down would initially appear to have solved the problem. If you haven't already checked for DC on the volume pot while the problem is happening that would be an easy next step."

    "Hardest to find issue I've had firsthand was the Princeton with a bad solder joint on the pilot light. Gave a crackle under thumping or vibration, but poking and prodding every last bit of the signal path and swapping tubes had no effect whatsoever. Found it purely by accident when snooping for stray voltages and I bumped the heater wires. "What the...."
     
  3. Whatizitman

    Whatizitman Tele-Afflicted

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    Hmmm. New symptom. Cutting in and out. Oh well. I’ll figure it out eventually. The output tube is new, and I biased it within reason. I recapped it with discrete filter caps, as I couldn’t find a match for the old cap can. My wiring and soldering leaves something to be desired. But it has been mainly for learning sake. It was not cutting out before, so I’m uncertain if the issues are related or not. Where should I start diagnosing? Tubes?
     
  4. peteb

    peteb Friend of Leo's

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    Old carbon comp plate resistors are known for making the sizzling bacon frying sound.


    If you want to iliminate the obvious, put in some new plate resistors of a different type.


    Metal film?
     
    Last edited: Jun 8, 2019
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  5. Whatizitman

    Whatizitman Tele-Afflicted

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    The load resistors are the originals from the 60s, and I assume are carbon film. They have some drift, but I haven’t changed them. That’s an easy and cheap fix, so I will go there next. Thanks!
     
  6. Whatizitman

    Whatizitman Tele-Afflicted

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    I also seem to recall a tab being a little loose on the pre amp tube socket. Maybe it was the plate? Could that also be a culprit for either sizzle or cutting out? Can that be secured without changing out the socket?
     
  7. flyswatter

    flyswatter Friend of Leo's

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    If by loose tab you mean that the lug jiggles around a bit within the tube base, that is not necessarily a problem provided the lug is solidly connected and the tube pin is snug. However, lugs on old tube sockets that shift laterally can snap off -- and it isn't always obvious until you tug on the leads. I assume you know the tube type and have a pinout diagram... that will help you figure out which connections may be involved. If a lug does snap off the socket, it is a fool's game trying to reconnect it... and the only method is tack soldering which isn't very solid. Better to just replace the socket in that case. You can also graft in new sleeves from an identical spare socket, but that's a bit trickier.

    You should also ensure the tube pins are clear of oxidization... which can also call noise and intermittent connection. Gentle rubbing with fine grit emory paper on each pin can remove it.

    You mention "carbon film" plate load resistors, but if they are the dark brown resistors typical of 60s amps, they are carbon comp -- which are the ones notorious for noise mentioned above. Carbon film will normally be tan in colour. For any amp of that vintage, it is worth replacing the plate load resistors anyway -- as they carry the high B+ voltage and so tend to suffer from heat over time.

    I don't know your comfort level working with open circuits, but if an amp on my bench had this problem, I would do a live circuit test, chopsticking all the solder joints and wires (usually noise will suddenly increase or disappear when you hit the sore spot), checking voltages throughout the circuit with a DMM, beginning with the output stage and working backward. Tube swaps but you've already done that. 9/10 these tracing methods will locate the problem if it's a cold or broken connection or an open or shorted component.
     
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  8. Wally

    Wally Telefied Ad Free Member

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    First question...has this amp been recapped and had good general service? If not, that is the first step. +1 on the power supply resistors...throughout the supply chain.
     
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  9. Digital Larry

    Digital Larry Tele-Afflicted

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    First amp I ever redid (a 5W tabletop thing) had phenolic sockets that had become completely carbonized inside. Was not visible externally. Vast amounts of DeoxIt did not help. Replacing the socket helped. By the time I figured it out I was "done" with this activity as a form of recreation. Several years have passed, I'm ready for another.
     
  10. Whatizitman

    Whatizitman Tele-Afflicted

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    Lugs. That’s the word I was looking for.

    I feel comfortable enough working on the amp, and have done a bit already. But I’m still a noob. It may just be my poor lead dress that’s to blame. I just wanted to get some ideas on what to focus on before I take it apart again. Y’all have been very helpful! Thanks!!

    I love TDPRI!
     
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  11. Wally

    Wally Telefied Ad Free Member

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  12. slider313

    slider313 Tele-Afflicted Silver Supporter

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    Did you follow the grounding scheme from the power supply cap can?
     
  13. Wally

    Wally Telefied Ad Free Member

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    What amp is this?
     
  14. Paul in Colorado

    Paul in Colorado Telefied Ad Free Member

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    My '1990 Mesa Boogie MK I RI is doing this (bacon frying background noise). Would replacing the input resistors with trim pots be a cause of that (a mod I did to dial in the gain), or is it serious tech time?
     
  15. Whatizitman

    Whatizitman Tele-Afflicted

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    Not sure. When I recapped i initially tried just tried to follow how the original scheme was with the old filter cap can, but bypassing the can with discrete caps. There is very little documentation on this amp, so I had to make some guesses. There was a ground loop, though. I changed it, and it really has very little hum now, so I think that worked. I can’t recall the exact scheme I went with, sorry.

    The static/sizzle sound occurs when playing. There’s otherwise no background noise or hum at idle.
     
  16. Whatizitman

    Whatizitman Tele-Afflicted

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    60s Multivox “combo bass amp”. It’s similar to the Premier 50 with SS rectifier. That’s about as much info as I have been able to dig up on it.
     
  17. slider313

    slider313 Tele-Afflicted Silver Supporter

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    The static/sizzle while playing sounds like a bad connection or a coupling cap leaking voltage.
     
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  18. Whatizitman

    Whatizitman Tele-Afflicted

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    All of the coupling caps are original.

    I think I will start with (1) rechecking my wiring/soldering and grounding scheme, (2) change out old plate resistors, then (3) double check and/or change out coupling caps. This is an ongoing learning project, so no hurry. I've already changed to 3 prong, new speaker and output tube, changed filter caps. The amp was a fairly a cheap pawn find, so I'm trying to keep it as cheap as possible, and learn as I go. It'll never be my main amp. It's just for learning about tube amp building and repair. Being able to play it every once in a while is just extra reward.
     
  19. slider313

    slider313 Tele-Afflicted Silver Supporter

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    A friend found one of these at the electronics disposal unit two weeks ago.

    His has a single 7591, a 12ax7 and a 12" Jensen C12R.
     
  20. Whatizitman

    Whatizitman Tele-Afflicted

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    Yup. That's it. I think everything was original in mine. The jensen was in bad shape, so I replaced it with a MOD cuz they're cheap. I got a new EHX 7591 and rebiased it - the cathode resister was way out of spec, and the old tube was redplating sumpin' awful. They were clearly cheaply made amps to begin with. The chassis has a tremolo tube cutout, and the "Combo Bass Amp" lettering on the panel is just a thick sticker laid over the cutouts for tone knobs for the tremolo version guitar amp. It's vintage, but clearly not worth sinking too much into to get working. It's perfect for a cheap-@$$ noob like me to learn how to work on amps.

    EDIT: the amp breaks up at 1:00. I can't imagine someone ever thinking this was even close to adequate for a bass practice amp LOL. The old C12R buzzed so bad on low notes at moderate volume. I never tried a bass through it. Too scared to.
     
    Last edited: Jun 11, 2019
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