Troubleshooting a volume fluctuation issue--looking for some tips

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by bender, Mar 29, 2015.

  1. bender

    bender Member

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    Hi Folks

    I am looking for a bit of help in my attempt to narrow down an issue I am having.

    Equipment:

    40W custom made brownface circuit, but with the tube reverb sitting on a separate circuit (I bridge it with a high quality cable). Has tremolo as well (on main circuit). Running Tungsol KT66, and a pile of other tubes in the rectifier, pre-amp, and reverb.

    Connected to a Marshall 4x10 cab (this is my home office setup, will troubleshoot in the next few days with a different cab at the jam space).

    Symptoms:

    - Volume will vary wildly. This is a LOUD AMP (normally). Goes quiet. Goes loud, with some 'scratchy, staticy sound'.
    - this typically does not happen immediately when I turn it on


    Current troubleshooting:
    All troubleshooting has been done currently with the reverb connected (so in through the reverb section, and then that connected to the rest of the amp). Tremolo is off for all the troubleshooting.

    Obviously, one step is to exclude that section next.

    - took it to the guy that built the amp (great, local guitar & amp tech, been dealing with him for a while, and he builds and repairs a lot of amps to a very high standard). Could not replicate the issue there :(-- used one of the guitars in the shop and one of his cabinets.

    - have tried different instrument cables. No apparent effect. I do need to spend a bit more time doing this perhaps.

    - tried two different guitars yesterday. It seems I can replicate it with my Les Paul Traditional Pro (with the quite hot zebra humbuckers), certainly more consistently than with a low output single coil guitar.

    - have tried other heads through the cabinet, no issues.

    Next steps:

    Try it at the jam space, varying the following (while having a recording device ready):
    - try 2 different cables + 3 different guitars (high output Les Paul, semi-high output P90 guitar (my main axes), low output guitar)
    - try through reverb section and
    - try different speaker cables
    - if all else fails, I have some 6L6 tubes I can shove in, but I am not completely clear on how else to to troubleshoot tubes (especially pre-amp).


    Last night, I managed to capture the issue on a recording (which I have yet to edit and publish, will aim to do if it doesn't become clear what the issue is).


    At the moment, I suspect the guitar (Les Paul)--but is this possible? I am not a guitar wiring expert by any means.

    EDIT: I had also plugged in an active pickup (EMGs) guitar last week for what I think was the first time. Is it possible to screw up an amp that way? I didn't think so, given all the pedals people use (and I use) that screw around with the signal coming in.

    EDIT: Just tried it when I got home, bypassing the reverb, and not with my Les Paul. Volume fluctuating. Loud sounds OK maybe with a bit of static when hitting a low string (like open low E if forceful), and variously just quiet sounding--like it loses a significant amount of power.


    Am I missing something obvious?

    Any tips appreciated.

    Best regards,

    Bender
     
    Last edited: Mar 30, 2015
  2. Blue Strat

    Blue Strat Member

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    It's probably an intermittent connection ANYWHERE in the signal chain.

    If you suspect the guitar, try another one. Troubleshooting is about minimizing variables so that's what you have to do.

    If/when the amp acts up again, try banging on the top and see if that changes anything. I'm completely serious about this. Try wigging the tubes in their sockets to see if this brings on the problem. Check all internal solder connections especially ground connections.
     
  3. bender

    bender Member

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    Thanks. It's not the guitar and not the instrument cable. It's hand wired and I was thinking something may have come loose in transport.


    Sent from my iPad using Tapatalk
     
  4. donnyjaguar

    donnyjaguar Member

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    I would eye-ball the tubes when it goes quiet to see if anything is glowing cherry red. Or isn't glowing at all.
     
  5. bender

    bender Member

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    Haven't seen anything odd in that department. I figured I can spot an obvious blown tube but that's about it.

    Thinking loose connection at the moment. I drive on a few dirt roads to get to and from the jam space. Possible some of the vibration did something.
     
  6. Blue Strat

    Blue Strat Member

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    Sometimes pulling and reinserting the tubes can clear up odd intermittents. Usually the go-no go varieties, but it only takes a few seconds to try.
     
  7. bender

    bender Member

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    Thanks going to try that, and will bring another head with me (a Prosonic I have) to A/B, and also allow me to swap some tubes as part of troubleshooting.
     
  8. TimmyP

    TimmyP Member

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    The only visual clues I know of that a tube is bad: It's white inside (lost vacuum), arcing inside , or the heater doesn't glow.
     
  9. bender

    bender Member

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    Okay, had a nice loud jam for a few hours. NO PROBLEMS. The only different variable I can think of is the cabinet.

    I'll have to check the wiring of my cabinet at home and try some other heads with it. *sigh*
     

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