Volume Drop in Tube Amp?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by guitars4god86, Feb 6, 2012.

  1. guitars4god86

    guitars4god86 Member

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    I have a custom made tube amp (5E3/5F1 clone-mix) that is really acting up... I was playing it the other day and everything was great - volume, tone, etc... But after about 10 minutes of playing, the volume all of a sudden decreased by about 90%. Just one minute everything was full power, and the next not.

    All of the controls still worked - the tone pot, the volume pots too...just everything was really quiet. The sound coming out of the amp retained the same overall character too - I was playing with the volume dimed, so a good amount of gain, and the gain was still there after the drop.

    So, I shut the amp off, let it cool and tried it again an hour later. Same thing happened - played great for about 10 minutes, then massive volume drop. So the next step was to replace the power tube - so I let the amp cool off for another hour and dropped a new 6v6 in, came back...same exact results, good for 10 minutes, then a drop. So I left the amp for the day. I came back the next day to play and now it just started off quiet. It never played again at full volume/full power....still hasn't...

    All the pots still work. I can still control the tone. I can still control the volume - granted, it's not much anymore, but I can definitely hear changes in the volume and gain when I move the knob around. I looked at the components inside - all the capacitors and resistors look good (none are bloated or have black spots). I tapped on every piece inside the amp with a chopstick to see if anything would pop or crack - nothing. I tried moving every solder joint to see if something came loose, but I didn't find anything. I tried reseating tubes, putting all new tubes in every position (except the GZ34) and that didn't do anything either. I know that my guitar, cables, cab all work because I tested them with another amp.

    Basically, I'm at a loss for what could be causing the volume drop. The builder suggests that maybe it's the output transformer...could that be the case? I always thought if that goes then no sound would come out...

    Any suggestions or do I just have to literally check voltages across every single piece inside the amp?
     
  2. guitars4god86

    guitars4god86 Member

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    No one has any ideas, huh? :(
     
  3. schmidlin

    schmidlin Member

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    Document all the DC voltages on every pin of the tube sockets when healthy. Then document the same when ill, and post the findings. It should point the finger where to look pronto. Then go to step 2 (more to come).

    Like any amp diagnosis: pin the problem down until it has nowhere to hide. Voltages are a great start.
     
  4. Blue Strat

    Blue Strat Member

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    Does "percussive therapy" (banging on the amp, SERIOUSLY!) change anything? I suspect an intermittent connection. It can happen in ANYTHING.
     
  5. Peteyvee

    Peteyvee Premium Platinum Member

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    Does it still power up?
     
  6. '58Bassman

    '58Bassman Member

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    Get the chopsticks! Be safe about it, though.
     
  7. guitars4god86

    guitars4god86 Member

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    @schmidlin - the amp doesn't get "healthy" anymore, so I'm not able to document anything compared to where it should be unfortunately...

    @BlueStrat - I'll try hitting it...that's what I'm feeling like right now :)

    @Peteyvee - yep, still powers up and still plays, just at a lower volume. All gain and tone characteristics are there (within reason, obviously not as much since it's quieter)

    @'58Bassman - I'll give the chopsticks another try, but I didn't find anything the first time...
     
  8. 900z1

    900z1 Member

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    a few thoughts -
    It could possibly be a uhf parasitic oscillation, but those are not usually intermittent.
    Check all your plate voltages against the schematic
    Check the negative feedback resistor to see if it is still in spec.
    check all the grounds on the board with an ohmmeter to the chassis including the speaker jack and output transformer.
    If you are comfortable disconnecting a transformer, disconnect the output transformer and check resistance between the primary and secondary - should be 100% resistance or overload - anything less is a short and it needs to be replaced. check center tap to each side to see if they are close to the same - it won't be exactly the same but close on a good transformer. if they are off by a lot, it may be a bad transformer.
     

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