Sound fade out, noise fade in

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mbratch, Sep 25, 2006.

  1. mbratch

    mbratch Member

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    I had a problem this evening at rehearsal I haven't encountered before...

    While playing, the sound of my amp began to fade out while a nasty hum of equivalent volume faded in. Then the hum stayed there until I toggled the standby switch off then on again (leaving the on-switch on the whole time). Then it was clean again until later in the evening it did it once again.

    Is this a tube problem? Maybe the rectifier? :messedup
     
  2. jmadill

    jmadill Gold Supporting Member

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    Are you using a compressor? If so, as the note fades out, it is possible that it is boosting the gain so that when the note is dead, the hum is most prevalent. This is very true of the Tone Press and similar pedals, with the sustain set high.

    -jm
     
  3. Blue Strat

    Blue Strat Member

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    Damn! You again?!?!?! :)

    As always, this is impossible to do over the internet.

    If I had to guess, I'd say that it's one of the power tubes loosing bias. Could be one of the tubes, but more likely a bad contact with pin 5.
     
  4. mbratch

    mbratch Member

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    I'm not using a compressor.

    I'll check the integrity of the pins.
     
  5. Blue Strat

    Blue Strat Member

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    Pins and sockets, have you retensioned the power tube sockets? If not, do that. Also, if the amp does this again, try rocking the power tubes in their sockets to see if you can make the problem go away. This would prove the pin 5 contact theory.
     
  6. mbratch

    mbratch Member

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    I thought I had retensioned the sockets a month or so ago, but I can easily double check that (I know I did it on at least one of my amps, just don't recall which). I'll also do some contact cleaner, and I'll try the experiment you suggested if the problem happens again.
     
  7. Blue Strat

    Blue Strat Member

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    Sometimes sockets get so worn that retensioning only holds for a short while.
     
  8. John Phillips

    John Phillips Member

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    Not quite. A cracked grid stopper resistor (which I've come across several times on older Fenders) will do exactly this even if the pin contacts are fine, because moving the tube about disturbs the resistor too, as it's soldered across between two of them.

    The first time I came across this, I retensioned the pins about three or four times thinking I must have not done it well enough, because I was so sure it was a pin contact problem - but it wasn't.

    I would replace the grid stoppers even if they look OK, with an amp of this age - they could be cracked internally. Use carbon-film or metal-film, they're a lot more stable under heat stress than the original carbon-comp (this change doesn't affect tone).

    If you haven't already, replace the screen resistors with 3W wirewounds while you're at it - it's easier to get at the grid stoppers with the screen resistors out of the way anyhow.
     
  9. donnyjaguar

    donnyjaguar Member

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    I remember playing with another guitarist years ago and every time he hit a hard chord one of the power tubes would fall out of his derelict amplifier. This was the guy who always had the worst equipment. I remember this amplifier because you couldn't touch the chassis anchors when it was on otherwise you got a shock. Coincidentally he was the least skilled player but gave 110% all the time. It was hillarious. :) Ironically he introduced the guitar to another friend of mine who was out-playing him after about 3 months.
     
  10. mbratch

    mbratch Member

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    I replaced the 470ohm resistors a few months ago. I don't recall if I did the 1.5K's so I'll check those.

    It's a little mysterious to me why turning the standby off and back on again (without waiting for any cool down) would make the problem go away so easily. I wouldn't have expected it to be that consistent (well.... 2 out of 2 times anyway).
     
  11. Blue Strat

    Blue Strat Member

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    Yeah, that's why I figured it was a positional/contact type thing. Snapping the switch might be vibrate the chassis just enough to let whatever it is, settle back into position.
     

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