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Thread: Tremolo comes on for a second and then fades out...

  1. #1
    Member kevtronic's Avatar
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    Tremolo comes on for a second and then fades out...

    I've got an old Guyatone GA-440s push/pull tube amp that uses the following tubes: (2) 6BM8, (1) 6VA6, and (1)12ax7. The reverb uses the 6AV6 triode for the driver tube and 1/2 12ax7 for recovery.

    The reverb tank is bad and needs to be replaced (working on finding a replacement at the moment). The tank is a small single spring reverb (kind of cool in its own right).

    The tremolo uses the triode of one 6BM8 for oscillation (the triode of the other 6BM6 is used for phase inverter). The tremolo comes on for a quick second and then fades out. If I turn the tremolo switch off, wait at bit, and turn the tremolo back on it comes again for a quick second and fades out. I tested all the caps, found a couple bad ones and replaced them. I tried swapping the two 6MB8 tubes. I've cleaned/tested all the pots, on/off switches, etc. I do have some replacement tubes on the way.

    Anyone have any idea's of what to test/double check at this point?
    I know I'm missing something here, but I just can't find it.
    I have attached a schematic for a GA-430 which is similar to the GA-440: GA-430.pdf

    I could be wrong, but I'm not thinking the reverb issue should be causing the tremolo issue?

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    The tremolo oscillator is pulsing and not starting up. Which caps did you replace? You could just shotgun the circuit and replace all of the feedback caps as well as the cathode bypass cap. Check the resistors as well.

    The reverb tank in these amps are ceramic transducers, just like the old Danelectro/Silvertone tanks. They don't sound very good, but the few that I've worked on all were at least still working, unlike most of the Danelectro ones.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by kevtronic View Post
    I could be wrong, but I'm not thinking the reverb issue should be causing the tremolo issue?
    No, it's old capacitors that are likely causing your tremolo to fail, regardless whether they test good or bad or what. First I'd suggest replacing the trio of caps in the lo frequency oscillator: C17, 18, 19. Additionally a fresh bypass cap on that triode's cathode will help: C20. While you're at it C22 which strips DC from the LFO wave is another candidate for replacement.

    It doesn't hurt to check all the resistors in this circuit, replace any that are far off spec, and clean the vibrato speed & depth pots.

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    Member kevtronic's Avatar
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    Quote Originally Posted by Leo_Gnardo View Post
    No, it's old capacitors that are likely causing your tremolo to fail, regardless whether they test good or bad or what. First I'd suggest replacing the trio of caps in the lo frequency oscillator: C17, 18, 19. Additionally a fresh bypass cap on that triode's cathode will help: C20. While you're at it C22 which strips DC from the LFO wave is another candidate for replacement.

    It doesn't hurt to check all the resistors in this circuit, replace any that are far off spec, and clean the vibrato speed & depth pots.
    Good call. I had the resistors for the oscillator circuit, so I replaced them one at a time - no difference. I had two .022 400v orange drops on hand (which measure at .019 and .020) so I replace two of the three caps in the oscillator circuit and it's working. I'll have to dig up a third cap to finish it out. The only thing a little weird is when I turn the speed up to its fastest setting the tremolo effect stops until its turned back down a bit and then after a second or two it starts back up. Thanks for the info as always...

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by kevtronic View Post
    The only thing a little weird is when I turn the speed up to its fastest setting the tremolo effect stops until its turned back down a bit and then after a second or two it starts back up.
    That's a standard feature in Fender amps, likely in many others besides, nothing to worry about.

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    Member kevtronic's Avatar
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    With the tremolo up and running it's on to the reverb tank, which I am going to start a new thread for. Again, thanks for all the help and input...

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    Member kevtronic's Avatar
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    Here is the link to the Reverb thread I started for the GuyaTone (tokyo sound) reverb: http://music-electronics-forum.com/t42057/

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