magnatone reverb feedback issues

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by jeancoltrane, Feb 3, 2015.

  1. jeancoltrane

    jeancoltrane Member

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    May 1, 2011
    hey all, i'm lucky to have a glorious sounding magnatone that has some strange issues that cant be tracked down. after several techs and over $1000 spent there is still an issue that cant be solved; the last guy i brought it to refused to even work on the amp when he saw it :hide2 its a pretty complex beast http://www.magnatoneamps.com/M20.html

    the power section has been recapped and all tubes replaced several times. my biggest concern is that the reverb will intermittently slowly self oscillate/feedback into a low tone, i think it was around a B note. It gets quite loud, about as loud as the guitar signal itself. It stops if i turn off the reverb, but will usually quickly return once its started. the thing is i cant reproduce it when i need to, like showing a tech - it wont happen for a week and all the sudden it will start every time i use reverb. It happens with no input signal, sometimes on a cold amp but more often than not after the amp has been on a while. also, i dont know if its related but turning the reverb on sucks a fair amount of treble and adds bass to my tone.

    i also seem to have a grounding issue on the bass input jack, as that channel has fairly loud hum unless a jack is inserted halfway - its the last channel in the circuit right before the reverb i believe. i feel like it could be related somehow but none of the techs ever fixed it.

    if anyone has had a similar experience or any advice on where to look it would be much appreciated.





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  2. donnyjaguar

    donnyjaguar Member

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    Canada
    The circuit complexity doesn't scare me - and it shouldn't scare any tech worth his salt, IMO. From my experience a reverb pan feeding back can be resolved with additional mechanical de-coupling. ie: add some more padding to de-Q the pan from the vibrations of the speaker.

    I couldn't find a schematic on-line. If you know of one can you post the link?
     
  3. jeancoltrane

    jeancoltrane Member

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  4. jeancoltrane

    jeancoltrane Member

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    will also mention ive taken the reverb tank out of the amp and tried to physically induce the problem with no luck, so i dont think its related to a mechanical/tank issue.
     
  5. Diablo1

    Diablo1 Member

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    If you can find another reverb tank, you might try swapping it out. Also you might try swapping out all the caps in the reverb circuit. Perhaps one of them is bad and is radiating noise. The first one I'd try is the electrolytic 1/450V, that couples the driver tube to the tank. Where are you located? I have an M13, that I fixed up myself. The M20 looks similar to a twin M13.
     
  6. jeancoltrane

    jeancoltrane Member

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    thank you for tip. i'm saving up for a tech visit and will be writing down my trouble shooting steps and the suggestions presented. unfortunately im up north near toronto, but have found a guy a few hours away that has fixed an m13 before. ive tried most of the techs in a 200km radius and now its time to move on haha.
     

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