Tweed Twin tube issues

Discussion in 'Amp Input - Normal or Bright' started by achar073, Aug 6, 2020.

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  1. achar073

    achar073 Strat-Talk Member

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    Just bought a Fender 57 custom Twin off reverb. Previous owner shipped the amp with tubes still in their sockets. Not great but it worked fine when it arrived so I kind of overlooked it.

    Now about a month later I’m noticing the amp kind “glitches” out when I play harder. Like when I strike a chord at first it sounds like TV fuzz for half a second and then cleans up again and sounds normal. Happens especially when I’m running a gain pedal.

    I’m thinking some of the tubes need replacing. Any idea which ones? Could it be power tubes? Rectifiers?

    Running a little low on funds since buying the amp and would prefer not to replace all the tubes right now if I can avoid it.
     
  2. Triple Jim

    Triple Jim Guy Who Likes to Play Guitar Silver Member

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    If that amp is based on one of the versions with a cathodyne phase inverter like the 5E8-A, you may be hearing the unpleasant frequency-doubling distortion that happens when you severely overdrive a cathodyne. It can be largely avoided by adding a 500 kohm to 1 megohm grid stopper to the cathodyne.

    I had that problem with the '65 Princeton Reverb I built, and fixed it with a 1M grid stopper. Because of the way a cathodyne works, that much resistance doesn't reduce the high frequency response, so there is really no negative effect of adding it.

    I don't know exactly what the circuit of that amp is though, so I'm sort of guessing here.

    Edit: I found the schematic: https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-57-Twin-Schematic.pdf

    Sure enough, it has a cathodyne inverter, V4B in this case, and they didn't include a grid stopper. I don't know if changing V4, like by swapping it with one of the others, has much chance of changing the distortion much, but I guess you could try it. Adding the resistor is the cure though. Or don't overdrive the amp so hard.

    You can read about the subject with an Internet search for "cathodyne grid stopper".
     
    Last edited: Aug 6, 2020
  3. achar073

    achar073 Strat-Talk Member

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  4. achar073

    achar073 Strat-Talk Member

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    Thanks, modding would be beyond my technical abilities at this point but it’s something to keep in mind.

    In terms of overdrive I’m playing a telecaster with vintage style pickups using an EHX Soul Food at about half gain. It’s a very low gain drive and the sound is still fairly clean.
     
  5. Triple Jim

    Triple Jim Guy Who Likes to Play Guitar Silver Member

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    I understand, but a decent amp technician would know exactly what I mean, and adding one 15 cent resistor to the tube socket pin shouldn't cost much, should you decide to try it someday.
     
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  6. achar073

    achar073 Strat-Talk Member

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    I might take to a tech. Thanks again.
     
  7. stratology

    stratology Strat-Talker

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    I have the early 'low powered' 5E8 version of the '57 Twin from the Fender Custom Shop. Never heard what you describe. The pre-amp section can take both 12AX7, or the original spec 12AY7, which don't overdrive as easily. I much prefer the latter.

    I would experiment with tubes, and have a tech look at the amp, but not start modding the circuitry right away. These are great amps out of the box.