1970 Twin Reverb - gut shot - comments welcome

Discussion in 'Amp Central Station' started by 791979, May 22, 2019.

  1. 791979

    791979 Tele-Meister

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    I've owned this amp for a number of years, I replaced the filter caps in the dog house when I got it, but it's only recently that it's been put to use. Today I replaced the tremolo roach, and now have working "vibrato" for the 1st time.

    I was thinking there may be some caps that should be replaced, and maybe some other stuff could be improved, and rather hoping someone knowledgable will be able to comment.

    It has some kind of MV connected to a pot on the back of the chassis. It also has a transformer in the cab to take it from Euro voltage to 110V, and I plan on replacing that original PT for an export one soon, really just to reduce the weight a little.

    [​IMG]
     
  2. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Looks like all the necessary caps have been replaced.

    I always disable the ground switch when I rewire the power cord as well, and make sure the accessory outlet is the correct polarity, which it looks like it is from what we can see in the photo.

    Everything else seems good from here :D
     
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  3. 791979

    791979 Tele-Meister

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    Thanks
     
  4. Wally

    Wally Telefied Ad Free Member

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    Clean. The next thing to do imho would be to add a bias voltage adjustment pot. Our Brother Robrob has it drawn out.....
    Rob Robinette amps in a search will get you there.
     
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  5. brogh

    brogh Assistant Admin Staff Member

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    very nice and clean indeed :)
     
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  6. schmee

    schmee Poster Extraordinaire

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    Looks good. None of the 100k plate resistors have been changed out (apparently) If it's not hissing etc then no need to I suppose.
     
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  7. Faceman

    Faceman TDPRI Member

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    Bias circuit looks funky to me. If you have the correct negative bias, it is not a big deal but it looks like two extra resistors and an extra cap. Should just be a 50uf cap, 10KL bias pot, and 27K resistor to ground to create the -52v for bias. Do you see the correct readings?
     
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  8. Musekatcher

    Musekatcher Tele-Afflicted

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    I'm having a July 1970 Twin serviced, all original with D120s. It also has a mystery knob on the rear chassis, possibly a master volume. I should get a call in a week or so on the verdict. It probably needs caps and tubes, may need a transformer. Its a valuable time window to have a period correct amp in good working order and understand/learn how they were being used.
     
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  9. Andy B

    Andy B Tele-Afflicted Gold Supporter

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    The bias circuit is typical for that year. Usually (2) 50uf caps. There is no reason to change it to the older style. When I service BF circuit I replace the bias cap with a 100uf/100v F&T cap.
     
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