68 drip edge twin reverb ac568 circuit-change to ab763?

Discussion in 'Amp Tech Center' started by muscmp, Oct 14, 2019.

  1. muscmp

    muscmp Tele-Afflicted

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    a friend bought a 72 tele and 68 twin reverb. i noticed that the amp had an ab763 label but upon opening i found the tantalum caps and resistor circuit indicating it was an ac568. i explained it to him and he thinks he wants me to take it to ab763 but i feel he is not going to get the amp loud enough to make the change worthwhile as he doesn't gig and the amp won't leave his house.

    it appears that i can just cut the caps and resistors and leave them in for possible redoing later if need be. i'd run the wiring from each pin8 of the 6L6 tubes to ground. the only other changes may or may not make a difference and seem to be resistor changes.

    i'm looking for recommendations as to whether it is worthwhile and does it make that much difference in sound. i don't know about any value change.

    enclosed schematics for each.

    thanks,
    mikeB



     

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    Last edited: Oct 14, 2019
  2. Bartholomew3

    Bartholomew3 Friend of Leo's

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    I have an original bought new 68 twin in my living room. I run it with two 6L6 power tubes instead of 4, usually have the amp volume on approx 3 with the guitar volume turned way down amp tone controls on approx 5 everywhere, reverb at 3.

    IMO a speaker change will do more good in a low volume situation than a change in wiring etc. My goal would be to eliminate as much idle noise/floor noise as possible and get it back to original specs.

    It probably won't sound the same after caps & tubes. Bottom Line is he hasn't heard it tuned-up yet but wants mods that may not be needed. I would play it first for a while after maintenance gets it running at full efficiency - can always make changes later.
     
    Last edited: Oct 14, 2019
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  3. avspecialist

    avspecialist TDPRI Member

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    I modded 2 silver faced Twins to AB763. a very nice tone improvement is to increase the negative feedback resistor from 820 to 4700. The harmonic structure improves a lot. It really helps the amp especially at low volume. I found this resistor by using a 1 meg pot in series with the 820, When I got the sound I like, I measured the pot and resistor and got 4700 ohms. Getting the bias correct for your playing helps a lot also. These are just some of the changes I make to amps I work on.
     
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  4. Ten Over

    Ten Over Tele-Meister

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    Probably could just tack each pin 8 to ground and leave everything else intact to see how he likes it.

    I sure would like to see some pictures of the guts before anybody mods it. I've never had my hands on one and it seems like all the pictures are post-mod.
     
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  5. Wally

    Wally Telefied Ad Free Member

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    Fender did not produce that circuit for very long and never revisited it. I pull those caps. The simplest thing to do then is to add a bias adjustment pot to the bias circuit while maintaining that balance pot.
     
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  6. muscmp

    muscmp Tele-Afflicted

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    wally: pull the caps and ground all pin 8s, and the resistors also, i assume.
    thanks,
     
  7. Wally

    Wally Telefied Ad Free Member

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    yes, pull the caps and resistors from the cathodes and either take the cathodes straight to ground or install 1 ohm resistors from the cathodes to ground for a bias measurement point.
     
  8. muscmp

    muscmp Tele-Afflicted

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    wally: perfect!
    i don't want to do any drilling into the chassis so if i install a bias pot, what size? and, where exactly to place it?
    thanks,
     
  9. Wally

    Wally Telefied Ad Free Member

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    Muscmp, I’ll refer you toRobRobinette’s site...he has examples, I feel sure. I treat each situation according to what is available. I am apt to install a terminal strip on which to situate the bias pot. Others mount it right on the existing bias balancing pot.
     
  10. muscmp

    muscmp Tele-Afflicted

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    dang! how could i forget about robrob's site? thanks again,
     
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  11. BobbyZ

    BobbyZ Doctor of Teleocity

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    If it was my amp, just to satisfy my own curiosity. I'd service it and install the bias voltage pot Wally mentioned. Nothing that changes the original circuit, just gets it working right. Then I'd play it like to see what that circuit really sounds like. If its as bed as we've been led to believe, changing at that point wouldn't be a big deal.
    Other than that my only TR tip is try to keep the speakers on the lower end of the efficiency scale. You really don't need ones that put out 100 decibels at one watt in a TR, especially for home use.
     
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