Restoring 1972 Twin Reverb

Discussion in 'Amp Tech Center' started by Keyrose, Sep 15, 2020 at 12:23 PM.

  1. Keyrose

    Keyrose TDPRI Member

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    Thanks for all your help guys! I'm feeling confident that I can do this as a project. Would anyone know a good electrolytic capacitor kit?
     
  2. Ten Over

    Ten Over Tele-Holic

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    Here are a couple of options. The kits don't seem to notice that the vibrato uses a 5uF bypass capacitor.

    Capacitors for Twin Reverb.png
     
  3. Jake Nelson

    Jake Nelson Tele-Meister

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    Built a 6L6 Blackvibe after playing a Twin Reverb and everything rattles at anything over low volume... this thing is LOUD!!
    [​IMG]

    Anyway carry on. Twins are my favorite.
     
  4. BobbyZ

    BobbyZ Doctor of Teleocity

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    Can't remember how many times I've forgotten about that darn 5uf cap and just put in a 25uf-25volt.
    Other than old Ampegs where I might order a kit from Fliptops I just order caps from someone like ACS. F&Ts for the filters and 100uf-100 volt bias and Sprague for the 25uf-25volt caps, about 50 bucks to do a Twin Reverb. Add a buck or two for the trim pot to add to the balance pot, and you'll need a resister for that too.
    Personally I'd probably go with a service first and try it before taking out the mod. I'd probably go ahead and yank the mods anyway but it's fun to try stuff.

    First amp I soldered on was a silverface TR, honestly wouldn't recommend one of those for a beginner. First thing you'll do is burn a bunch of spots in that Easy-Melt insulation on the wires and it'll look like hell. Don't ask me how I know. . . .
    But if you're going to do people here will help.

    Two ways to change a cathode bypass cap.
    Right way. Desolder the eyelets and solder in new caps.
    Easy way. Clip leads right next to old POS white Mallory, bend eyelet on old lead, solder new caps there. That "hack" method cuts your time in there with hot things way down. So you melt little or none of the wax on the board and less chance of burning spots all over the wires. The electrons won't care, but other techs will call you a hack. I already use a soldering gun and really don't care. :)
    Yeah I do it the easy way on silverfaces. Blackfaces are just so much nicer to work on so I do those the right way.
    I do pull the filter cap board out for a complete desolder and cleaning on either though. That's simple enough.

    Oh in case no one told you.
    Twins are loud and heavy.
    (I've got two)
     
  5. Keyrose

    Keyrose TDPRI Member

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    So the value for the tremolo bypass doesn't make a difference? Which one is the balance pot and why should I add a trim pot? Thank you for the advice on the caps. Some areas do look rather hard to get to without burning some wire. And yes, I am aware the twins are very loud, that's why I got a quad ;):D
     
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  6. BobbyZ

    BobbyZ Doctor of Teleocity

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    The blackfaces used a 25uf caps across the whole board, like I said i just forget to order the 5ufs and haven't had any trouble.
    Balance pot is the one buy the power transformer you use a screwdriver to adjust. As originally wired they raise the bias on two tubes and lower it on the other two. You had to swap a resister to change the overall bias, kind of a pain.
    At some point they put a hum balance pot on the back of the chassis, in the heater filament string. Seems to be late 100 watters that got that, right before they went to the 135 watt ULs.
     
  7. Keyrose

    Keyrose TDPRI Member

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    Oh ok, I was under the impression that the pot was a bias pot. There is a small blue resistor that is soldered from the back of the pot to the middle lug, would that be the one that I change to a trim pot?
     
  8. BobbyZ

    BobbyZ Doctor of Teleocity

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  9. BobbyZ

    BobbyZ Doctor of Teleocity

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    Adjustable_and_Balanced_Bias.png
     
  10. BobbyZ

    BobbyZ Doctor of Teleocity

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    What I do is measure the bias, voltage with no tubes, say it's -40 and tubes aren't redplating or whatever. When you get done and turn the pot you should get the same voltage at some point, maybe now it goes from -35 to -45. Then you're in the ballpark.
    For matched tubes take the voltmeter and set the balance pot so you've got about the same voltage coming out both sides. Hard to hit the exact center point but something within reason.
    If you're actually trying to set bias for mismatched tubes it's more fiddly. It doesn't have to be exact anyway.
     
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  11. Keyrose

    Keyrose TDPRI Member

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    Thanks a bunch! This is all very helpful. I'll be sure to add this. Would I need any special instruments to check the bias or can I do it with a multimeter?

    Edit: Just read your last post
     
  12. Ten Over

    Ten Over Tele-Holic

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    The 5uF bypass cap on the tremolo evens out the intensity across the speed range. A 25uF cap causes the intensity to increase as the speed is decreased.

    If you are ever inclined to delete the bias balance in favor of a bias level, here are a couple of ideas:

    Twin Bias Mods 3 .JPG Twin Bias Mods 4.JPG
     
  13. avspecialist

    avspecialist Tele-Meister

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    To answer a question “ does anybody know of a good capacitor kit”. I like F&T for the electrolytics. Mojotone has small parts kits they sell also.
     
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  14. Keyrose

    Keyrose TDPRI Member

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    I ordered a kit with F&T and Sprague caps
     
  15. avspecialist

    avspecialist Tele-Meister

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    Good to hear. I hope all goes well reinstalling. I would do 1 part at a time noting the orientation of especially the electrolytics. How are your soldering skills?
     
  16. Keyrose

    Keyrose TDPRI Member

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    Yes, I'll definitely take precaution with the orientation. The amp hasn't been turned on in a while but I'll go through the process of draining the filter caps in case there is some juice left in them. My soldering skills are pretty good. I have done mods on my guitar and pedals before.
     
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