Restoring 1972 Twin Reverb

Discussion in 'Amp Tech Center' started by Keyrose, Sep 15, 2020.

  1. Keyrose

    Keyrose TDPRI Member

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    I got a 1972 Twin Reverb (actually a Quad, so same amp chassis) a while back with the goal of fixing it up. Its functional but it has a few anomalies:

    • The normal and vibrato channel are drastically different tones. The normal channel is overly bright and no matter the settings, while the vibrato channel is dark. I suspect that this is some sort of phasing issue. Am I correct?

    • The reverb and tremolo are functional on both channels, which isn't a problem in itself, but I wonder if that is what is causing the first issue.

    •V1 producing a lot of rattle no matter the tube. I have swapped outmany tubes in that position and is still rattles at anything over very low volume. It will also feedback at times at high volume. I can even hear the rattle through the speakers when I tap on the cab.

    •There is a wire connected to a resistor that goes back into the board (see pictures). This seems odd to me as I couldn't find any pictures of a twin chassis that had the same thing.

    The filter caps look like the originals so I'm assuming that they should be swapped out. For everything else, would I just go though the amp checking every cap and resistor with a multimeter to make sure it's in spec? I do know how to solder and have done mods on my guitars and pedals before but never on an amplifier. Would this project be too ambitious for me or is it something that I can do with proper guidance. Please look though the pictures and let me know if you see any red flags, or anything the looks unusual. Any comments, help, or suggestions are appreciated!

    http://imgur.com/a/LbV3kJ7
     
  2. pbenn

    pbenn Tele-Afflicted

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  3. hepular

    hepular Tele-Meister

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    electro caps def need replacing.

    i think i saw 2 unoriginal resistors--1 by the bias adjust pot & that weirdo attached to the wire. (where does the wire that resistor's hooked up to come from? @pbenn mighta figured that out)

    re: v1: deoxit the socket & look up tightening the holders in the socket: could be a loose connector: if that doesn't solve problem, start working backwards.

    then check voltages
     
  4. MrCoolGuy

    MrCoolGuy Tele-Meister

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    I don't think it's too ambitious of a project. Gotta learn somehow.
    But it's not as simple as just checking values on every cap and resistor in the amp. It would be hard to do that because they measure differently in circuit than alone...
    But you could start by returning it to stock condition with new electrolytic caps then checking voltages...
     
  5. Keyrose

    Keyrose TDPRI Member

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    The wire is the green one the is wrapped around the yellow wire. On the other end, it is connected to the blue cap that is third from the right. You can see it more clearly on the picture above the one where I am holding the wire
     
  6. Keyrose

    Keyrose TDPRI Member

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  7. schmee

    schmee Doctor of Teleocity Silver Supporter

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    Sounds like a plan. The Normal on most Fenders is thicker sounding and less bright. So maybe someone modded it?
    You can check resistors but will usually find that the majority are out of spec. I wouldn't change anything that is within 20% really.... unless you are just into completely rebuilding it for he heck of it.
    Better to spend the time checking, resoldering etc wire ends, especially on the tube sockets themselves.

    Just follow the Fender board layout to reverse the mod and clean things up. Maybe even the Twin layout. http://www.thevintagesound.com/ffg/
     
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  8. Keyrose

    Keyrose TDPRI Member

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    How would I go about checking voltages? Could I do it with a multimeter? And where would I check it from?
     
  9. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    That resistor with wire you mentioned is a mod. It could make for a phase issue. I would have to see more of the circuit to know just what has been modded. To reverse that mod may take more than just removing that wire/resistor.

    If it were my amp I would do a cap job. I would most likely get rid of the tone mods.
    Safety first. If you do not know how to be safe with high voltages, either learn or take it to a tech.
    Once you know how to be safe, do the cap job. Any questions? Just bring it to the TDPRI shock brothers. It may be a bit head spinning to have several shock bros giving advice about different parts of the circuit but you can weed through what needs to be done.

    Basically it seems you have a functioning amp. When the cap job is finished, it should still be a functioning amp. You will then have a base from where you can proceed.

    If you live in an area where parts are not readily available, you may want to buy some parts that may be needed to keep from having multiple shipping charges.
    Remove the added bits and put back the removed bits. The shock bros will help when you get to that point. Promise.
     
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  10. Tim G

    Tim G Tele-Meister

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    new filter caps are a no brainer, must be done. As far as the normal channel being much brighter, look for a treble bypass cap on the normal channel volume control, functions much like the bright switch. It was a popular mod 40 years ago with guys wanting to make their Fenders sound like a Marshall Super Lead bright channel.
     
  11. MrCoolGuy

    MrCoolGuy Tele-Meister

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    Yes, a multimeter, and you would look at a schematic or layout for your amp. Certain points will have voltages stated.
    Does yours have a master volume?
     
  12. Keyrose

    Keyrose TDPRI Member

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    Part if me would like to restore it completely to spec but, if within 20% spec won't make a much of a difference in tone, then I could live with that.
     
  13. Keyrose

    Keyrose TDPRI Member

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    Yes, it has a master volume but no push-pull pot like later ones.
     
  14. Ten Over

    Ten Over Tele-Holic

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    Looks like the Normal channel cascades into the Vibrato channel immediately before the Vibrato channel tone stack. Removing the added mixing resistor and reconnecting the green wire to the 220K mixing resistor over by the P.I. will return it to stock.

    Twin Reverb '72.png
     
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  15. schmee

    schmee Doctor of Teleocity Silver Supporter

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    I'm mostly just saying that most my old BF's are way out on a lot of resistors up to 20% and I dont chase it unless I have a problem with the amp.
     
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  16. Keyrose

    Keyrose TDPRI Member

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    That's good to know. I heard of treble bleed mods on guitar but was unaware that they were common mods on amps. Would there be any mods that make the tone darker? The vibrato channel seems to be darker than usual. It's to the point where there in no interlap in tone between the two channels.
     
  17. Keyrose

    Keyrose TDPRI Member

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    Thank you, this helps a lot. Where would the phase inverter be? Sorry if this is a repetitive question, I have a hard time understanding circuit schematics. And I would completely remove the resistor, right? Or would I use it to connect the wire to it's original place?
     
  18. Ten Over

    Ten Over Tele-Holic

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    Yes, you would remove the resistor altogether while leaving the other end of the green wire connected to the .047uF capacitor. The free end of the green wire is then connected to the 220K mixing resistor as shown. The green wire was originally twisted around the yellow wire on their way to the 220K mixing resistors.

    Quad 220K Mixers.JPG
     
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  19. Ten Over

    Ten Over Tele-Holic

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    Pictures of '72 Twin:

    72 Twin Gut Mid.JPG

    72 Twin Gut Input.JPG
     
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  20. Keyrose

    Keyrose TDPRI Member

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    Thanks! I'll do this first thing when I get home.
     
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