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1976 Fender Twin Reverb Overhaul

Discussion in 'Amp Tech Center' started by keithb7, Feb 13, 2017.

  1. keithb7

    keithb7 Friend of Leo's

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    1976 Twin Reverb customer's amp specs:

    - Non-pull boost
    - Has Master Volume
    - Not Ultra Linear
    - Has Heater Hum Balance
    - Bias Balance circuit
    - No Power Tube Hum Balance
    - Chassis SN B60387

    Thought I would start a thread for those interested in following along. A local young musician bought this amp 5 or so years ago. It had been heavily modified by someone before him. Some type of pull boost fuzz. Vibrato circuit was all ripped out. An FX loop added. A line out added. I was asked to bring it back to stock.
    I started with the original layout and schematic and traced everything in the amp. I used 2 colour hi-lighters. Everything stock was coloured in yellow, everything that was missing or modded was coloured pink. I put together a parts list and ordered everything I think I need from Doug Hoffman. Then I got to solder sucking and yanking all the mods out. Everything on the bench here in front of the chassis was what I pulled out. I have no idea what the heck the mini added power transformer was for. Customer said
    amp seemed to play ok but was a little lifeless. I discovered original B+ caps, bias and cathode caps still in the amp.

    [​IMG]

    Today my order from Doug arrived.

    [​IMG]

    This may be a little out of my comfort zone. We'll see. I may need some help from some of you kind folks. Here we go, time to get to work. I'll start by measuring all resistor values and label them for quick access.
     
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  2. noah330

    noah330 Friend of Leo's

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    Seems like it would have been cheaper to leave this asis or sell it and buy a stock one.

    Cool project for the right person.
     
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  3. keithb7

    keithb7 Friend of Leo's

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    All parts were $150.88 shipped. I work for cheap!
     
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  4. Wally

    Wally Telefied Ad Free Member

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    It sure would have been of interest to have seen all of the stuff you pulled out while it was wired into the circuit. Got 'before' pics?? Is there a particular reason for replacing the power tube sockets?
     
  5. keithb7

    keithb7 Friend of Leo's

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    [​IMG]

    [​IMG]
     
  6. keithb7

    keithb7 Friend of Leo's

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    Far left side you can see transformer mounted. Also Canadian version thermal breaker.

    [​IMG]
     
  7. keithb7

    keithb7 Friend of Leo's

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    The main power sockets pins are very sloppy. Offered to re-tension them. New at $1.54 each owner said replace them. He had metal hose clamps found in cars, around the tube bases in an attempt to retain the power tubes.
     
  8. Sandhill69

    Sandhill69 Tele-Holic

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    LOL I guess some people are a bit less OCD than others...
     
  9. 24 track

    24 track Doctor of Teleocity Ad Free Member

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    keep us poosted I love this stuff
     
  10. Wally

    Wally Telefied Ad Free Member

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    Thanks, Keith....I figured you would have pictures. What a mess, right? To where did that white wire from the added transformer lead? Someone got creative with that one.
     
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  11. SacDAve

    SacDAve Poster Extraordinaire

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    Looks like a fun project to me.
     
  12. mgreene

    mgreene Tele-Afflicted

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    You'uns may not believe this but the best sounding Twin I ever had had all orange drops in it. It was a blackface - so imagine my surprise when I opened it up to make that discovery. When I sold it, the dealer agreed that it was one of the best sounding twins he had ever heard.
     
  13. BobbyZ

    BobbyZ Doctor of Teleocity

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    Extra tranny at first I was thinking filiment tranny for an EL34 conversation. But is that some kind switching of relay?
     
  14. keithb7

    keithb7 Friend of Leo's

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    It appears a relay was installed in vibro roach and cap area. Push/pull pot installed in vibrato channel guitar input. It activated some type of anemic fuzz I was told. I saw the amp, pulled chassis and got to work. I never even attempted to turn it on. I was disgusted that someone defaced this Twin in such a manner. Lol.
     
    Last edited: Feb 14, 2017
  15. CoyotesGator

    CoyotesGator Tele-Holic

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    Control voltage for the relay.
     
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  16. keithb7

    keithb7 Friend of Leo's

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    I'm tempted to just rewire the whole thing with cloth push back, solid core wire. It holds its place so well! I'm sure owner won't object. I'm also pretty sure though it'd turn into "Pro Bono" work. I do surely enjoy it though. We'll see.
     
    Last edited: Feb 14, 2017
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  17. keithb7

    keithb7 Friend of Leo's

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    V2 was a 6EU7 tube. Thats got to come out...

    Seems to me this next problem problem is very common with 6L6 power tube Fenders. Heat and old... I have fixed several like this. Guessing metal film was not invented yet by 1976? Putting 3W MF in all 4 positions.

    [​IMG]
     
    Last edited: Feb 14, 2017
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  18. BobbyZ

    BobbyZ Doctor of Teleocity

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    Be alot of work rewiring with new wire. But not having to work with that easy melt 70s crap, might make it worth while.
     
  19. keithb7

    keithb7 Friend of Leo's

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    4 new power tube sockets with new resistors, and new bias caps.
    Does anyone have a suggestion for removing old dry crusty solder flux? It looks like crap. I can scrape it off with a fine tool, but it still leaves some dust. Compressed air? Q-Tip and?
    These SF plastic covered wires are sure fun to work with 40 years later.

    [​IMG]
     
    Last edited: Feb 16, 2017
    24 track likes this.
  20. Wally

    Wally Telefied Ad Free Member

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    A stiff toothbrush with a vacuum sucking up the crud as you work might get that crusty stuff off??
     
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