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1976 Fender Twin Reverb Overhaul

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

  1. BobbyZ

    BobbyZ Doctor of Teleocity

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    You can eliminate the heater hum pot with two 100 ohm resisters. Like the ones on old Fenders, with no center tap for the filiment wiring, there on the pilot light, but can go other places too.
    Only schmatic I've seen with that on it is for the 135 watt TR, might help.
     
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  2. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The two outside terminals of the hum pot are connected to the heater wires. The wiper is connected to ground so as you turn the pot you alter the balance of resistance and adjust it by ear for minimum hum.
     
  3. Round Bale

    Round Bale TDPRI Member

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    I have had nightmares that weren't this scary looking
     
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  4. cboutilier

    cboutilier Tele-Afflicted

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    I don't believe my 75 CSA SR has the circuit breaker either.
     
  5. oldfish

    oldfish Tele-Holic

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    fantastic work your doing there ill be following your progress closely good luck
     
  6. Andy B

    Andy B Friend of Leo's Silver Supporter

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    Looks like a fun project as long as you aren't rushed. I'm waiting parts to do a PA135. I haven't decided if it is staying stock or if the channels are getting revoiced to make things more interesting.
     
  7. keithb7

    keithb7 Friend of Leo's

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    Well, it works. However I have more work to go. I have not yet double checked all my wiring to match the layout and schematic. However I wanted to test the circuit, so I used a light bulb limiter to fire it up. No shorts it seems. The limiter bulb dims nicely. If you'll remember earlier in this thread I said that I didn't even plug the amp in when I got it. I just pulled the chassis and got to work. The owner brought the amp to me with only 2 power tubes. Claimed he just ran it this way because he heard it was ok, and was only about 40W instead of 80W. That is fine, from what I have read. However you need to pull one tube from each pair. My understanding is if you plan to do this long term, you should know that the OT impedance is off one step. The OT's used in Fender amps can take this mis-match I am told. However best practice would be to run an 8 ohm speaker load with 2 power tubes in, instead of 4 ohm load when all 4 tubes are in. Anyway, it turns out the 2 power tubes that came in the amp are stone dead. I tested them in my tube tester to confirm. No heater filaments working. Put in 4 known good tubes. We get power and sound, but not a lot of volume. So now I will double check all my wiring, test all the pre-amp tubes and then start taking voltage measurements. I am encouraged so far.



    [​IMG]
     
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  8. Blue Bill

    Blue Bill Poster Extraordinaire Ad Free Member

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    Wow, fantastic job, thanks for the fun thread. The owner is a lucky man, he's gonna love it.
     
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  9. BobbyZ

    BobbyZ Doctor of Teleocity

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    For low power a real simple check to make sure everything's ok in the power section. Just use one good 6L6 in each of the four spots for a quick check. That'll tell you if each socket is ok and both sides of the OT are ok. Also if the phase inverter is doing its thing feeding both pairs of power tubes.
    You'll be running it class A single ended while doing it won't hurt anything for the length of the testing. (probably wouldn't hurt an old TR running it like that all day, for all I know)
    Also possible to have a dead triode in the phase inverter tube.
    The way those are wired the pair of 6L6s on the left are on one side of the OT and of course the pair on the right the other.
    Anyway that tell you if that section of the amp is doing its thing pretty fast.
     
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  10. 2 Headed Goat

    2 Headed Goat Friend of Leo's

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    +1 re BobbyZ's suggestion. + taking voltage measurements. The coupling capacitors are a good place to start - the ones that link each gain stage. As you may already know their job is to pass the signal (AC) but block DC voltage… any leakage will attenuate the gain of the next stage. Really cool project and very impressive work you're doing!
     
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  11. FenderLover

    FenderLover Poster Extraordinaire

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    Also use a magnifying glass to look at the leads of all the blue coupling caps and look for cracks at the component body. You could have a walking wounded part in there that still sorta works.
     
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  12. keithb7

    keithb7 Friend of Leo's

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    Reviewing all my wiring here now. I see V4 pin 7...Ahem. Missing a wire to its connection at resistors. I am very surprised you guys never picked that up in my photo above. ;)
    I also see where I missed a 2000pF cap off the power tube pairs, pin 1 of each pair to ground. 1 off V7 and 1 off V10.
    6L6 pin 2 is connected to pin 5, control grid it appears. What purpose do these caps serve? Cancel potential oscillation? I don't see those caps in some other TR layouts.

    Volume is up especially when you remove the lightbulb limiter. :rolleyes: But still not quite where it should be.
    It also helps when you roll up all the eq knobs in an AA270 circuit. LOL. Turn them all down you have no volume.

    Voltages seem good. Something is up in the reverb channel. Turning reverb setting up or down controls volume of that channel. Right now chassis is just on the bench. Some crackling too in the reverb channel. Getting closer. Channel 1 is sounding pretty good but bright switch is having no effect at all. More to come.

    ***Update, reverb circuit plugged into tank. Oh yes. Volume is way up and working. Tons of reverb on tap. But Reverb pot still effects volume control. Hmm. More to check.

    I lifted the board and had a look underneath with a mirror. I don't see any hidden under-board connections.
     
    Last edited: Feb 24, 2017
  13. keithb7

    keithb7 Friend of Leo's

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    With someone's suggestion on another forum, the error was found in the reverb circuit. A 470K resistor separates the reverb circuit from the input grid mixer. I mistakenly put a .470K resistor in there. I had double checked the values but missed the decimal on the DMM. Drat. A simple 470 ohm reading would have caught my attention. .470K threw me off. I saw the K on there and moved on. Humbly moving on. :oops:
     
  14. keithb7

    keithb7 Friend of Leo's

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    Amp is sounding killer tonight. Just like SF TR should. With help from an on-line connection, I located a 100K resistor where there should have been a 100 ohm resistor. In the phase inverter. So two for two, human error. Twice I missed a decimal point. Dang easy to mix up one little thing like a resistor value and the whole amp sounds like shat. Patience and determination are key traits found in an experienced amp tech. I'm learning the hard way, but this is how I seem to retain info. Still big smiles here over this project. Enjoyed every roadblock. One more yet. Vibrato. Nothing. Could be a simple thing like a tube. Who knows. Will flush this out next.
     
  15. grolan1

    grolan1 Friend of Leo's

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    I've had a couple of roaches go bad before in BF/SF amps.... that could be your issue as well.
     
  16. brogh

    brogh Moderator Staff Member

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    you know as we say here .. video or it didn't happen ( yeah i gave it a little twist :D ).

    Glad you got it up and running, you will get the vibrato fixed don't worry looking forward to hear this one.

    Cheers
     
  17. BobbyZ

    BobbyZ Doctor of Teleocity

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    Just skimming through here today. I'm sure you know already Keith. The trem on these needs to be shorted (switch closed) at the RCA jack to turn on. Easy to forget because alot of Fenders work the opposite way.
     
  18. keithb7

    keithb7 Friend of Leo's

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    You bet. I'm familiar with the Trem system. If you recall in the original amp pics the stock trem was gutted. All gone. All the trem parts are new. Everything. Will get to it eventually here and check back in.
     
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  19. keithb7

    keithb7 Friend of Leo's

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    All good to go. Last finishing touch is the Wally Mod. 5K pot in MV slot, for variable negative feedback. I'll give it a try.

     
    Last edited: Feb 26, 2017
  20. theprofessor

    theprofessor Poster Extraordinaire

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    Way to go, keithb7! What a great thread! I don't know much at all about the insides of amps, but I've been following your progress. I'm certain that the owner will be happy. One more notch in your belt for preserving and restoring the mojo of vintage Fender amps.
     
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