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1976 Fender Twin Reverb Overhaul

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

  1. mherrcat

    mherrcat Tele-Meister

    250
    Dec 12, 2013
    California
  2. 24 track

    24 track Friend of Leo's

    Nov 6, 2014
    kamloops bc
    Keith , silly question here,
    I notice electrolytics on the board wouldnt they be prone to failure and drying out? just a curiosity question , nice work so far , I did a 74 super that was one of the last of back face circuits in a silverface cab, the ownwer let a monkey with a soldering iron in there , I had to remove all sorts of weird mods as well.
     
  3. keithb7

    keithb7 Friend of Leo's

    Age:
    45
    Jan 9, 2010
    Out West
    24track I have lots of work to go on this amp. I'm just posting progress as I move along. All E-caps are to be replaced top and bottom side of chassis.
     
    24 track likes this.
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  5. keithb7

    keithb7 Friend of Leo's

    Age:
    45
    Jan 9, 2010
    Out West
    The further I get into this amp the more and more I see where it has been hacked up. Today I started at V1 of the circuit board and am working my way toward the power tube end. Going over everything very closely and replacing parts, and re-routing wires where needed. Mainly just working on board components. Wires will all be traced and put right after the board is re-populated. The owner and I discussed taking the Master Volume out of this Twin Reverb circuit. I am using the AA0270 layout and schematic and attempting to wire this TR up as such. Will still have the bias balance, but no MV as per the AA270 circuit. There are a few differences in the circuit board eyelets. So far I have been able to work around this. I have a good array of close-up circuit pics of my 1973 Twin to help me with ideas and as a reference for some areas. Slowly chugging along here in the evenings. The board is starting to resemble a TR again I think.

    [​IMG]
     
    brogh and 24 track like this.
  6. TheZ

    TheZ Tele-Meister

    276
    Nov 13, 2009
    Northwest USA
    Cool project! Thanks for sharing.
     
  7. 24 track

    24 track Friend of Leo's

    Nov 6, 2014
    kamloops bc
    Thanks , they were part of the cluster.... I see now, I had never seen electrolytics in that circuit before
    I really like posts like this I can learn alot as I stand in the sidelines, Thanks again
     
  8. Wally

    Wally Telefied

    Mar 17, 2003
    Lubbock, TX
    Good work, Keith. Talk of removing the MV leads me to observe that that hole is a great place for an adjustment pot to vary the negative feedback voltage. And...then I think about making this amp into a two channel amp that has two distinctly different channels...with effects on both channels. Those differences would depend on the players needs. Or...they might like having two nearly identical channels as the amp was designed.
     
    24 track likes this.
  9. keithb7

    keithb7 Friend of Leo's

    Age:
    45
    Jan 9, 2010
    Out West
    I like the varying NFB option. Hmm. 820 is the stock AA270 NFB resistor. Would you recommend I use a 1K pot? Would adjust from zero to 1K, correct? And the effect would be not unlike like the presence knob on my 5F4 Super, correct? It does add some nice adjustable bite.
     
    24 track likes this.
  10. Wally

    Wally Telefied

    Mar 17, 2003
    Lubbock, TX
    I have I have used 5K's. This adjustment is different from the Presence control in that the Presence control is a frequency specific action. Increasing the resistance in the loop decreases the overall cancellation of signal that the loop creates.....harmonic content increases as does the output...slightly. Things get richer.
     
    24 track likes this.
  11. FenderLover

    FenderLover Friend of Leo's

    Jun 11, 2009
    Minnesota
    I was just reviewing the before pics again - yikes. You are a good steward of the Fender Amp Preservation Society for putting things right on this one. The owner probably couldn't justify the hours a shop would have to sink into it.
     
    jimash likes this.
  12. cboutilier

    cboutilier Tele-Holic

    Age:
    25
    851
    Oct 28, 2015
    Halifax, NS, Canada
    I should try this out with my 75 Super Reverb. Either that or a PPIMV
     
  13. keithb7

    keithb7 Friend of Leo's

    Age:
    45
    Jan 9, 2010
    Out West
    Thx Fenderlover. I too am a Fender Lover I guess. I do enjoy working on and keeping vintage Fender amps alive. Its fun and keeps my brain challenged. I suspect you are right, a professional tech's charges would exceed real street value on this amp. Without a doubt. My goal is to have this classic amp chiming again as it was intended.
     
    24 track likes this.
  14. brogh

    brogh Super Moderator Staff Member

    Jun 26, 2010
    italy
    Hey Keith !

    Another great project here :) the stuff you got rid off in this amp is just ridicolous, looking forward to see it finished :)

    Cheers !
     
    24 track likes this.
  15. Bill Moore

    Bill Moore Tele-Holic

    Thanks for the thread!
    I just picked up a 70's MV Twin that I will need to go through, I don't think it's been modded though.
     
  16. keithb7

    keithb7 Friend of Leo's

    Age:
    45
    Jan 9, 2010
    Out West
    That first pic of all the stuff I pulled was at the beginning of this project. The pile just keeps growing bigger as I move through the amp. It's crazy.
    Someone spent a lot of time modding this amp. Someone paid someone else, a lot of money trying to make an amp, something that it's not. Silliness today,
    but I am sure there were years where hot-rodding old Fender amps was a huge craze. Thus the cycle ends as I take it back to what it was designed to do.
    I am an little weary about initial plug in and test, I must admit. We'll see. That's a long ways away yet. The light bulb limiter and fire extinguisher will be
    near by. LOL.

    Wally I just ordered a 1K pot to try out your idea.

    Look at my latest pic above. See the .01 (x2) and .022 caps in the vibrtato circuit? The small round ones I believe are mica disc caps. They are rated at 500V. The original TR had 1KV rated caps, and are much larger in size. Looking at an AA270 the vibratro voltage is about 245V DC. Is there any cause for concern here? The new ones just look pretty small compared to the original ones from the 70's. Thx.
     
    Last edited: Feb 18, 2017
    24 track likes this.
  17. brogh

    brogh Super Moderator Staff Member

    Jun 26, 2010
    italy
    cmooon don't even say that ! LOL !

    you should make a nice frame and put all the extra pieces in, it would be a great conversation piece .. " what you can find in a silverface twin " :lol: really make a picture of the whole extra stuff when you're done, should be interesting to see

    Looking forward to see it finished, and most of all hear it :)

    cheers
     
    24 track likes this.
  18. jazzguitar

    jazzguitar Tele-Holic

    990
    Mar 17, 2003
    Germany
    Interesting project!

    If I were you, I would keep the bias balance and the hum balance. Some of the very few good ideas of the CBS engineers ...

    These are the same as 12AX7 but with a different pinout.


    I am not sure that this was not intended. Better replace a $1 resistor than fry more expensive parts. I doubt if high power resistors are a good idea here ...

    They normally only fry with a screen short in a power tube. The carbon comps are fusible resistors (Navy mil spec): instead of burning they just crack open. I have seen that a few times. Not as much with reliable power tubes.

    Re those vibrato caps: yes the old ones are huge. Modern day components just are much smaller, and no, they do not need to be 1000V rated.

    Oh, and by the way, those old solder flux blobs dissolve in alcohol, eg isopropylene.
     
  19. keithb7

    keithb7 Friend of Leo's

    Age:
    45
    Jan 9, 2010
    Out West
    Today's update: The re-civilization of the board components is complete. The brown dust on the board that I was picking off with a small pick, turned out to be old dry crusty wax. I went over the board with my soldering iron in the bad areas, melted the wax and things look much better. Direct localized heat from the soldering iron might be better here than using a heat gun? Either way, I'm happy. There are just a couple of areas where eyelets were different on the AA270 vs the original 1976 circuit. You'll see a few extra eyelets not needed for the AA270 conversion. Next step is to tackle all the wiring. It's a mess. I decided to mount the cathode cap V4, first stage reverb recovery side ,right on the board. Plenty of room. I am not sure why, but Fender often mounted the cap right off the reverb RCA jack. Anyone know the reason? Heat? Interference? I can move it if needed. If you look, you'll see that on the original layouts Fender showed it placed right on the board. You can actually see the original reverb recovery cathode cap still in place in my pic below. The old white mallory cap, off the 3rd from right, RCA jack. I will be removing it soon. No breakfast yet and only 1 coffee. I need to take a break.

    [​IMG]
     
    Last edited: Feb 18, 2017
    24 track likes this.
  20. 24 track

    24 track Friend of Leo's

    Nov 6, 2014
    kamloops bc
    Nice work Keith, very clean, Impressive!

    Leo built these like tanks and the ptp wiring really helps as well, cool post!
     
  21. keithb7

    keithb7 Friend of Leo's

    Age:
    45
    Jan 9, 2010
    Out West
    Several hours later, some more progress. The control panel has all been re-wired. Tons of modded wiring in there. V1 and V2 re-wired. Having a ton of fun here. Honing my skills. Building confidence. This takes a considerable amount of time. I'm not complaining but it's crystal clear to me that paying a tech an hourly bench rate to do this would not be economical. Seeing as I decided against the Pro-Bono work, and am not going to re-wire the entire circuit (LOL, earlier rookie comment...Now I know better) I have been re-using some bits and pieces. Like the shielded cable for example. I replaced the entire vibrato circuit including new pots and wiring. Re-wired input jacks. Ignore the wiring around V3 through V10, I have not touched it yet.

    A couple of questions:

    The heater wire balance pot is there, but not hooked up. It was hooked up to a 6EU7 that was in V2. I had to rewire that tube socket completely to go back to a stock 12AX7 in there. I have no guide or schematic of this heater hum balance pot. Does anyone have a schematic to share? Or instructions on how to wire it up?

    Another question. By 1973 the Canadian version of these Twins came with a thermal breaker that ran in-series with the mains AC power from the fuse, then carried on to the power switch. I guess if things got real hot and sweaty the breaker melts and cuts all power. Not sure why it is there, when there is also a 3A fuse. My understanding is the domestic USA models did not get this thermal breaker. I am thinking of leaving it out. My 1973 TR also had one, but a 1971 TR that I serviced did not. It is my belief these 3 Twins that I have been in, were all built for the Canadian market. Any comments? Concerns about leaving it out? Any colour to add around the use of it? Thanks, Keith.

    [​IMG]
     
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