Fender Twin Reverb issue

Discussion in 'Amp Tech Center' started by lonewalk78, Feb 18, 2020.

  1. lonewalk78

    lonewalk78 TDPRI Member

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    New here to the forum...I have a 1978 Fender Twin Reverb which looks all original from what I can tell. I just got it from the original owner who didnt think he ever had it worked on. I installed new JJ tubes ( 4-6l6 matched, 4 atx7, and 2 at7), cleaned.all the pots and sockets. I'm having issues with it distorting or flabby bass when I am at 4 volume and 4 master volume. This amp has no bias pot and I am new to amp tech. Any help would be awesome
     
  2. lonewalk78

    lonewalk78 TDPRI Member

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    20200130_171103.jpg 20200130_163558.jpg
     
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  3. Musekatcher

    Musekatcher Friend of Leo's

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    You need to find a reputable tech. It likely needs caps, and possibly a resistor or two, etc. Budget a couple hundred bucks, it will probably be less.

    PS - that will be a fantastic amp when its in good operating order. A Twin with JBLs is one of the best sonic experiences there is!
     
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  4. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Yup - cap job for sure. No point in trying anything else before that gets done.
     
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  5. dented

    dented Doctor of Teleocity Gold Supporter

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    mmmmmmmm D120s....mmmmmmmmm:twisted:
     
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  6. moosie

    moosie Doctor of Teleocity Ad Free Member

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    As mentioned, your power caps are old, in dire need of replacement. But... it's not just flabby bass you need to worry about. They can 'go' without warning, and when they do, are likely to take out the power transformer with them. Then you're up the creek.

    Get it fixed before turning it on again. Please.
     
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  7. TelenTubes

    TelenTubes Tele-Holic

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    Question... hopefully not a thread detailer...

    If not played... how long will caps and tubes last? I have a 61 fender bassman that I had cleaned up and put in working order. But it's TOO LOUD to play much, so it's been sitting. Caps and tubes were replaced 15 years ago. Played for about an hour and it's been sitting. Just curious.
     
  8. RottenTheCat

    RottenTheCat Tele-Holic

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    See where its says "output tube matching", on the rear panel? That's the bias control, which is "balance" not "level". It can be converted to "level" if you like, fairly easily. The hum balance control moves the relative position of the artificial ground for the filaments. That one, you can set by ear. The output tube matching.... gotta use a meter.

    That amp looks like, to me, one of the last, say 81-82 "Ultra-Linear" Twin Reverbs, at 135watts. These are CLEAN to the max, don't have a good grind of any sort at all. Difficult to set back to Blackface spec, and really you don't want to. Your amp is sought after by professional jazz and steel guitar players, who want full tone, and clean clean clean. As I recall, the reverb on these is pretty damn nice too. Has the tightest bottom end of any Fender amp chassis... a real thumper if you let it. But a clean thump, not flabby or futzy. Top end just glistens. Its really a wonderful "full tone" almost "Hi-Fi" amplifier, and was the ultimate Fender made, probably ever.

    Ultralinear amps use a output transformer tap to run the screen grids... something like 42 or 43 percent of the full winding. Don't crucify me, I'm goin on late nite brain cells, should I be wrong on the percentage. Running the screens as a function of the OT primary allows you to push the tubes harder, keep them cleaner, less distortion.

    Recap is fairly easy, insist on Sprague/Vishay TVA ATOM capacitors, as they're huge for their rating, will take the heat.

    And, find a good surgeon that does hernia operations on the cheap.... you may need him.
     
    Last edited: Feb 18, 2020
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  9. moosie

    moosie Doctor of Teleocity Ad Free Member

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    They're getting ready for replacement. Whether or not you played them, but even more so since you didn't.
     
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  10. Teleguy61

    Teleguy61 Friend of Leo's

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    Beautiful amp, good luck.
    The symptoms you describe can be caused by failing filter caps.
    Caps in the doghouse, and all the preamp bypass caps, and the bias supply filter cap probably need replacement.
     
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  11. Milspec

    Milspec Friend of Leo's Silver Supporter

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    A tube amp should be allowed to run at least 30 minutes to an hour every month to keep the caps fresh. It will not effect the tubes outside of maybe some oxidation on the pins. I think caps take the most damage when you fire up the amp with fully discharged caps from sitting idle for months. Do that enough times and the life will be greatly shortened.

    I have a very similar Twin ('79 with the newer version of JBL speakers) and a '64 Bassman....the Twin is played every day while the Bassman does sit for weeks at time between sessions. Service that Twin and enjoy...they do make an excellent home amp in my view, but some of that reason is that you never want to move one again once it gets there.
     
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  12. BobbyZ

    BobbyZ Doctor of Teleocity

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    Hopefully you didn't toss the old 6L6s they could still be in good shape. That amp shouldn't be distorting at anything less than earth shaking volumes. As already said it needs electrolytics, all of them ASAP.
    In fact I wouldn't plug it in again !
    Real easy to add a voltage pot and leave the bias balance pot as is. Definitely worth doing. Right now those JJs could be way off and probably are.
    A few people like the master volumes and pull boost on these but most people just set the MV on ten and never pull the boost out. It's really an awful sounding circuit but when I was a teenager I liked them. If your goal is a distorted sound at usable volumes, you need a pedal or a different amp.
     
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  13. dickey

    dickey Tele-Afflicted

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    003.JPG I have the exact same amp with the exact same speakers & tubes. Had some "fuzz" or "hair" when I hit low notes, but cleaned the pots & it stopped it & been great ever since.
     
    Last edited: Feb 19, 2020
  14. Alter

    Alter Tele-Meister

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    That's typical behavior of an old tube amp in need of maintenance. Take it to a tech who knows Fenders. Have components checked and measured. Don't go down the route of replacing everything old with new stuff that is of inferior quality and might fail in months or a year instead of lasting a lifetime. Replace only faulty stuff.

    Once restored, don't let the amp sit idle for months, but try to play it a bit every now and then. As mentioned, it's what's needed to keep it in shape. Enjoy!
     
  15. lonewalk78

    lonewalk78 TDPRI Member

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    Thanks everyone, I know it needs a good cap job. I'm pretty handy and good with mechanics so I'm gonna get all of those soldered in. I found Sprag caps for all the eletrolitic ones, I think it's a total of 13 of them. I've been doing research and cant figure out why it has no bias pot, I also have a 1975 Twin Reverb Silverface and it does have the pot inside. I cant find the correct schematic and chassis print for the 1978 Twin which stinks.
     
  16. moosie

    moosie Doctor of Teleocity Ad Free Member

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    A lot of changes took place in the Twin from '75 to '78. Namely an ultralinear transformer.

    Your '75 won't have had a stock bias pot, either. Those were only stock on the blackface amps. But adding a bias pot is a common mod, so not surprising it's there.
     
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  17. Jon Snell

    Jon Snell Tele-Meister

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    There is no bias pot in the 1976 Fender Twin with the drawing number 010182 known as the Twin Reverb 100 or indeed the Twin Reverb 135. There is a balance pot and this design doesn't require adjustment if the correct valves are fitted.
    The first to have a bias pot was the AB763 then the 65 re issue in 1991.
     
  18. moosie

    moosie Doctor of Teleocity Ad Free Member

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    Check Rob Robinette's site for the mod to leave the balance pot in place, and add a bias trimpot to it. Best of both.
     
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  19. Musekatcher

    Musekatcher Friend of Leo's

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    And its a hi-fi sound that can handle the peaks and transients, unlike actual hi-fi equipment with insufficient transformers, that distort nastily with something as dynamic as a tele as the input. 6L6s and dozens of lbs of transformer just cannot be beaten for that crisp clean sound - great for steel, jazz, and telecaster guitars. There's a reason Fender successfully designed and marketed these, and it wasn't to save money.
     
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  20. Milspec

    Milspec Friend of Leo's Silver Supporter

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    Definately add the bias pot and re-bias to blackface specs....it made a big difference to the tone on mine.
     
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