Advice to save a SF Twin Reverb

Discussion in 'Glowing Bottle Tube Amp Forum' started by monkeybanana, Dec 8, 2019.

  1. monkeybanana

    monkeybanana Tele-Meister

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    My bud and his dad have had a Twin Reverb, among other cool things, sit in their music room and I'd like to get it working for them. They actually play out from time time instead of nerding out on gut shots and TDPRI :)

    Here's why it sat. Before I took it they showed me how it played quiet, sounded good, but a Twin should raise hair, no? Also, after about 10 mins a high frequency squeal comes in.

    I have opened it up to see if there was anything obvious that was out of whack. I don't see any bad solder joints. Everything that should be grounded is grounded. Tubes are good and sockets were cleaned. I don't think anyone has been in the chassis before.

    Okay so now I'd like to replace the electrolytics. If it was my amp I would only swap whatever is broken but if I do that here and something else goes out I am afraid the amp would sit in the music room again. Here is my question. The values on the filter caps do not all match what's on the schematic I found online. Would you replace them with the same values or go by the schematic? I know that some values are now more common in a few digits away but a 220mf vs a 100? Was Fender just subbing with that they had on hand?

    I'm including a photo. Looks like three eras in the cap house!

    I'll include a shot of the inside too while I'm at it.

    If you've made it this far I thank you!
     

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  2. Musekatcher

    Musekatcher Friend of Leo's

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    Squeal can be a tube-socket issue, or might be a cap issue. Are the existing caps good? If it were me, I'd just take it to a tech and have them tune it up, and let them know your budget.
     
  3. rolandson

    rolandson Tele-Holic Silver Supporter

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    I'd say it's time for a (filter) cap job, but save that for later.

    First off, tap each tube with a chopstick and listen for microphonics...it could be as simple as a funky tube (the squeal is what one might encounter with a microphonic power tube).

    Next up, tension the sockets.

    Then move on to the filter caps (which ought to be done every decade or so anyway).

    Lastly carefully trace the signal path looking for the not so obvious.

    Failing all of that, or, if one lacks proper tools and expertise...

    Doctor visit.
     
  4. Tonetele

    Tonetele Poster Extraordinaire

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    I had an SF twin Reverb and it sounds like a tube/valve issue that needs checking out.
     
  5. Andy B

    Andy B Friend of Leo's Silver Supporter

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    From your pictures it looks like an ultralinear twin. 135 watts. You are looking at the wrong schematic.
     
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  6. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    It also appears all original from a quick look. Replace what's there with the values that are there (or close enough) for the electrolytic tune up.
     
  7. mojavedesert

    mojavedesert TDPRI Member

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    Turn off the reverb. Make sure the master knob is not pulled out, that amp uses the reverb tube as a boost for overdrive. Try channel one first turn everything off on channel two, the side with reverb and tremelo. See if it still squeals.

    This explains the tubes.

    http://fenderguru.com/amps/twin-reverb/

    I think V3 and v4 are the problem, my amp was doing the same thing, easy fix.
     
    Hiker likes this.
  8. monkeybanana

    monkeybanana Tele-Meister

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    Thanks for the replies. Since most of you mentioned a tube issue I cleaned the sockets a third time and it seems to have done the trick. I am going to put it back in the cab and try with with the reverb. Darn. This was less exciting than I thought. At least I can play it until my bud comes and picks it up.

    BRB
     
  9. Ten Over

    Ten Over Tele-Meister

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    All of the capacitors were manufactured in 1977. All of them are in desperate need of replacement. I wouldn't even turn the amp on until they are replaced.

    You mean that the amp has 220uF caps in it and you saw 100uF in some schematic, right?

    Man, that thing is pristine on the inside. Don't let anybody talk you into modding it.

    Note how the ground switch isn't connected to anything. It came that way from the factory.

    Twin Reverb 135.png
     
  10. Ten Over

    Ten Over Tele-Meister

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    I would strongly discourage you from returning that amp without replacing the doghouse caps. They'll take it to someone else and that person will badmouth you for not replacing the caps. What's that white stuff all over the caps and board?
     
    King Fan likes this.
  11. monkeybanana

    monkeybanana Tele-Meister

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    This place rules.

    So that schematic explains a lot. I didn't notice the disconnected "death cap" but that's interesting.

    This amp is not my amp so I will not make any mods. Too bad the cab doesn't look as good the inside but this amp was played out and my friend has pictures of his dad playing it at a show in the eighties or early nineties.

    The white stuff is from the amp! Not recreational. I do want to replace the caps but I want to make sure that's what they want.

    The reverb sounds really good too. Makes me want a Princeton. ahhh
     
  12. monkeybanana

    monkeybanana Tele-Meister

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    Also, should I do all the electrolytics or can I only change the filter caps?

    And can I use 100uf caps instead of the 70 and 80? (Looking at F&T's on sale at Antique).
     
  13. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Yes, do them all/yes
     
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  14. Wally

    Wally Telefied Ad Free Member

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    If it were my amp, it would be in the hands of an experienced tech. No offense intended, but your questions indicate a lack of experience. Ymmv.
     
  15. monkeybanana

    monkeybanana Tele-Meister

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    I always appreciate your input Wally so not offended. I can do basic tasks and want to help my friends out for free plus I got to see the inside of an old amp.

    If it were one of my 5 watt amps I would leave the cathode caps alone if they worked. I know folks are more conservative here. But I’m thinking of doing everything in here as these guys would leave the amp sitting and probably wouldn’t see a tech anytime soon.

    Beyond the basics I will recommend a tech if the amp has serious issues which I don’t think it does.
     
    Wally likes this.
  16. King Fan

    King Fan Friend of Leo's

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    Monkeybanana, you're a good guy, both to fix your friends' amp and to take expert advice with good grace.

    The guys above saying to replace all the e-caps are some of the smartest, most experienced guys here. I've never seen any of 'em set a foot wrong.

    Those e-caps are 40 years old. There's no such thing as a failing cap that somehow sounds better than a replacement. And you make the best point of all -- your friends aren't likely to have it done in the *next* 40 years either. :)

    Check with them first, though.
     
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  17. Wally

    Wally Telefied Ad Free Member

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    Yeah, I am Still waiting for someone to offer me professional services for free. I am waiting, but I am not holding my breath.
    Good luck with it.
     
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  18. monkeybanana

    monkeybanana Tele-Meister

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    Well I’m obviously not a pro but again this amp has been sitting for a long time. And they never get to rid of stuff. Actually I just remembered before this thing was in their music room it was buried in the garage until I saw it and asked about it about a year ago. I know once they hear it they will be very happy. I’m not going to do anything invasive.
     
  19. Axis29

    Axis29 Poster Extraordinaire Ad Free Member

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    I'd replace the electrolytics in the doghouse and the bias caps and see how it goes from there.

    As for values, as someone mentioned try to get close to what's in there already.
     
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  20. D'tar

    D'tar Friend of Leo's

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    Oh, it's never FREE! At minimum, the price of a good meal and some beverage of choice! With any luck some good conversation as well!
     
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