twin reverb cuts out help

Discussion in 'Amp Central Station' started by adjason, Apr 18, 2019.

  1. adjason

    adjason Friend of Leo's Silver Supporter

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    Hi, I recently picked up a late 70's twin reverb and it really sounds good. It plays fine for about 15 minutes then the volume almost cuts out to a thin quiet tone. I have noticed if I flip the standby switch off and then on again it seems to work fine for about another 10-15 minutes before repeating. Obviously I should take it to a tech - but any easy things I could try first? Someone changed some things if there but I am not super knowledgeable


    ATTACH=full]601849[/ATTACH] twin amp 2.jpg twin amp 3.jpg twin amp 4.jpg twin amp 5.jpg twin amp 6.jpg twin amp 2.jpg twin amp 2.jpg twin amp 2.jpg twin amp 3.jpg
     

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  2. jondanger

    jondanger Poster Extraordinaire

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    That cap job in the photo labeled Twin Amp 1 is highly suspect. Before anything else, I would address that.
     
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  3. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Yeah, that's kind of a disaster. There's not point in troubleshooting anything else until the sketchy cap job (that was only halfway done to begin with) is addressed.
     
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  4. keithb7

    keithb7 Friend of Leo's

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    Certainly it has had some questionable work done to it. It needs an experienced tech. Looking at the photos posted its hard to see much related to a quick easy fix. I'd also be doing cathode caps next time I was in that amp.
     
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  5. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Be happy the cutting out made you ask, and good job posting the good pics. The smart pros above nailed it: Your amp badly needs an overhaul anyhow. Beyond the doghouse (ow-ow-ow) the caps on the board look really old and toasty. It's all good: if a competent tech says some need to go, late-era SF caps aren't the 'special mojo' kind that people hate to replace.

    Don't feel bad; any amp this old really needs service. If you bought a late-70s classic car you wouldn't try to run it with the original oil, tires, belts, hoses, and brake pads.
     
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  6. Ten Over

    Ten Over Tele-Meister

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    Mid-seventies at best. Late-seventies were 135W UL's with a whole collection of bad ideas.
     
  7. adjason

    adjason Friend of Leo's Silver Supporter

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    Yeah I figured that looked like crap but I thought I'd throw it out to the pros here...thanks everyone
     
  8. Silverface

    Silverface Poster Extraordinaire Ad Free Member

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    Yep - one original filter cap was left in place and a mix of others used? Amateur work. But cutting out sounds like one or more dirty tube sockets, and the amp looks a bit dirty, like it's been stored in a dusty area (crap blows in through the jacks).

    Take it to a QUALIFIED tech.
     
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  9. LuthRavin

    LuthRavin Tele-Meister Ad Free Member

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    ^^ Like Silverface alluded to..I wouldn't even turn it on again..till it was in a shop..no telling what the last one in there was doing..
     
    Last edited: Apr 19, 2019
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  10. clintj

    clintj Friend of Leo's

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    I don't know where to start. Ok, maybe I do. Find a competent pro. Several people already mentioned the caps, that's a no brainer. That doghouse work is a hack job. Several of those ground points also look sketchy, and that era Fender where they used the transformer bolts is bad about having loose grounds at those points, which cause static, pops, and sound drops. It needs to be evaluated closely from end to end, and should sound fantastic once given some love.

    Sent from my Pixel 2 using Tapatalk
     
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  11. Milspec

    Milspec Friend of Leo's

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    I agree that this is likely mid-70's, but disagree about the UL Twins being full of bad ideas. That has been a common statement in forums, but I have one and find it wonderful. Leo progressed over the years towards making cleaner and louder amps at every stop and CBS continued on that path to end with the 135 watt of pure clean tone with seriously high headroom. I think Leo and company would have ended up in a similar place if still in charge. The master volume is always called out as a bad thing, but really it works pretty good for home usage. It is differnt than the earlier Twins, but not really worse.

    Anyway, your point is correct on the age of the amp pictured....just felt the need to dispel some of the internet legend surrounding UL Twins.
     
  12. adjason

    adjason Friend of Leo's Silver Supporter

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    Yes I think its a 75 or 76 - 100 watt version. I've owned a couple of other 70s twins through the years but this one sounds the best...that is why I don't want to change anything but yes I'll take it in....No problems with me and a master volume
     
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  13. Ten Over

    Ten Over Tele-Meister

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    Well, don't take it personally.

    Using two big-ass resistors to reduce the preamp voltage while consuming just as much current as the preamp itself isn't a bad idea? Using a ground switch with a death cap on a three-prong amp isn't a bad idea? Idling a 12AT7 at 9mA isn't a bad idea? Squatting down V4B with a 12K/.01uF load isn't a bad idea? Wiring up a simple 4/8 Ohm secondary in a manner that no guitar player and even some techs can't figure out isn't a bad idea?

    I'm not speaking from internet lore. I had one of those back in '79, maybe '80. It didn't sound good. It ran hot. Real hot if I turned it up. I don't know what that MVS was supposed to do, but it made it sound really bad. Reverb was killer, however.

    Who knows about Leo? Maybe he would have gotten into 'shrooms and designed the Red Knob Twin.
     
  14. Ten Over

    Ten Over Tele-Meister

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    Make sure that you tell the tech not to alter the main board unless absolutely necessary. Some of those guys replace all kinds of resistors and capacitors that don't really need changing and it just might ruin the mojo. The power supply can be completely replaced without altering the sound.
     
  15. Milspec

    Milspec Friend of Leo's

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    I don't take it personal, but then I just go by what I hear and not by circuit designs. The only thing I have done to mine is set the bias to blackface specs and nothing else. It doesn't provide a lot of break up, but I love clean tones and the late '70's Twin provide that in spades. It just seems like once the reputation has been established in forums it gets parrotted at every turn. Many players would love the UL Twins, it just depends on what you like to hear.
     
  16. BobbyZ

    BobbyZ Doctor of Teleocity

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    Just going by the coupling caps I'm thinking it's older than that, but maybe they used brown caps later than I thought? Normally see all blue blobs after the early 70s but that's a master volume amp so it's got to be after about 73 or so.
    At any rate it looks like the bias balance set up is still there, easiest thing to do is add a trim pot and leave the rest as is. That way you can set bias and you don't affect the sound like the old way where you'd convert the phase inverter to blackface style.
    Hopefully a normal service and all new E-caps gets rid of the intermittent issues.
    On my 76 SFTR it didn't, thankfully it was my amp because it was a real hair puller!
    At first it was like you turned the volume down, so I'm thinking I'll check voltages. Well as soon as I'd touch the damned thing with a meter it'd pop back to normal, clicking the standby switch would do it too.
    Ended up basically guessing and trying things that seemed likely. It'd work on the bench then do it again in the cab, so I put a board over the chassis to keep the heat in like in the cab. Finally got it without too many parts. I think. . .
    It's actually my favorite Fender now and I've got five blackfaces including a 1965 BFTR. Unless I'm going someplace, then my favorite is the Princeton Reverb because it's lighter. :)
     
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