Fender 57 Tweed Twin just lost volume

Discussion in 'Amp Tech Center' started by MASONish, Feb 15, 2014.

  1. MASONish

    MASONish Tele-Holic

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    Playing with my 57 Twin tonight and the volume slowly went down to complete silence within 4-5 seconds. I checked the amps fuse which appears to be fine, not blown. I suppose it could be blown but doesn't give the appearance of a blown filament. Any suggestions?
     
  2. songsmith1950

    songsmith1950 TDPRI Member

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    That sounds like you had a tube filament go out for some reason. The volume would have gone down slowly like you said and then would have gotten a little distorted before dying. Turn it back on and see if any of the filaments remain dark instead of lighting up.
     
  3. MASONish

    MASONish Tele-Holic

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    Power tube maybe? I've had a set of Winged C's in there for a year at best. The original tubes were in it since I purchased this amp new in '07 & they didn't go bad. Just wanted to replace.
    The Jewel light is still working so I assume the fuse hasn't blown.
     
  4. songsmith1950

    songsmith1950 TDPRI Member

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    No, the fuse hasn't blown. The pilot light should also prove the filament string is good. But since the filaments are parallel wired it does not mean all of the filaments are good or are lighting. Again, I would turn the amp on, which should be safe since the fuse didn't blow, and then look to see if all filaments light.
     
  5. MASONish

    MASONish Tele-Holic

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    The 2 rectifier tubes are not lighting up. On the board are 2 fuses. The fuse going to the rectifier tubes is white. Could be a blown fuse maybe?

    [​IMG]
     
  6. songsmith1950

    songsmith1950 TDPRI Member

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    That fuse is white by design and is not your standard type fuse. Have been looking at original which had only one fuse. Wait a few. I am going to Fender support to look at the schematic for the newer one.
     
  7. songsmith1950

    songsmith1950 TDPRI Member

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    F2, probably the white one since I can not read what it says by it. The 6.5 Amp fuse beside it can not be the problem. It is for the preamp and output tube filaments and if they light it is good. The other fuse, F1, is a 3 Amp fuse and must be good or nothing at all would be happening.
    If F2 is bad it could be the tube itself. Secondly, in order of expense and ease of repair, it could be either of the rectifier tubes. In this amp you have two tube rectifiers for the sake of the sound and the characteristics. Meaning that you can run off one of the tubes, but would just have more sag and sponginess feeling in the sound of the amp. It would run fine for testing though. So if F2 was bad you could get a little box of five and then just pull one rectifier tube to see if operation was restored. If not the fuse probably blew again. Put a new fuse in, pull the other rectifier tube and replace the one you pulled first to see what happens.
    If none of this stops the fuse from blowing you probably have a bad rectifier filament winding in the transformer. That, my friend, is worst case.
    If that is the case, before replacing the power transformer itself, you could try a "Copper Cap" or other brand solid state replacement rectifier. They can have the same sag characteristics as the rectifier tubes. They need no filament voltage as they have no filament at all. That might just isolate the bad winding enough to allow the amp to work for thirty years, I can not say for certain.
    These are my best guesses. Hoping it is easy and fast and cheap to fix.
     
  8. MASONish

    MASONish Tele-Holic

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    Thanks so much for the info & all your time and help!
    May be the fuse. I put 2 different rectifier tubes in to make sure. Still nothing. The tubes didn't even light up. I'll start with a fuse to see. Good project for tomorrow! Thanks again!
     
  9. songsmith1950

    songsmith1950 TDPRI Member

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    You are quite welcome my friend. Hoping all goes well. Will be on tomorrow night if you are around and looking.
    The fuse will probably be necessary first as that fuse will cause the filament voltage to be disconnected. Buy three fuses anyway. Then try one Rectifier tube at a time. If it lights up it will work. If not try the other one. Like I said, one rectifier tube will run that amp. Not as beautifully as it is with two, but for test purposes.
     
  10. MASONish

    MASONish Tele-Holic

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    A big THANK YOU to songsmith1950 for the help. The F2 fuse was blown. Not only blown but it actually melted a bit of the end solder of the fuse! It was a 12A 250v fuse so I put a 10A in so it wouldn't get that hot again, hopefully, and it will blow faster before getting that hot. Seems to be working fine now. Amp is 7 years old now so maybe this is common with its age. Killer, killer amp!
     
  11. songsmith1950

    songsmith1950 TDPRI Member

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    You are so welcome. I do like that 10 amp fuse better. Rule of thumb in fuses and most of electronics is to double the rating so the fuse (12 Amps) was double the rating of the actual filament usage. (2 Amps each times two tubes.)
    I agree, you don't want that fuse to last until perhaps even the transformer is blown.
    It is possible that fuse became a little brittle with age. That can and does happen.
    Beware those rectifier tubes though. It is also possible that one has become either microphonic or can have an intermittent short due to an element being jarred by the sounds into coming out of it's designed place just enough to short. If possible and it was my amp I think I would change them.
    Yes, that is quite an amp my friend. There are legends about the low power twin rectifier Tweed Twin and from what I have heard in person and on recordings it is very well deserved.
    Have fun, my friend. If you get a chance post something played through that amp and send us a link!

    Tom
     
  12. MASONish

    MASONish Tele-Holic

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    Thanks Tom for your help! And yes, there is something very special about the tweed Twin. There's a reason almost every pro player, like at Clapton's Crossroads Festival, is using the Twin. Of all my amps I'd have to say this is my favorite. And it takes all OD pedals beautifully.
     
  13. CoolBlueGlow

    CoolBlueGlow Tele-Afflicted

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    Hi Masonish,
    I don't want to sound like a naysayer, but fuses blow for a reason. This fuse which blew is the rectifier filament fuse in your amp, but since 5u4 are directly heated cathodes, the HT DC also flows through this fuse. (See schematic, here)

    http://support.fender.com/schematics/guitar_amplifiers/57_Twin_Amp_schematic.pdf

    The factory fuse is a T12a 250v rated fuse. Therefore, the 10a fuse you have selected is inadequate, according to Fender.

    Some may question how a 12 amp fuse blew in a circuit which at first blush appears to be flowing six or eight amps, but in this circuit, it is not that simple.

    A couple things to note which might help unravel the mystery.

    1. 5U4 rectifiers are directly heated cathodes. Rectifiers with directly heated cathodes may short out, causing fuses to blow, which I'll explain in a moment. Such cathode shorts can be intermittent. In unfused circuits, the rectifier tube operates until it fails (often spectacularly). In fused filament circuits, such as your twin reissue, (we hope) the filament fuse will blow. Replacing the fuse does not solve the problem...therefore you or your tech needs to look deeper and ask "why did this fuse blow?" and "why is F2 getting hot".

    2. The Fender 57 Twin has a standby switch. For reasons know only to Fender (Perhaps nostalgia, or perhaps fear of being accused of tampering with the original design?) they did not improve the standby design. In any case, back in '57, Mr. Fender originally placed the standby switch immediately in series upstream from the choke. Now I freely acknowledge that Leo was a genius, and further declare that by comparison I am just a Padewan, but this is a terrible location for a standby switch. When the amp is moved from standby to operate, the choke develops what Valve Wizard describes as "massive flyback voltage". This phenomenon can cause the choke to fail. More significantly (in your case) it can cause the phenomenon of rectifier tube flashover. Such rectifier tube flashover can lead to blown fuses - exactly as you have experienced.

    Read about it here to understand what's going on.

    http://www.valvewizard.co.uk/standby.html

    If that doesn't convince you, then read this tube data sheet to see RCA's stern warning about "hot switching" the 5U4. (Starts at pp. 2 bottom). Leo must have missed that one, since this sheet was published in 1954! :)

    http://tubedata.milbert.com/sheets/049/5/5U4GB.pdf

    3. O.K., so enough rectifier tube gloom and doom. now, the other thing you mentioned as that F2 looked like it had been hot. you need to know that directly heated cathodes like the 5U4 place an additional hidden strain on the secondary winding of the power transformer, as the full B+ flows through the filament circuit. Therefore, the HT winding and filament insulation must be able to stand the strain of "floating the B+ and the filament current" above ground (as noted on the Fender schematic - see the term "elevated", again, on the Fender schematic). As the HT winding does its work, it also does double duty as an isolation transformer between the 5U4 cathode and ground. Because of this, and because of your description of the "hot spot" on F2, IMO you should check B+ at TP1. It should be 460vDC, +\- 10%. If it is significantly below +460VDC, that is a hint that the secondary HT winding insulation in your power transformer may be partially breaking down. Such a breakdown would cause an increased current draw, which could explain the "hot spot" on F2. Measuring the current draw through F2 should reveal this.

    Hope the above isn't too murky. If the above sounds too deep for your comfort, take it to your tech and discuss it with him, along with the following suggestions.

    If I were servicing your amp I would:

    1. Replace the suspect rectifiers and measure filament ACV. Should be 5 VAC, +\- 5%
    2. Verify the integrity of the wiring and solder joints at F2 (the "hot spot")
    3. Reinstall the correct T12a/250V fuse
    4. Operate the amp and measure quiescent and full power operational current through F2, (as well as B+ at TP1)

    If all was normal, I would then:

    5. Strongly recommend the standby switch mod described by Valve Wizard, in this link http://www.valvewizard.co.uk/standby.html, under "improved standby switching"
    Valve Wizard's mod is simple, it is absolutely sound engineering, and it will have zero effect on the amp's operational tone, whilst significantly reducing the strain on the entire power supply.

    If the owner did not wish to do #5, for originality reasons, I would strongly recommend that he simply discontinue the use of the 57 Twin reissue standby switch. By design, tube rectified amplifiers gain no lifespan benefit from the use of the standby switch, other than as a convenience option to mute the amp. (I would still do items 1-4.)

    Hope this helps...

    CBG
     
  14. MASONish

    MASONish Tele-Holic

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    CBG-

    Thank you kindly! I will. Being an engineer I want to know why the fuse in F2 failed. There is a reason as you stated. I'm going to take the Twin to my tech along with what you have stated to get to the bottom of this. Thanks again for you time & resource. :respect
     
  15. Wiley

    Wiley TDPRI Member

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    Hi MASONish, what was the final outcome? I have the same amp and the same problem and wanted to get some insight before I lug it over to the tech. Thx.
     
  16. MASONish

    MASONish Tele-Holic

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    The tech that worked on it said the fuse holder wasn't making good contact with that F2 fuse. Installed an inline fuse holder for that particular fuse, which is the rectifier filament fuse. Has seemed to fix it.
     
  17. Wiley

    Wiley TDPRI Member

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    Thx for the reply. I'm glad that you didn't come up against some of the possible worse case scenarios. It is a well designed amp.
     
  18. MASONish

    MASONish Tele-Holic

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    Be sure and let me know what was exactly the problem with your Twin. '57 Tweed Twin RI right?
     
  19. Wiley

    Wiley TDPRI Member

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    Ok
     
  20. Wiley

    Wiley TDPRI Member

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    Yes, '57 TT reissue from 2009 or so. No mod's so the comments about the Standby switch were curious to me.
     
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