71 SF Vibro Champ, should I mess with it?

Discussion in 'Shock Brother's DIY Amps' started by Skish, Aug 3, 2019.

  1. Skish

    Skish TDPRI Member

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    9C7E5F6F-E9DD-4593-B89D-6DAEEE3142F7.jpeg Looking at this ‘71 Vibro Champ, I was planning to swap out all the capacitors. But I’m hesitating because it doesn’t sound bad. I’m not sure if it’ll improve it or not.
    Can anyone speak to pros and cons?
    I’ve got Mallory 150s for the tone and coupling caps, orange drops for the 3 Vibro caps and 4 less expensive 25uF caps for the cathode caps.
    Also noticed one cathode cap is 10uF on the original, but it seems all clone kits use 25uF for all 4. Is there an issue replacing that with a 25uF?
     
  2. Dacious

    Dacious Poster Extraordinaire

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    Don't change the tone caps. Your amp but there's no reason to. The white Mallory bypass caps are probably toast, along with the cap can.

    Replacing poly and ceramic caps with modern equivalents will do nothing except destroy value. The ceramic caps control the trem oscillation via bias wiggle but are not in the signal path. There's nothing wrong with the original tone caps in these. There's nothing wrong with the new ones you have - but you won't hear any difference. 'Orange Drops' are not the old ones but rebranded Vishays. Mallory, too

    Change the cap can if it hasn't been renewed and the 25uf bypass caps - keep the 10uf or you could negatively impact the trem.

    I've owned two - a 68 and 78, they sounded great with the original caps. So do all the originals.

    The electrolytic caps are like spark plugs in a car - they wear and go bad with age. Good electros are important for clear tone especially with bass response.

    The rest, unless they break there's nothing to gain from changing them.
     
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  3. tweedman2001

    tweedman2001 Tele-Afflicted

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    ^^^^^This
     
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  4. sleazy pot pie

    sleazy pot pie Tele-Afflicted

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    I know nothing about working on amps, but I have listened to all of the episodes of The truth about vintage amps podcast with skip Simmons (not james). He mentions not changing anything unless they are bad in every episode.

    Great podcast if you haven’t found it yet.
     
    Last edited: Aug 3, 2019
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  5. bftfender

    bftfender Friend of Leo's

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    Every amp i get..we fire it up[ at my techs bench....listen...(have a bunch of classics for comparison if need be) most sound pretty good...we figure speaker out for application..then He will go over every single component...i get an amp back that is good to go prob the rest my life with reg maintenance. 71 Pro, 74 Super & Super RI are amazing(along with a bunch of M's) The Silverface amp responded so well..i am now a convert for the studio & live with both. Speakers were key along with making Normal a lil more aggresive
     
  6. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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  7. Skish

    Skish TDPRI Member

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    Thanks for all the quick replies, especially Dacious.
    Sounds like good advice!
    I had already swapped the speaker with a Weber 8SigA-S. There was a bad speaker in there (not original).
     
  8. Skish

    Skish TDPRI Member

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    image.jpg image.jpg Pulled the three 25uF bypass caps. They all measured between 32 and 34. Will have to order a Cap can. I wasn’t sure it was original so thought I might not need to. Looking at it now, I’m sure it’s original so I should order one.
     
  9. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Dacious nailed it.

    You're right. Mallory = 235, 7108 = exactly 48 years ago. The cap can change is most needed but toughest to do. I suggest a 100w or bigger soldering iron — then move all wires as far back as possible and protect them with a 'heat dam' made out of a strip of silicon hot pad.
     
    Last edited: Aug 3, 2019
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  10. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    The issue with electrolytic caps isn't what they measure, it's that they just physically can't do the job after 20 years.
     
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  11. muscmp

    muscmp Tele-Afflicted

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    i have a 75 vibro champ. i replaced the can cap and other electrolytics with F&T caps. one of the white mallory caps had exploded. it still worked but sounded much better afterward.

    skip james was an old blues guitarist. you are thinking of skip simmons and here is the link to his posts.

    https://www.fretboardjournal.com/?s=skip+simmons

    play music!
     
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  12. sleazy pot pie

    sleazy pot pie Tele-Afflicted

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    Thanks muscle. I fixed my post above.
     
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  13. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Good one, @muscmp . Skip James wrote "I'm So Glad", influenced Robert Johnson, and was the most famous recorded exponent of the haunting minor-key 'Mississippi Hills' or Bentonia style. His Wiki entry makes fascinating reading.

    Skip Simmons is a respected tech who writes / podcasts quite a bit. But he's at the far end of the spectrum with his 'leave it be' style in old amps.

    E-cap replacement may not be absolutely vital for safety. There was a good thread on here a while back where experienced folks were having trouble actually recalling a failing filter cap taking out a transformer. But for function and sound? I'm guessing 90-95% of pros and top techs like to replace old e-caps.
     
  14. DavidP

    DavidP Friend of Leo's

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    What's the hum level like?? I'd consider lifting those filament chassis grounds and running a two wire filament supply to the heaters (as in bigger Fender amps), using two 100 ohm resistors off the two legs of the pilot light for a ground. This helped tremendously in a SFVC I had.
     
    Last edited: Aug 3, 2019
  15. muscmp

    muscmp Tele-Afflicted

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    unfortunately, i'm one of them. i have an early 64 bassman that i brought up on a variac for 3 hours. i then played thru it for 10 gorgeous minutes when all of a sudden there was smoke. i pulled the doghouse and one of the caps had indeed exploded and immediately took out the PT. proper fuse did not save it. ended up paying out the nose to have mercury magnetics rewind it. i couldn't tell the difference with the rewind but it had been a couple months until i got the PT back. now, i replace them anyway. they weren't meant to last that long.
    play music!
     
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  16. muscmp

    muscmp Tele-Afflicted

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    i wish it was muscle!:lol:
     
  17. Dacious

    Dacious Poster Extraordinaire

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    I wouldn't. My 68 had one leg of the heaters through the chassis. It was whisper quiet. Even with trem cranked.

    If it's not broken I wouldn't screw with it.
     
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  18. Skish

    Skish TDPRI Member

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    I was just remembering why I questioned if that cap can was original...I have reviewed AA764 and AB764 Schematics for the Vibro-Champ and this ‘71 amp has components from each. It has a 10uF bypass cap instead of a 2uF on the second stage, but it has the cap can with only 20uF instead of a 40uF and the rest as 20uF.
    An amp in transition I guess.
     
  19. tele_savales

    tele_savales Tele-Meister

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    A good story about when it's ok for a man to cry...
     
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  20. King Fan

    King Fan Friend of Leo's Ad Free Member

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    How’s the cap can wired? AFAIK, despite the first schematic, Fender early on started pairing the first two 20s in the cap can to function as 40.
     
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