Need help with Fender SF Vibrolux Reverb repair

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by muntizeppa, Apr 10, 2015.

  1. muntizeppa

    muntizeppa Member

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    Hi, I have a SF mid-70s Vibrolux Reverb that I've pulled from my grandparents garage. The original 5U4GB rectifier tube was missing, so I bought a new one. I plugged it in and turned on the amp. I strummed some chords but nothing was coming out from the speakers not even a silent hiss. Then, after nearly two minutes, the rectifier starts to red plate followed by a small blue spark, I switch the amp off immediately and noticed the slo-blo fuse was fried. I guess I've busted the rectifier too.

    Since it's over 35+ years old, I'm gonna recap all the electrolytic caps. The dead cathode caps might've not supplied the correct amount of voltage to the rectifier, which in turn, destroyed it.

    So here's what I'm planning to do:

    Under the dog house, there are five 16uF/450V caps. I'll be replacing them with F&T 16uF/475V caps. Also there's a 1 Watt 2.2K carbon comp resistor which should've been a 1 Watt 4.7K. That's what the schematics say.

    Inside the chassis, there are five small 25uF/25V caps, One in pairs, 3 single and the other is connected to the vibrato jack and one of the 12AX7 pins. This isn't how it was drawn in the schematics and Fender seriously need to update the drawing. Gonna replace them with Spragues of the same value. There's also a 5–50V cap, and according to the schematics it's also a 25uF/25, but Imma replace it with a 25uf/50V. And there are two caps (I think it's popularly known as cathode caps) which read 80-75V, and in the schematics, it's a 50uF/70V. Gotta replace them with the recommended 100uF/100V.

    Once done, a new rectifier would be plugged in and hopefully it should be in fighting condition. I'm guessing that a bad rectifier could be the cause for the amp not emitting any sound. And the cathode caps causing the rectifier to fail.

    I'm hoping to get advice from anyone who has experienced with the same particular amp or have experienced the same symptoms. I was also informed that there could be something else that needs to be investigated. By the way, I live in Brunei, and I usually order the parts from tubedepot.com.
     
  2. tlpruitt

    tlpruitt Member

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    Take it to a good tech to get it serviced properly. You will save money and time in the long run.
     
  3. pdf64

    pdf64 Member

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    Old electrolytics which have been sitting unused for years are best being slowly reformed back to their working voltage, using a current limited source.
    Hit them with full working voltage and they'll likely pass dc, overheat and short / blow out.
    I strongly suggest you build a light bulb limiter, eg http://ampgarage.com/forum/viewtopic.php?t=20341
    What is the type / model reference of your amp (eg tube chart, back panel)?
     
  4. RiftAmps

    RiftAmps Member

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    This comment scares me. Take the amplifier to a qualified technician before you hurt yourself.
     
  5. schmidlin

    schmidlin Member

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    He lives in freakin' Brunei!! :waiting

    Sounds like the OP has a good handle on things but I must say living in a garage can tear apart the best of amps. Much more so in a place that has 4 true seasons, not sure where yours was stored.
     
  6. swiveltung

    swiveltung Member

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    1) make sure you got the rectifier tube in properly... sometimes they can go in rotated wrong.
    2) Check the Screen resistors on the power tube sockets, do they look physically burned or broken?
    3) yes replace the filter caps.
    4) the Cathode caps wont do that in my experience.
    5) you may be working to the wrong schematic for your amp, IIRC that 5uf 50 cap is in some models...
     
  7. muntizeppa

    muntizeppa Member

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    The amp looks like it's in good condition. Very minor rust. Tolex needs polishing. Grill cloth intact and aged nicely. The speakers are working, but the cardboard cone had torn. I've already set my eyes on some vintage Webers.

    The technicians here would replace bad tubes and recap the doghouse at most. Finding a dedicated one is close to impossible. It's the reason why I decided to get this beauty functional myself. But it seems I'm hitting a dead end in every direction I turn.

    Anybody know what might cause the rectifier to red plate? Are that any good after that? I've also observed at all the related schematics; AA964, AA864, AB568 & AA270. The drawings that the small electrolytic caps have three in pairs and one single. But based on my research, in reality, most Silverface VR have only one in pair. The rest are single and one is hidden at the back.

    My amp's serial is A32061, which puts it in the year 1975. And anybody know how to post pics? It would be better if I could provide you guys with some visuals.
     
  8. jay42

    jay42 Member

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    sounds like there is a story here...hip grandparents too. What's the AC voltage there and does it match the amp?

    You don't polish tolex, however, Johnson's Baby Oil helps.

    OP, there are tons of safety disclaimers here and elsewhere on the Internet. Guitar amplifier electrolytic caps are sort of like guns. If you always assume they're loaded, you won't shoot yourself.

    Don't get hung up on Fender schematic differences. They were created for repair technicians, not the manufacturing process. Keeping the schematics up to date wasn't that important. I like blackface spec amps, however, that's just an opinion. Get your amp up and running first...without any 6L6s loaded. See how the supplies function first.
     
    Last edited: Apr 12, 2015
  9. smolder

    smolder Gold Supporting Member

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    The only time I've seen a rectifier do that is when the alignment pin was worn and the tube inserted incorrectly.
     
  10. fezz parka

    fezz parka Member

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    +1.
     
  11. muntizeppa

    muntizeppa Member

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    The voltage here is 220-240V. So I set the amp in the middle at 230V. I've not cleaned the tolex yet and +1 for your advice. I'll do just as you recommended.

    There's a groove at the rectifier which allows it to be inserted in only one position. Could this be a bad transformer? The input transformer has a small brown color in one area. Probably the black paint peeling off.
     
  12. pdf64

    pdf64 Member

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    As described, old ecaps that lie dormant for a long time may pass damaging levels of dc.
    The rectifier may be ok but you need a 'known good' spare for every tube type in the amp anyway, so consider buying another.

    The voltage selector should be set according to the upper end of the expected voltage range, so 240 in this case.
    Always best to check the line voltage.

    That's possible but unlikely.
     
  13. muntizeppa

    muntizeppa Member

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    I like this +1
     
  14. Steppin' Wolfe

    Steppin' Wolfe Member

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    muntizeppa wrote: "And there are two caps (I think it's popularly known as cathode caps) which read 80-75V, and in the schematics, it's a 50uF/70V."

    Those are the biasing capacitors.

    +1 with RiftAmps on the ''find a good tech' suggestion. Imho, that is too much of an amp for a beginner to start learning on.

    Note: The placement of those 25mfd/25V cathode bypass caps.....at a certain point, Fender moved those two caps' contacts for ground to the back panel. They ran out of room on the board for them...probably about the time they instituted the pull boost???
    I know that the MV amps have this placement. YOur amp doesn't have a MV....the VR's never got the 'big changes' that other 6L6 Fender guitar amps got like MV or ultralinear OT....but they may have instituted a uniform assembly process whereby all amps got those caps moved to the back panel for grounding purposes. Fwiw, the change is not shown in a schematic but in a layout. Increasing the size of that 5mfd/50V cap will increase the gain at that point in the circuit....and that is in the vibrato curcuit.
    Nothing 'right or wrong' about either value...or something in between...just different.

    Nice amp...good luck with it.
     
  15. muntizeppa

    muntizeppa Member

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    Thanks, will do just that.

    Thanks for spotting that the bias caps is a 5uF/50V. I'm guessing that the 80–75V means 80uF/75V. What might be a good replacement for them? 25uF/50V and 100uF/100V is what most people recommend. But I thought messing with the value is a bad idea as they should be exact or close to exact.
     
  16. Baxtercat

    Baxtercat Member

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    Go for a 100V bias cap in that position. 50V bias caps blow out sometimes.
    It's essential to get the positive/negative orientation right.
    [​IMG]
     

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