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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    I'll verify when I get the amp in front of me again, but I think you nailed it.
    Looks like from the photo above that it has two 20s wired in parallel.
    I overlooked that earlier.
     
  2. Commodore 64

    Commodore 64 Friend of Leo's

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    Clip the wires to the cap. install some small modern filter caps using a tag strip or something. Nobody will know what's in there. Save money and the new caps have better specs.

    Sounds harder than it is, wish I had a pic to show ya. (Found one, the cap is under it thx jonrpick)

    [​IMG]

    And another from keithb
    [​IMG]
     
    Last edited: Aug 9, 2019
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  3. Skish

    Skish TDPRI Member

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    What’s the effect of the 40uF vs 20uF for that first filter cap? Will there be a difference in the sound?
    I’m just wondering about switching back to 20uF.
     
  4. Commodore 64

    Commodore 64 Friend of Leo's

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    Might be a little bit tighter bass with 40uF. Probably better hum filtering with 40uF than 20uF.
     
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  5. King Fan

    King Fan Friend of Leo's Ad Free Member

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    ^^^ Commodore 64 is right, of course.

    More broadly: 'Back to 20' isn't actually a thing. AFAIK it's not vintage correct for these amps.

    My BF VC needed tons of circuit / value restoration, so I looked through dozens of online photos of early-year VCs. That jumper connecting the first two caps was the way Fender did it, almost from the start, despite the original schematic. Fender is infamous for evolving their designs but not their drawings or even their tube charts; it's actually hard to find original VCs with just 20, though some end up that way after 'reverting to the schematic.' The Bronco schematic eventually showed the actual wiring.

    My guess is they designed the AA764 on paper after the original Champ, with its 16uF first filter, but quickly realized that 20 didn't give a modern sound, and that they were wasting a cap in their 20/20/20/20 cap cans.

    Many posts over the years have warned that 40 is 'too high' for the 5Y3 per its data sheet, but thousands of functioning AA764s say in reality it works fine. For that matter, a 40uF reservoir cap is a popular upgrade to tighten up the sound of 5Y3-recto'd tweeds...
     
    Last edited: Aug 12, 2019
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  6. zook

    zook Friend of Leo's

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  7. Commodore 64

    Commodore 64 Friend of Leo's

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    I use 47uF on 5Y3 all the time. YMMV.
     
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  8. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Yeah, even a Tweed Princeton was using 2x 16uf caps in parallel for 32uf at the first node.
     
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  9. King Fan

    King Fan Friend of Leo's Ad Free Member

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    OK, sorry, here's me:

    duty_calls.png

    Re the 5Y3, *most* data sheets don't actually cite a max. The 10uF figure originated with the first RCA data sheets, when big single caps were hard to find, and both that and the later 20uF figure *were in the table for 'typical operation.'* If max was really 10uF or 20uF, most 5Y3 amps, and all of them from the 5F2a onward, would be over-spec.

    The factors that can blow a rectifier tube are a complex combo of current, plate voltage, *and* high first-filter capacitance. Yes, a 30 or 32uF 'max' can be found, but as we're seeing here, experts routinely go as high as 40. No less an expert than @Wally tells me he runs 40uF in a 5e3 and gets notable bass tightening. I've also seen many other pros, here and on other forums, using 40 and even 47uF reservoir caps.

    But the long-term evidence that 40uF was Fender-approved and time-tested comes from actual '60s and '70s AA764s. Here's the Bronco schematic:

    upload_2019-8-15_9-32-39.png


    When did they start jumping the 2x20s? Fender never did anything just one way. I've seen a couple of virginal '66s with non-jumpered caps, which suggests they did build some to the early schematic. Now here's a clearly vintage '69 with the jumper.

    upload_2019-8-15_9-59-26.png

    Let's go earlier. Here's the original can in my '67 BF:

    FullSizeRender 2.jpg

    And... wait for it...

    Here's a '64. Conceivably re-canned, but also very possibly original.

    upload_2019-8-15_10-5-44.png
     
    Last edited: Aug 15, 2019 at 12:19 PM
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  10. Skish

    Skish TDPRI Member

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    Any chance you have more info on the heat dam? That concept is new to me, but I can see that I may need help to solder that can to the chassis.
    I’ll start searching around the forum as well...
     
  11. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Heh, suggested to me by a tech who does cap can replacement. He actually used a tuna can with both ends out wrapped with 1” adhesive weather strip, but I found I could cut two 1” strips off the edge of an 8” silicone hot pad and form a similar shape with duct tape, overlapping the ends to get the right diameter to sit down on the board with the wires all scooted back. The flexy shape was helpful in fit. Thought I had a pic, and kept the gizmo, but no luck finding either one.


    Sent from my iPad using Tapatalk
     
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