Silverface Vibro Champ: please help with voltage-tranny Q

Discussion in 'Amp Tech Center' started by trouserpress, Apr 6, 2018.

  1. trouserpress

    trouserpress Tele-Holic

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    I've just bought a Vibro Champ that did not come with the typical voltage selector switch of the export models (like my Bandmaster Reverb has). It lacks the Fender logo and the foot switch and has a 'continental' power cord. Otherwise it seems to be a complete and all original (umodded) USA amp from early 1973. The power transformer .....
    upload_2018-4-6_14-59-8.png
    does look original to me.

    I was prepared to install a new, propriate 240V transformer but to my surprise
    the previous (and electronically clueless) owner said he ran it by 240V and WITHOUT a step down transformer. Futhermore the amp's still equiped with the 125V-1A fuse!

    upload_2018-4-6_15-25-12.png

    Please take a look at this gut pic:
    upload_2018-4-6_15-18-37.png

    Could it be that Fender in 1973 used trannies with additional 'export-coils' that have been activated by some tech?

    What steps would an expert take to safely check out what I have here?
    By the way I'm no expert but am cautious and already have (with the help of you fabulous guys) successfully overhauled 2 Silverfaces.

    Any advice, comments, hints, ..., are very much appreciated!
     

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  2. elpico

    elpico Tele-Holic

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    I'd pull the tubes, leave this amp unplugged, and apply 6.3v to the heater winding (you can just jump it over from another amp). Either 120v or 240v will appear on the plug and you'll know where you're at.

    edit: I should probably be explicit that this will energize the mains and high voltage windings so behave as carefully as you normally would with a live amp. With the amp unplugged the chassis won't be grounded so be aware of that as well (you can clip it to your donor amp's chassis if you want it to be. There's no need to handle the amp at all though, just set up the jumpers and clip the meter leads to the plug before you switch anything on, read the number off the meter, then turn the juice off again)
     
    Last edited: Apr 6, 2018
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  3. RLee77

    RLee77 Friend of Leo's

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    After a few google searches...
    The transformer is a Schumacher A010020... which is the “optional” export transformer, likely with dual primaries for 110/220. Didn’t dig deeper than that, but I bet with some more searching online you could find a wiring diagram.
     
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  4. trouserpress

    trouserpress Tele-Holic

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    Thanks a lot elpico and RLee! I checked the transformer as you proposed. It's got to be that 'Schumacher' and it is properly connected for 220v. As I then plugged it into 220v I read 6.29v on the heater wire.:)
     
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  5. trouserpress

    trouserpress Tele-Holic

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    The cabin of my champ (without baffle, speaker and electronic compartment) weighs about 1.2kg. To me that's seems to be relatively light. Furthermore I can see structures shining through the paint that faintly look like solid wood:

    upload_2018-4-8_11-12-4.png

    According to the serial numbers (caps too) the amp is from early 73 or late 72. I wonder if the cabin is made of solid pine. Would that be normal or unlikely or rare .. or just wishful thinking?

    Please share your opinions!
     
  6. Dacious

    Dacious Poster Extraordinaire

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    That is likely a pine cabinet with pressboard baffle. Yes it looks like pine, but that looks like a dadoed in baffle. I was under the impression that construction came later. My 78 had that.

    The earlier amps had a screwed in baffle.

    The multivolt selector would be pretty big in a Champ, I doubt there were any made like that. I had an export Vibrolux Reverb. The selector was massive and the primary wires a rat's nest.
     
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  7. trouserpress

    trouserpress Tele-Holic

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    Yes, you're right, it's pressboard and glued in. I confused 'baffle' with 'grill cloth panel'

    Same with my other two silverface amps. But these have no solid pine on them.

    Thanks for the info.
     

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  8. Dacious

    Dacious Poster Extraordinaire

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    On a lot of later Silverface amps the top and bottom were pine and the sides pressboard. That's what my Princeton and Vibrolux Reverbs had. I think my 78 Pro Reverb was, too but it's a long time since I owned it
     
  9. peteb

    peteb Friend of Leo's

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    That PT is listed on the bronco schematic as a dual primary alternate.





    image.gif
     
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  10. trouserpress

    trouserpress Tele-Holic

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  11. peteb

    peteb Friend of Leo's

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    Yes, a Bronco is a vibro champ.

    And the bronco schematic is the de facto schematic for the Silverface vibro champ.
     
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  12. trouserpress

    trouserpress Tele-Holic

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    Bronco = SF Champ. Right, that's what I was already guessing but had not been sure about.

    Looking at the schematic I see the lead between speaker terminal and NFB resistor marked as 'yellow'. In my amp it is red!! Oh no - I'll have to fix that ......not, just kidding.

    De facto I plan to put a 2x6way rotary 'Soul switch' into that lead to give me different degrees of negative feedback combined with some capacitor filtering. Thing is, although I read quite a few threads I'm still clueless about which 5 additional resistance values to choose for the Champ. Tone stack design and speaker impedance seem to play an important role here.

    I will read some more but may have to put forth that question in a new thread (nudge, nudge :rolleyes:).

    Thanks to y'all!:)
     
  13. elpico

    elpico Tele-Holic

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    You can use a pot as an adjustable resistor, just turn to where it sounds good.
     
  14. trouserpress

    trouserpress Tele-Holic

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    Yes, but such a switch lets you..
    .. find and reproduce specific amounts of nf much easier
    .. add nf
    .. add different capacitors to eliminate unwanted side effects in the region of low amounts of nf
    and provides firmer contact (although I'm not sure if this is relevant).

    Playing the amp with open guts and a provisional pot ..... probably difficult and calling for more than one guitar player and more than one set of ears to reveal the 'good' intervals.

    I guess I'm gonna do as you say: Install a 50k pot, put it all together and take my time playing with it - making pencil marks on the chassis.
     
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