BlackFace your SilverFace Twin Reverb

Discussion in 'Tech Tips' started by Chocol8, Aug 12, 2011.

  1. Chocol8

    Chocol8 Well-Known Member

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    Someone asked me to help with what they need to convert a SF Twin into something that actually sounds good, a BlackFace with the AB763 circuit. Other than these circuit changes, a BF may or may not have better speakers and that is something that should never be ignored when modding any amp. Speakers can make a bigger difference than most of the circuit changes, but that is another topic.

    If all works out there will be two PDF files that show the circuits with my annotations and then two that are in original condition attached to the post.

    IF YOU SEE SOMETHING I MISSED, PLEASE LET ME KNOW AND I WILL FIX IT!

    On to the circuit changes.

    Changes from a SilverFace 100w Twin Reverb with push pull MV to an AB763:

    I circled the various locations that need to change on the circuit diagrams so you can see the before and after. The layout of the drawings is different but the electrical connections and the layout is essentially the same. I also put the circles on the AB763 layout so you can see where physically the different components sit. The two blue circled resistors not on the layout are contained in the can with the big filter caps underneath the chassis.

    Master Volume –Remove with extreme bias, it sucks tone and in general just sucks.

    Magenta: This is the master volume and the push pull switches. Just cut this stuff out.

    B+ -This affects the high voltage to the preamp tubes and their bias

    Medium Blue: Change this resistor from a 2.2k to a 1k ohm rated for at least 1 watt ***with filter caps
    Dark Blue: Change this resistor from a 10k to a 4700, again 1+w rating ***with filter caps
    Light Blue: Change the two 47k resistors to a 100k and 82k, both +/- 5% or better

    Phase Inverter -This affects the frequencies cut off before the signal is split and sent to the power section.

    Red: Change the two 330k resistors to 1 meg, and the 33k to a 22k
    Dark Red: Change the coupling cap from .01 to .001

    NOTE: The cap and resistors work together to make a filter. The AB763 cuts off bass below 80 hz and the SF design cuts base of at 40 hz. The low E on a guitar is roughly 80 hz and one octave down would be 40 hz, so the SF passes an entire additional octave. The extra bass can burden the amp and speakers and detract from the sound, but you may want it or some of it to pass through if you drop tune. By choosing different values for the resistors and the cap, you can set the cutoff point to be 40 hz, 60 hz, 80 hz etc. depending on your desires.

    Reverb Bias of the reverb tube -This will run the tube cooler and keep you from having to replace the reverb driver tube so often.

    Dark Green: Change the 470 ohm resistor to a 2.2k and add a 25/25 electrolytic bypass capacitor.

    Bias -This sets and allows you to adjust the power tube bias.

    The AB763 design lets you change the bias of all four tubes up or down. The SF design lets you adjust the balance, so if you raise one pair, the other pair drops and vice versa. They both suck. My advice is to wire this so you can set the bias of each pair independently, basically two of the AB763 design. This way you can buy matched pairs of power tubes and then get all 4 properly biased. If you go with the straight AB763 design you need to buy matched quads to get them all biased, but these sets are available today. With the SF design you need dumb luck to get the tubes biased right since any adjustment you make to get one pair biased will move the other pair in the opposite direction which is likely to throw them out of bias. DON’T KEEP THE SF DESIGN.

    Orange: Do something here to allow you to set absolute bias rather than balance, or do a dual setup, or do a hybrid with an absolute adjustment and a balance. Just change it, it’s hard to make it worse.
    Yellow: Change the 68k resistors to 220K 5%. This ties in to what you do above, but I point this out because some people believe the 68k resistors reduce blocking distortion and therefore they like the SF design better. It’s up to you what you choose here.


    Modified PDF's Note that Page 2 of the AB763 schematic shows the physical chassis layout. If you pull out your chassis (CAREFUL, HIGH VOLTAGE EVEN WHEN UNPLUGGED) you should be able to locate all of the circled components on the circuit board.

    http://www.filedropper.com/twinreverbsf100schem-chocl8

    http://www.filedropper.com/twinreverbab763schem-chocl8

    Originals:
    http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/fenderamps/twin_reverb_sf_100_schem.pdf

    http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/fenderamps/twin_reverb_ab763_schem.pdf
     
  2. gilmourjunkie

    gilmourjunkie Well-Known Member

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    You rock, thanks for this!!
     
  3. Chocol8

    Chocol8 Well-Known Member

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    No prob, I think I am done with edits now, until someone finds an error.

    [hr]
    You also asked about brands and so forth. There are a lot of opinions on these things, but I will say that IMHO older is not always better.

    The old carbon comp resistors added noise and not the good kind, but people today pay extra to get them. I prefer to use the more accurate, more reliable and quieter alternatives. Note that Fender speced them all at 1/2 w and +/- 10% unless otherwise noted, and some of the ones you are changing are the otherwise noted ones so pay attention to that.

    For the coupling caps in a Fender I generally use Mallory 150's and for the electrolytics Spragues. I avoid ceramics and paper in oil for caps, especially within an amp like the Twin.
     

  4. dragonfly66

    dragonfly66 Guest

    Thanks for writing this up. The filedropper files do not work, do you have those PDFs somewhere else or could email them to me.

    I have 1976 MV that I want to blackface. Yours is the only good resource that I have found for removing the MV. I have found BF conversions for the non-MV but only this one for the MV.
     
  5. Chocol8

    Chocol8 Well-Known Member

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    I sent you a PM. Send me your email addy and I can forward on the files.
     
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  6. LoFandy13

    LoFandy13 New Member

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    Can I get those files too?
     
  7. LoFandy13

    LoFandy13 New Member

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    Can I get those files too?
     
  8. PsychoCid

    PsychoCid Well-Known Member

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    Hey this thread is 6 years old... And @Chocol8 has disappeared from the board.
     
  9. voodoorat

    voodoorat Well-Known Member

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    it says he last logged in last november, maybe your mention will summon him though.
     
  10. PsychoCid

    PsychoCid Well-Known Member

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    Nah, there have been many attempts to summon since then.
     
  11. uwmcscott

    uwmcscott AGF Survivor Champ

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    Mention a gold-foil-face Twin Reverb version and see what happens.
     
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  12. rmfroyd

    rmfroyd Well-Known Member AGF Registered Dealer

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    Something about a dustup concerning healthcare I think. I am pretty sure he was told to fuck himself. That would probably end it for me as well.
     
  13. LoFandy13

    LoFandy13 New Member

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    Does anyone have clear pics or a layout diagram that includes the push/pull master volume for a 1974 twin?
     
  14. PsychoCid

    PsychoCid Well-Known Member

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    Hm.
     
  15. Tyrannocaster

    Tyrannocaster Well-Known Member

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    You can blackface it all you want, but if you don't change the PT (and OT too, depending on which one you have) it will always be a Silverface. I have a '69 Silverface Dual Showman Reverb (it's identical to a Twin, just the head - mine is in a Twin cab w/ 2 12" speaks) with Blackfaced circuitry but the original trannies. My buddy has an original '65 Twin. Through the same speakers his sounds better to both of us. The Silverface is louder, but more sterile. Higher voltages are part of it, but probably not all. On its own, my amp sounds pretty good, but not next to his.
     
  16. LoFandy13

    LoFandy13 New Member

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    I' not trying to blackface this amp. I want to return it to original operation.
     
  17. Tyrannocaster

    Tyrannocaster Well-Known Member

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    Go for it. I would have left mine original too, but I got it the way it is.
     

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