Silverface Twin Balance to Bias Mod

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Re: Silverface Twin Balance to Bias Mod

Postby pdf64 on Sun Feb 07, 2016 4:31 pm

jaywalker wrote:I'd be inclined to replace both of those white 50uF/70v bias caps while you're in there. (looking at aa270 schemo).
(Positive to ground) and you may end up deleting one after conversion. I like the suggestion to keep the balance.

Just to note that I think it's the later master volume, pull boost model http://bmamps.com/Schematics/fender/twin_reverb_sf_100_schem.pdf
So the bias supply caps may be 80uF 75V.

The pull boost circuitry / wiring on those can degrade the regular tone a little; in particular that long white wire from the reverb drive RCA to the pull boost switch on the master. You can assess that for yourself by disconnecting it at the RCA socket (and turning the pull boost off!).

The 120pF cap on the master volume can add an unpleasant edge to the tone, especially at lower settings, so consider checking if you prefer that lifted too.
My band:-http://www.youtube.com/user/RedwingBand
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Re: Silverface Twin Balance to Bias Mod

Postby Jamie on Sun Feb 07, 2016 6:04 pm

Yes, it was the pull master volume boost model. That was removed long ago by a local tech. Thanks!
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Re: Silverface Twin Balance to Bias Mod

Postby pdf64 on Sun Feb 07, 2016 7:17 pm

Jamie wrote:Yes, it was the pull master volume boost model. That was removed long ago by a local tech. Thanks!

The photo linked to shows that wiring still seems to be in place http://s1233.photobucket.com/user/bfmcgee/media/sft_board_zpsukogb149.jpg.html
My band:-http://www.youtube.com/user/RedwingBand
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Re: Silverface Twin Balance to Bias Mod

Postby Jamie on Sun Feb 07, 2016 8:16 pm

Hmm, not sure which wire that is. I'll get back to this later to refresh when I'm ready to make the fixes. Thanks again.
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Re: Silverface Twin Balance to Bias Mod

Postby supershifter2 on Thu Aug 18, 2016 10:31 am

I know a guy that converted his silver face twin to the AB763 bias circuit. I am going to convert my 71 twin reverb to it. Its really simple to do. Here is the AB763 and the 100 watt with boost schematics.

http://el34world.com/charts/Schematics/ ... ematic.pdf

http://el34world.com/charts/Schematics/ ... layout.pdf

http://ampwares.com/schematics/100W_Mas ... in_Rev.pdf
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Re: Silverface Twin Balance to Bias Mod

Postby Jamie on Thu Aug 18, 2016 5:33 pm

Thanks for the refresh on this, I need to get to this mod.
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