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FDP Forum / Amp Mods, Repairs, and Projects / What is involved in "Blackfacing"

swampyankee
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olde New England

If you can't play good, play loud
Aug 13th, 2019 11:31 AM   Edit   Profile  

..and I'm not talking about makeup that'll get you labeled as a racist.

When I got my '70 Twin I sent it to a well-known local tech for cap replacement and a general tune-up. Among other things, he "reworked bias to blackface bias", and also changed the value of a couple caps to 80uf/500v saying the ones in there were the "wrong value and caused excessive bass".

I may be a pretty good solder-jockey, but I'm not electronics-savvy by any means. But in general, what constitutes a total change to blackface spec on a circa '70 Twin, and are these changes any step toward that spec?

Leftee
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VA

Aug 13th, 2019 12:47 PM   Edit   Profile  

I think that, by the time of the production of your amp, the “biasing” was only a hum balance pot. It was not a bias adjustment in the traditional sense. At least that’s the way I remember it.

pdf64

UK

Aug 14th, 2019 09:11 AM   Edit   Profile  

Yes, I think all SF TRs only had balance adjustment for power tube bias, rather than the shared 'overall' bias control of the TR AB763.

Need to be more specific on the TR model you've got, I think there were about 3 or 4 SF variants, the last being the 135 watt.

For a given control setting, they're generally more bassy than a TR AB763 because of the bass pot taper (30& audio rather than 10%) and the coupling cap value between pre and power amp (10nF rather than 1nF).

swampyankee
Contributing Member
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olde New England

If you can't play good, play loud
Aug 14th, 2019 10:00 AM   Edit   Profile  

"Need to be more specific on the TR model you've got, I think there were about 3 or 4 SF variants, the last being the 135 watt."

As said, Mine's a 1970. Non-master volume. I believe it's the AA769 circuit.

FDP Forum / Amp Mods, Repairs, and Projects / What is involved in "Blackfacing"




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