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

swampyankee
Contributing Member
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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
Contributing Member
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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.

Steve Dallman

Merrill, Wisconsin

Age is just a number...mine is big
Oct 15th, 2019 07:04 PM   Edit   Profile  

Your amp had a bias balance, which balances out the current draw of the two pairs of power tubes. He changed that to an actual bias adjust. Easy mod. I prefer to leave the bias balance and just add a bias adjust....best of both worlds.

As for filter caps...more capacitance increases headroom and prevents sag. Sag is not a concern in a Twin Reverb. Most can’t drive that amp to sag levels. I don’t see increased capacitance in a Twin an issue.

Steve Dallman

Merrill, Wisconsin

Age is just a number...mine is big
Oct 15th, 2019 07:18 PM   Edit   Profile  

Blackfacing is not a difficult thing. It involves changing a few resistors in the PI circuit to increase the impedance, changing the bias circuit from a balance to a bias adjust (or keep the balance and add an adjust), and remove a couple snubber caps on the power tubes that were put in to stop parasitic oscillation that the sloppier wire layout might cause. (After removing the snubbers, let the amp run and monitor the temp of the chassis. If it oscillates, the temp will climb. If it stays the same as before, you’re fine.)

There are a couple other things but they are not important. There was a SF change to the reverb but it’s considered an improvement. There may be filter cap changes...not important.

Leftee
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VA

Oct 15th, 2019 07:34 PM   Edit   Profile  

Nice to see you back, Steve!

SoK66

USA

We had the hit but Van got the money
Nov 8th, 2019 06:59 AM   Edit   Profile  

I'll add that there are two basic SF circuits. The later one, post late '69 on;y requires what's been noted above. The early one, sort of June '68, is a dog that requires what has been mentioned, plus removal of some capacitors on the power tubes.

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




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