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Old April 27th, 2012, 02:17 AM   #1 (permalink)
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Converting SFTR bias...

At last I'm ready to do a cap job on my 1970 SFTR, and while I'm at it I want to change the bias balance to a "proper" bias adjustment circuit.

However, I have found two apparently different ways of doing this, and I don't understand which would be best. Perhaps someone here could comment.

At the moment it looks like this (from Hoffmans website)


And after conversion (using Hoffmans guidance), note the cap from the pot to ground is removed.


So all good so far, however in Gerald Weber's book, he shows the cap is still present, but moved to the lower right tab on the pot (see fig 2 here)

So, what would the practical and sonic differences be between these two approaches?

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Old April 27th, 2012, 11:35 AM   #2 (permalink)
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I am going to think that if you leave the first resistor in that chain of bias voltage...the 1K resistor on the rectifier/bias board to the left....in circuit, then you are going to want to follow the circuit that WEber shows. IF you want to revert to BF specs throughout, you are probably going to want to change that first resistor to AB763 BF specs...and wire the bias pot as in the AB763 schematic. Weber's approach is simpler, it seems. The main point is to arrive at the end with an appropriate bias voltage adjustment range. I don't think there is any sonic difference as long as the two circuits yield the same range of voltage adjsutment.
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Old April 27th, 2012, 12:16 PM   #3 (permalink)
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Hi Wally,

Thanks for clearing that up, I'll go with Weber then.

Thanks again!
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