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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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In Reply to: Re: A NEW Dynaco ST-70 posted by Janos on April 6, 2007 at 21:47:49:
Janos,You are right that the original bias supply is not a tight circuit. With the VTA driver board that I use on the new amps that I build the bias supply is ON THE BOARD and not centered around the 7 lug terminal strip. The original bias supply and those 10K potentiometers didn't really give enough range and as the tubes wore they didn't have enough adjustability. On the old amp with the old bias system the trick was to RAISE the value of R1 and LOWER the value of R2. Joe has some bias expanding instructions on his web site but what I have found is that if I parallel a 27K ohm resistor with the 10K on R1 you get about 7300 ohms and if you parallel a 7.5 K resistor with the original 10K on R2 you get about 4285 ohms. With an "effective" 7300 ohms for R1 and 4285 ohms for R2 you get a wider bias range and can more easily bias tubes that have some time on them. This is a little more range than the instructions on Joe's web site but works well for me on a customer's amp especially if the selenium rectifier has been replaced with a diode.
Bob Latino
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- Bias issues on the ST-70 - Bob Latino 06:48:31 04/07/07 (0)
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