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In Reply to: Re: Bias Potentiometer resistance path posted by Satelliteman on September 20, 2006 at 13:04:52:
-is set up with the 25K pot and the accompaning resistor. In the old days we had a much wider range on the control which occasionally lead to damaged tubes. Since there was no need for that kind of range, we limited it to a more useful range, which had the side benefit of offering very good resolution on the part, making the bias very easy to adjust. The Bias tends to be very stable, so multi-turn pots are a useless extravagence.
Follow Ups:
JUST SET UP FIXED BIAS! Is that way to go?
The fixed bias is a convenience measure, and has proved to be very popular. Even though setting bias on an Atma-Sphere amp is quite trivial, when someone sees a chassis with 8, 14 or 20 power tubes, and they are told it is manual bias, they tend to flinch a bit.The new fixed bias arrangement also allows us to replace the old DC ammeter with an AC VU meter & range switch. In low range, 0VU represents 1 watt; in the high range, 0VU represents full power.
If you wish to convert the amp to fixed bias, warm the amp up fully, set the bias somewhat conservatively (say 500mA for an M-60) and then shut down. Measure the combined resistance of the bias pot & tail resistor (removed from circuit), then replace both with an equivalent resistance. You'll probably have to parallel a couple of resistors to trim the exact value. Depending on your B+ transformer, the output grids will end up somewhere in the range of -60 to -65V. Obviously, standard high-voltage safety practices must be observed when doing the work.
Make sure you have fairly new power tubes in the unit when doing this, else the bias will be too high when you later install a new set.
What is output transformers va. An is it good to pulldown v from 155v to 138v. My point is it made the amps sound better to go from 500va too 1kva?"
Yes - very stable indeed . Thx Ralph .
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