Notes on fixed bias operation

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Posted by Legendre (A) on September 21, 2006 at 07:12:56

In Reply to: Re: Bias range limiting posted by tubevibb on September 21, 2006 at 05:10:29:

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.

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Follow Ups Full Thread

Topic - Bias Potentiometer resistance path - Satelliteman 22:21:53 09/19/06 (11)

Re: Bias Potentiometer resistance path - Legendre 04:39:51 09/20/06 (6)

Re: Bias Potentiometer resistance path - Satelliteman 13:04:52 09/20/06 (5)

Re: Bias range limiting - Ralph 14:44:13 09/20/06 (4)

Re: Bias range limiting - tubevibb 05:10:29 09/21/06 (2)

Notes on fixed bias operation - Legendre 07:12:56 09/21/06 (1)

Re: Notes on fixed bias operation - tubevibb 08:21:52 09/21/06 (0)

Re: Bias range limiting - Satelliteman 15:13:36 09/20/06 (0)

Re: Bias Potentiometer resistance path - tubevibb 03:57:23 09/20/06 (0)

I understand that a fixed resistor around the pot is used so that if the pot fails... - jeffreybehr 23:44:13 09/19/06 (2)

Re:If the pot fails, the amp shuts down in failsafe mode. - Ralph 09:20:11 09/22/06 (0)

Re: I understand that a fixed resistor around the pot is used so that if the pot fails... - Satelliteman 00:34:55 09/20/06 (0)

Follow Ups

Notes on fixed bias operation - Legendre 07:12:56 09/21/06 (1)

Re: Notes on fixed bias operation - tubevibb 08:21:52 09/21/06 (0)

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