Re: Bias Potentiometer resistance path

Posted by Legendre (A) on September 20, 2006 at 04:39:51

In Reply to: Bias Potentiometer resistance path posted by Satelliteman on September 19, 2006 at 22:21:53:

Stu (and others),

The fixed resistor is not strapped across the pot, it's in series with it (to ground).

The reason for the resistor should be fairly obvious - it limits the range of the bias control to the useful region (you'd never want to set the grids at 0 volts!), and improves resolution on the control. Since all pots have roughly the same 270 degrees of range, you can see that a pot dropping 25V has twice the control resolution as one which is dropping 50 volts. Since the bias control doesn't really need that much range, we can gain resolution.

Also, it should be noted that if the bias pot or tail resistor open up, the bias voltage defaults to full negative - putting the output tubes in cutoff, as opposed to allowing a runaway condition. Same situation for the V4 driver tube - if this tube burns out, opens up or otherwise stops conducting, the same cutoff condition results.

While nothing is perfect, the bias circuit in the Atma-Sphere amps is inherently fail-safe under most all fault conditions.

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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

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

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