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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.
Follow Ups:
Ledgendre - I think my original post wasnt clear enough it seems .
Firstly , I dont think I mentioned any term " Strapping " or meaning parallel ..but if I did , I appologise for the confusion .
However ..if its resolution only that this is for , well then thats very simple to cure with a 10 turn pot ( Spectrol type ) like ive used . They work brilliantly . I realise the fixed resistor allows you to place the standard pot somewhere near the ideal range , so adjustment can be found without being at the end of the pots electrical range ....
I was just checking that it wasnt for some other purpose - hence why I asked . For instance , I couldnt see why the pot wasnt a 10 turn linear spectrol with approx 50 k ohms . Ive experimented with a standard 270 deg linear pot , and found the sensitivity not to my liking . Its far to easy to move it well off target . However , a 10 turn pot , mal adjusted by one turn , doesnt make the setting wildy different . Ive proven this , as one amp I have fitted 10 turn pots to , and the 2nd has for now , the standard style 270 deg pots . I'm definately changing these out for the 10 turn ,, and accompanying them with the Vernier adjusters to boot . They , at a glance , can be checked , and also referenced for where & how much shift is happening over time . I m just applying my common sence to the controls , but at the same time , asking to clarify the original intentions - just to be sure . Thankyou
Stu
-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.
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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