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Old 20th April 2012, 01:28 PM   #1
bigwill is offline bigwill  United Kingdom
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Default Wirewound bias pot problem.

Hi there,

I've tried some military NOS wirewound 20k potentiometers for bias in an amplifier I'm building. They appear to be of very good quality and extremely well made. Unfortunately, I encountered some oxidation along the tracks of one of then and the amp temporary lost bias as I was adjusting it, which was disconcerting. A few back and forth turns of the pot (with the amp off ) cured this, but on another of the pots, the extreme 'off' position causes the wiper to go open.

Obviously, a 100k resistor from the wiper to the negative supply will solve this. If I install 'catch' resistors and give them a good spray with some contact cleaner, do you think I'll have anything to worry about? I am not going to be the owner of this amplifier.
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Old 20th April 2012, 01:43 PM   #2
lexx21 is offline lexx21  United States
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Can you open the pot to see inside or is it open already? If so, spray it out with Deoxit and clean it well. If it is going to be sold, I wouldn't "jury rig" something like this. It could come back to haunt you in various ways.
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Old 20th April 2012, 02:07 PM   #3
bigwill is offline bigwill  United Kingdom
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I have a bunch of these, I think I'm going test each one with a scope and a voltage, and cherry pick the ones that have absolutely no noise along their travel. And clean them too. And add the catch resistors.

I just took them out of the amp and tried this, one of them clearly doesn't work properly. Don't worry, I'm meticulous and wont send out an amplifier with a dodgy biasing circuit.
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Old 20th April 2012, 04:46 PM   #4
tomchr is offline tomchr  Canada
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Contact cleaner like Deoxit Faderlube is your friend. It's pretty common for pots (wirewound or otherwise) to develop some oxidation along the track. On older pots, I've also noticed that the wiper will sometimes go open at the extremes of the track.

In military and avionics designs, these things are accounted for by electronically limiting the adjustment range of the pot and providing a "default" setting should the pot fail. This is done with resistors from the wiper to either end of the pot as I recall.

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Old 20th April 2012, 06:03 PM   #5
bigwill is offline bigwill  United Kingdom
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Just sprayed some contact cleaner in them and it seems to have cured even the worst example I found, phew!
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Old 20th April 2012, 07:55 PM   #6
DF96 is offline DF96  England
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'Catch' resistors should always be included, although many circuits omit them. No pot can be entirely trusted.
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Old 20th April 2012, 08:04 PM   #7
bigwill is offline bigwill  United Kingdom
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Originally Posted by DF96 View Post
'Catch' resistors should always be included, although many circuits omit them. No pot can be entirely trusted.
Indeed, I was going to add them anyway, but I got tempted to test the amp before I installed them. In some ways it was lucky because it highlighted the problem.
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Old 20th April 2012, 08:39 PM   #8
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I had exactly this problem recently on an amp I built several decades ago. IMO, wire wound pots are not the best choice for biasing. I had installed catch resistors, so it was no big deal, but the amp was running misbiased for way too long before I noticed how warm it was. I don't know the best pot to use in this application, but I'd now put wire at the bottom of the list. Save those for something where they get turned every now and then. Hmmm... I could make a pretty good case for setting bias, measuring the resistance of the pot, and soldering in a fixed resistor or pair, as necessary. Problem solved!
I may be barking up the wrong tree, but at least I'm barking!
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Old 21st April 2012, 11:58 AM   #9
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I use Bourns wirewound resistors and a "catch" resistor to the negative bias supply (not to GND!). No problem so far.
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Old 22nd April 2012, 07:43 AM   #10
nazaroo is offline nazaroo  Canada
Join Date: Feb 2012
There really should be no BIAS adjust for balance.
Its a cheap and suspicious way of balancing the current through the OP transformer (and thats all that matters).

There should be a direct adjustment to match current,
and an indicator to always show a match or expose an imbalance, right on the front of the amp.

Instead of a pot on the negative bias, putting the tube at risk,
there should be a variable resistor in the cathode circuit, with catch-resistors.
That way, even if there's a pot failure,
the only thing that happens is the current is shut down a little,
and the BIAS voltage remains robust and keeps its integrity.

Why go to all the trouble of having fixed bias, with all the added expense of another power supply etc.,
then cheapen the whole operation and put everything at risk,
by trying to use the bias to make crude adjustments to current balance?

Even better, would be to redesign or select the proper HV xformer based on self-bias, eliminate the fixed-bias supply,
and sink the money somewhere more effective,
like an automatic quiescent current-balance circuit.

Using the BIAS to adjust transformer balance is ad hoc,
and a stupid hack.
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