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.
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.
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.
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.
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.
~Tom
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.
~Tom
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.
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!
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.
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.
Maybe because cathode bias performs worse than negative grid bias? I have not 'sunk' money into a negative voltage supply. I get my transformers wound and it doesn't cost any more money to have a bias winding.
With various protection measures in place, negative grid bias is the best performing way to bias valves, especially with DC coupled followers driving the grids.
Obviously balance is important, though I'm not using toroidal output transformers so it isn't absolutely necessary to balance down to the last mA, and it is nice to be able to independently set the current of each valve.
With various protection measures in place, negative grid bias is the best performing way to bias valves, especially with DC coupled followers driving the grids.
Obviously balance is important, though I'm not using toroidal output transformers so it isn't absolutely necessary to balance down to the last mA, and it is nice to be able to independently set the current of each valve.
There's no doubt that this is the way most people do it,
and it can be done safely with appropriate protection.
I'm wondering about the wisdom however, of even having caps at all in a driver coupling, with the further design limits and constraints that imposes.
I think there must be far better ways to get grid BIAS stability than another power supply. Why can't driver stages be designed to give stability?
But I sympathize with the many who struggle with the problems and solve them in their own way.
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