Bias pots

[BobD]

I’ve been using standard Alpha chassis mounted 25k pots for bias controls and I think that vibrations are making them drift.

Does anyone else have this problem ?

Is there a 25k linear chassis mounted pot available that won’t move under vibration – maybe has detents or something ?

[Normster]

Maybe try multi-turn pots like Komet uses? Mouser has a 10-turn Bournes for about $10.
http://www.mouser.com/index.cfm?handler=displayproduct&lstdispproductid=274924&e_categoryid=315&e_pcodeid=65215

[BobD]

Thanks for the tip.

What are "10 turn precision" pots anyway ?

Do they have "detents" or any other anti-rotation feature ?

[Normster]

You can get them with a locking feature, but since it's "geared" the chances of it drifting are pretty slim.

[Enzo]

And just because the bias current drifts, don't assume it is the pot moving. The tubes drift over time.

Try this. Set the bias and measure the grid voltage. Then measure the plate current. Now later when the current has drifted, check the grid voltage to see if it too has drifted. If it has not, then it is the tubes.

Furthermore, around here, the wall outlet does not always provide a steady 120VAC. A change of 5 volts on the mains will become a change of 20 volts on the B+ rail. THAT will certainly cause a different current reading.

SO always verify the mains voltage too.

I suspect the pot is fine.

[BobD]

This is more than a little drift. I set the bias around 40ma and it has gone down as low as 22ma in a Super Reverb clone. The reason I checked is because it was sorely lacking in power.

[Steely]

Those 10-Turn pots look like they would provide excellent fine-tuning.

Less exact, but providing a positive "locking bushing" to prevent mechanical drift, would be the locking bias pots available from Weber:

https://taweber.powweb.com/store/potsord.htm

I found a similar pot at my local electronics store, with a locking nut. Haven't installed it yet, so I can't report on it's performance.

George

[Enzo]

Again, monitor the bias voltage to see if it is drifting. If not, you will still have the problem with the new pot.