What basic specs for a BIAS pot

[Curranproducer]

hey there, I am trying to figure out what the BASIC specs for a bias pot should be, voltage handling, and the like, I see some amps with the CTS/Alpha type pots and then I see some with the chumpy little plastic trim pots. The plastic would make me very nervous in a HOT Hi Voltage tube curcuit. what is the right way to go or to look for?!

[VacuumVoodoo]

Any trimpot in the range 10k to 50K rated 0.1W and above will do.
These here are good up to +100 C.

[img:150:136]http://se.mouser.com/images/bourns/imag ... _print.jpg[/img]

Of course, this being TAG, you will be drowned in a multitude of alternative suggestions

[Jana]

Ohm's law (V=IR, I=V/R, R=V/I) is what you can use to determine if your bias pot is up to the task.

Washington and Colorado law is what you can use to determine if you want to bias pot.

The first law will make you happy, the second law will make us all happy.

[pdf64]

Depending on the overall voltage magnitudes and how the adjustment circuit is arranged, it may be a good idea to also consider the pot's voltage ratings, eg across the track, between track and case (if metal).
Review it under start up and steady state conditions, at max, min and intermediate settings.
Pete

[Leo_Gnardo]

Those occasions when I can use the Fender style 10K bias trim, I do that. Wish they came in different ohm denominations. On rare occasions I use an ordinary Alpha or CTS pot, cut the shaft short, file a screwdriver slot in it.

Most of the time I use the chumpy little Bourns blue cubes. I think they're rated for half a watt. 10K, 20K or 50K as appropriate. Can't say I've ever had one fail.

I like Jana's solution too. Should be available everywhere.

[Phil_S]

Bias supply draws almost no current, so those inexpensive plastic ones are fine. You can buy a lifetime supply (unless you are an assembly line producer) on eBay for under $5. Examples:
http://www.ebay.com/itm/30pcs-20K-ohm-V ... 485ba8ba29
http://www.ebay.com/itm/20pcs-50K-ohm-V ... 485b8ece64

To make the variable resistor, just jumper the wiper with one of the outer lugs.

[iknowjohnny]

On a side note, why jumper the wiper and one outer lug? I see that done all the time, but why? I just use the wiper and whichever lug i want depending on direction. Other than that i don't understand why some pots uses as a VR use that other outer lug with the wiper. I understand that with a bias pot it will be a safer way in case the wiper fails. But they use this in all sorts of places where thats not a issue like say a bass pot in the tone stack.

[martin manning]

Most of the time I use the chumpy little Bourns blue cubes. I think they're rated for half a watt. 10K, 20K or 50K as appropriate. Can't say I've ever had one fail.

Yes! These are 1/2W Cermet units and very good quality. The only down side is that they don't have the accessibility or gorilla-proof aspects of chassis bushing mounting that the Fender-style bias pots have.

[Jana]

IKJ, You answered your own question.

It also affects the tone.

[iknowjohnny]

IKJ, You answered your own question.

It also affects the tone.

Well, what i meant to ask is why they do this on areas where it's NOT a reliability issue as with a bias pot. You say tone? Like with the bass pot in a marshall? Can you clarify how that affects the tone? I could see how it might affect taper but i can't figure how it would be different tonally. Just curious really.

[Jana]

I'm glad you asked.

The bass electrons get all bunched up in the unused section of the bass pot. They are flying down the wire and then all of a sudden, "whoa, the bridge is out and no where to go!" So they just pile up in there and eventually start leaking back into the circuit. That's why when playing gigs and the people start piling in, you have to turn up the treble. It's those bass electrons sneaking out and muddying up the sound.

I would also add that the effect is mitigated somewhat if you use red wire instead of brown.

[NickC]

I'm glad you asked.

The bass electrons get all bunched up in the unused section of the bass pot. They are flying down the wire and then all of a sudden, "whoa, the bridge is out and no where to go!" So they just pile up in there and eventually start leaking back into the circuit. That's why when playing gigs and the people start piling in, you have to turn up the treble. It's those bass electrons sneaking out and muddying up the sound.

I would also add that the effect is mitigated somewhat if you use red wire instead of brown.

If you pound down the wire feeding the pot, into a shape roughly approximating an airfoil, the excess electrons will go flying around the interior pot space at high speed until they jettison out the miniscule openings around the casing.

[Leo_Gnardo]

Most of the time I use the chumpy little Bourns blue cubes. I think they're rated for half a watt. 10K, 20K or 50K as appropriate. Can't say I've ever had one fail.

Yes! These are 1/2W Cermet units and very good quality. The only down side is that they don't have the accessibility or gorilla-proof aspects of chassis bushing mounting that the Fender-style bias pots have.

True 'nuff but also good 'nuff for many applications.

Anybody have a source for Fender style bias pots in denominations other than 10K, like 25K & 50K? Sure would be good to find some.

[eniam rognab]

ill quit screwin around, sorry

[VacuumVoodoo]

This thread is definitely going potty.