Rheostats

[leadfootdriver]

A while back, I plugged into my Rivera Rockcrusher attenuator backwards (like a dummy), and got a burning smell that I traced down to this Rheostat. I'd like to replace it, but know nothing about them; like power rating. There are no numbers stamped on it other than B20ohm in print.

Does the B stand for anything? I should replace a discolored 2W resistor that's connected to it that I can make out to be a 40ohm 5% if that is any clue.

[R.G.]

B = linear. It's a 20 ohm, and probably 1W at least. It probably ought to be 2W if you can get it. Big-power pots get expensive because there are so few of them used.

Actually, from the discoloration of the 2W 20R resistor it's in series with, I would look at what the pot does in the circuit and figure out some other way to do that adjustment that could either withstand more heat or didn't produce so much. Got the schematic?

[leadfootdriver]

Thanks for the reply. That pot is the studio mode pot. It allows you to turn a blaring amp down to a whisper. But that's all I got. No schematic.

I should call Rivera and ask!

[Chuck H]

Indeed a schematic would be ideal. IMHE attenuator schematics can be tough to get. Everyone wants to keep their secret sauce. There's a few around and none would use a small rheostat for overall volume control. Unless in "studio mode" the volume is already well cut and that little pot just trims an already trimmed signal.

It looks like the plain ol 5W Alpha wire wound pot available at Mouser along side the other Alpha 24mm pots. Look at figure F.

http://www.mouser.com/catalog/645/usd/824.pdf

[R.G.]

Hey - Mouser now carries wirewound pots!

It being a part of an attenuator makes sense with it being burned out. If that's the use, all the watts you can get would be best.

[Chuck H]

It's probably used in an already attenuated "mode". The little wire wound pots have much better control from zero than the big rheostats. Still, if anything in line blows (or something is plugged in wrong) having the part rated to the amps watts would avoid a unit failure or having no load on the amp.

[olddawg]

What makes you think the pot is bad? The resistor is dis colored and could have given you the smoke? Is it crunchy when you turn it? Does it measure out oddly or have dead spots? A cooked wire wound rheostat usually will be obvious.

[leadfootdriver]

Thanks for the replies. That pot was scratchy and the volume would cut in and out when I would rotate it. I think I was burnt too. I cleaned it out about a year ago but it was never as good as new.