Trim pot for PI plates

[Tdale]

On some schematics there is a trim pot between the two PI plate resistors. What voltage does it need to handle? First, I assumed that it would need to handle several hundred volts...but since the resistance is about 10K, I guess the voltage across the trimmer is much smaller than across the plate resistors, so 100V or 200V should be more than enough?

Tommy

[Bob-I]

On some schematics there is a trim pot between the two PI plate resistors. What voltage does it need to handle? First, I assumed that it would need to handle several hundred volts...but since the resistance is about 10K, I guess the voltage across the trimmer is much smaller than across the plate resistors, so 100V or 200V should be more than enough?

Tommy

I use the cerment trim pots from mouser. Never had a problem, like you said there's not much voltage across them.

[skyboltone]

On some schematics there is a trim pot between the two PI plate resistors. What voltage does it need to handle? First, I assumed that it would need to handle several hundred volts...but since the resistance is about 10K, I guess the voltage across the trimmer is much smaller than across the plate resistors, so 100V or 200V should be more than enough?

Tommy

I have rebelled on this one. For both the OD trimmer and the PI plate divider I'm using some little bitty pots I got at a yard sale at Tektronics years ago. One's an Allen Bradley and the other is a Clarostat. They both have 1/8th inch shafts and fit neatly under the board. When I run out of these I'll buy the little alpha and bourns pots. I just don't like my OD signal passing through one of those little square jobs.

FWIW
Dan

Dan

[Bob-I]

I just don't like my OD signal passing through one of those little square jobs.

I'm starting to agree. I've been balancing the PI using 91K and 110K Vishay's and leaving it at that.

[Tdale]

I might go for that too...

When we're on the subject of trim pots.. Is there any reason why the OD entrance pot should be an internal trim pot and not a panel mounted pot that can be adjusted from the back?

The way it is now, there seems to be only one "correct" setting for this pot, since you have to disassemble the amp to adjust it, something that you don't do very often..

Tommy

[Bob-I]

I might go for that too...

When we're on the subject of trim pots.. Is there any reason why the OD entrance pot should be an internal trim pot and not a panel mounted pot that can be adjusted from the back?

The way it is now, there seems to be only one "correct" setting for this pot, since you have to disassemble the amp to adjust it, something that you don't do very often..

Tommy

Nope, I've done that to most of my amps. Look at the Princeton thread and you'll see that the pot is mounted next to V2 so that I can reach in and adjust it.

[MarkB]

I might go for that too...

When we're on the subject of trim pots.. Is there any reason why the OD entrance pot should be an internal trim pot and not a panel mounted pot that can be adjusted from the back?

The way it is now, there seems to be only one "correct" setting for this pot, since you have to disassemble the amp to adjust it, something that you don't do very often..

Tommy

I'd think of that pot as a classic "set and forget" unit. Not the kind of thing you'd need to change for different guitars or different songs. No harm in putting it outside, but if any trimmer could be inside, I'd think this is one of them. Just a matter of taste I guess.

[Funkalicousgroove]

The nice thing about the trimmer in the PI is you can use it to either balance the sine wave for max output, or throw it off balance till it sounds best. I know that I get an extra 6 watts out of mine with the PI balanced (Not that 6W amounts to a hill of beans). I've been using piher carbon comp or NOS CTS Carbon Comp Trimmers with no issue I think they're 1/4W