Multiple turn trimpots

I wonder why multi-turn PCB trimpots aren't commonly used to set cathode bias current? Cost, physical size and power dissipation don't seem to be issues. I suspect it has to do with their failure modes. What has your experience been?

j beede wrote:I wonder why multi-turn PCB trimpots aren't commonly used to set cathode bias current? Cost, physical size and power dissipation don't seem to be issues. I suspect it has to do with their failure modes. What has your experience been?

A couple of things:

a) Cost: For us, the hobbyist, the difference between a $0.90 part and an $1.95 part is negligible. Times 1,000 units, not so much.
b) And, here is a good example of the difference between 'accuracy' and 'precision'. Setting bias need only be accurate. Precision is wasted. So, a pot that has enough range to be accurate is all that is required.

If we wish to populate our equipment with "better" parts, that is a great idea, and I am as guilty as anyone else along those lines. But, let's be sure that those parts we use are appropriate to the use and do not introduce needless complexity instead.

in my experience, a $2 or $3 multi-turn pot will very often fail after being turned to one extreme or the other, and where is the end of turning???
I've never had a $1 single turn pot fail, and it has the same adjustment range, and I'd rather turn one 10 degrees than 4 or 5 360 degree turns.

My application would benefit from the narrow width of four multi turn pots mounted side-by-side. My suspicion is that there is a reason why these pots are not commonly used. tubes4hifi addressed the issue that I was trying to raise.

tubes4hifi wrote:in my experience, a $2 or $3 multi-turn pot will very often fail after being turned to one extreme or the other, and where is the end of turning???  
I've never had a $1 single turn pot fail, and it has the same adjustment range, and I'd rather turn one 10 degrees than 4 or 5 360 degree turns.

Yup!