Power supply trim Q

[markusw]

Would it work to replace the 22k in the ps string with a 1W cermet trim in order to more easily adjust plate voltage when swapping preamp tubes? I suppose there will be some crackle when turning because of DC but otherwise I guess it shouldn't harm??

Markus

[glasman]

You had better check the voltage and power ratings on the pots.

Given about 440 on one end and 320 or so on the other. You have about
.65 watts of dissipation. Most cermets are in the .25 to .3 watt range.

May work , but there is a reason they are 2W at a minumum.

Gary

[markusw]

Thanks a lot for your help!!

Since the toroidal xfmr I'm going to use gives 270VAC (it's for a preamp only) I have to change the ps string to get the proper voltages for the preamp tubes. The cermet trim I thought to use has 1W and 315V rating. According to the Duncan PSU sims it would "see" about 365VDC on one side and 305VDC on the other. With the trim set to 10k this should be about 0.36W (provided my calcs are OK ).

Do you think it's safe this way??

Markus

[glasman]

It would probably work for a bit. What I would do is to insert a pot temporarily, set it to the desired voltage, remove and measure the pot and replace it with a fixed resistor.

Gary

[markusw]

Great idea! Thanks.
Why do you think it would work for a bit only??
Sorry for my questions but I'm keen on learning

Markus

[glasman]

Great idea! Thanks.
Why do you think it would work for a bit only??
Sorry for my questions but I'm keen on learning

Markus

Even at 0.36W you are exceeding the power limits for most smaller trimmers.

Also take into account that the current through that trimmer is going to change as a function of the single applied to the preamp.

Gary

[markusw]

OK, I have to admit the trimmer I intended to use is not really small with 16mm in diameter but it has 1W rating.

Also take into account that the current through that trimmer is going to change as a function of the single applied to the preamp.

Sorry, what do you mean with "single"?

Markus

[glasman]

OK, I have to admit the trimmer I intended to use is not really small with 16mm in diameter but it has 1W rating.

Also take into account that the current through that trimmer is going to change as a function of the single applied to the preamp.

Sorry, what do you mean with "single"?

Markus

Sorry, too early in the morning here. I meant signal.....

[markusw]

I meant signal.....

Ah, makes sense
Will check the current with maximum signal present.

Markus