"Digital" bias pot?

[tedmich]

I hadn't seen these 10k Bourn pots before, normally $40!

10K PRECISION POT W/ DIGITAL DISPLAY | All Electronics Corp.
a bit of a big panel cut out, do these resolve well enough to really set precise bias current?

[Enzo]

So they are 10k, three button "thumbwheel" switched pots. Several thoughts.

First, you have three buttons, so unless I misunderstand, that means each button press changes it 10 ohms (or 9.99 ohms). That is 0.1 percent, but maybe not as precision as you thought. 10 ohm steps.

Second, bias is not that precise an adjustment.

From an eye candy point it ought to look great. But from practicality, when you change your tubes and need to change the setting from 962 to 443 or something, that seems like a lot of button pushing to me.

I used to use a similar product in some test rigs I built.

[R.G.]

I had to take a freshman-level course in instrumentation and measurement. I thought this was a complete waste of time and money at the time, but that thing has come in really handy over the years.

In this case, the pots are, as Enzo says, limited to the steps you can set. Bias is a voltage that gets converted to a current by the DC gain of a tube, and the whole useful bias range is much smaller than the total bias voltage. If you use this one pot for the whole range, then exactly as Enzo notes, the useful range of measurements is much smaller than the whole range of the pot.

This can be fixed somewhat by making the entire range of the pot be the useful range of adjustment. You put resistors on either end to make the amount of bias voltage available from the pot be a fraction of the total bias voltage. So if you have -60V available, but only use tubes that need -37 to -50, then you put a resistor on each end of the pot so the top of the pot is at -50 and the bottom at -37. This maximizes the resolution of the pot steps.

However, each tube has a somewhat different DC gain (that's why "matching" even happens) so the step on the pot for one tube is not necessarily the same change in current as the step on a different tube. So part of the apparent precision flies out the window.

I have a preference for measuring what you actually are controlling - in this case, current - and then indicating that so you can adjust for it. In my case, I like making a small window of current visible, say something like +/-0.2ma on top of the desired range of tens of milliamperes, maybe.

That's the chain of reasoning that led me to the "stop-light" style bias indicator: use LEDs to say "too cold" "too hot" and "just right" and make the "just right" be a small sliver. It's fast and it sidesteps the issue of resolution and variable tube gains.

It's easy enough to have a switch to set the desired current for different tubes. In the Workhorse, I put in a four way switch for different tube current targets.

[teemuk]

Am I right that this doesn't show the actual bias voltage or quiescent current but the set resistance value of the pot?

Which parameter you think is more important to know?

[Pedro Vecino]

When different pairs of tubes previously measured in the amp are used this may be interesting. Are changed in a moment.

[Enzo]

I can wax pedantic now and then. I find the discussions about accuracy versus precision to be illuminative. The two things are not the same. I get the idea the OP considers this a real precise way to set the resistance. Since each little button push seemingly would result in some quantum of change. Of course the mains voltage hops around all the time and so every secondary voltage in the amp does as well. B+ drops as you play louder, which also affects the "precision" of the bias. So the repeatability of the SETTING is tremendous, the repeatability of the resultant voltage... not so much.