Valve Stem OD Presence B20K pot sub

[notTHEwillienelson]

The Valve Stem OD (MI Audio Tube Zone) calls a B20k for the presence control (this was an internal trimpot on the original). With B20k pots not being available from my usual parts sources, I'm just thinking about using a B25k I have on hand with 100k resistors to mod the value to 20k and wanted a second opinion on if my logic checks out here.

Output section:

Based on how's it's connected in the circuit, I'm thinking a 100k resistor in parallel with lugs 1 & 2 AND another 100k in parallel with lugs 1 & 3 would effectively make it a B20k in the circuit. Like so:


Anyone agree or disagree with that?

 

[jimilee]

You can use a B25k with no issues, don’t even worry about the 100k resistor.

 

[thomasbe86]

yup, you can safely do as in your snapshot with 2 100k, it will change a bot the way the taper works but should not be too audible

 

[notTHEwillienelson]

You can use a B25k with no issues, don’t even worry about the 100k resistor.

Thanks! I might just try it both ways for the helluva it. I know it won't change much in the control behavior either way.

 

[notTHEwillienelson]

yup, you can safely do as in your snapshot with 2 100k, it will change a bot the way the taper works but should not be too audible

Thanks! Since it's a linear taper pot, the taper should be identical to a B20k, I think.

 

[lowpitch]

You can also try a single 100K across the outside lugs. Using a 25K pot will affect the transistor bias slightly since the presence pot also acts as the collector resistor, and so will using two taper resistors for any settings other than 100% or 0%. The taper is going to change either way, and the frequency response is also not going to be 100% the same but pretty close. Just try it out and see what works for you Your diagram is wrong by the way. The taper resistors should go from lug 1 to 2 and from lug 2 to 3. In your drawing it's one from lug 1 to 3 and the other from 1 to 2.