#183 Bass Pot Approximation

[mdroberts1243]

I was frustrated at not being able to find / make a 312kB pot for the bass in my 183 build. I tried scraping a high-ish 250kB to get 312k but a slip of the exacto blade and I had a non-linear 1.3M ohm pot... reliability would be suspect in any case.

I've seen people propose paralleling resistors with a higher value pot to get a 312kB equivalent and so I took a 589kB pot from the junk drawer and put 2 680k resistors between the lugs 1 - 2 - 3. I ended up with 317k pot measured from 1 - 3 and taking some samples got kind of a s-shaped curve that seems to parallel a linear 312k fairly well. Weird curve but it gives me a reliable resistance and covers the proper range for the critical tone-stack circuit.

Just an idea I thought I'd share.

[UltraHookedOnPhonix]

I was frustrated at not being able to find / make a 312kB pot for the bass in my 183 build. I tried scraping a high-ish 250kB to get 312k but a slip of the exacto blade and I had a non-linear 1.3M ohm pot... reliability would be suspect in any case.

I've seen people propose paralleling resistors with a higher value pot to get a 312kB equivalent and so I took a 589kB pot from the junk drawer and put 2 680k resistors between the lugs 1 - 2 - 3. I ended up with 317k pot measured from 1 - 3 and taking some samples got kind of a s-shaped curve that seems to parallel a linear 312k fairly well. Weird curve but it gives me a reliable resistance and covers the proper range for the critical tone-stack circuit.

Just an idea I thought I'd share.

Couldn't you get a perfectly linear sweep with your B589K pot and a 665K resistor tied across lug 3 and 1? This will give you 312K but won't mess with the tapering. Right?

Mouser part#: 273-665K-RC

[dogears]

Why? The pot is a variable resistor with the PAB off. Tone is the same with a 312k or a 500k. Just use a 500k 30% taper and the settings will be quite similar. IMHO.

Or the above suggestion is good as well.

Or, scrape another 250k. Easy and no reliability issues. I scrape Alphas all time.

More important, what ae you doing about the 30% taper pots in 183?

[martin manning]

Skyliner tone stack, no? In "Jazz" mode it's not wired as a variable resistor. Maybe nobody cares... In "Rock" mode, when it is a variable resistor, the wiper is shorted to one end, and the taper is indeed non-linear, with 65% of max resistance at 50% rotation. PAB on, it's just the element (and the shunt across it, if any) that's in the circuit, so no problem there as long as the parallel combination yields the right value.

[mdroberts1243]

Couldn't you get a perfectly linear sweep with your B589K pot and a 665K resistor tied across lug 3 and 1? This will give you 312K but won't mess with the tapering. Right?

Mouser part#: 273-665K-RC

DogEars pointed out the obvious... that the PAB relay makes the bass pot a varistor (in Rock mode, as Martin clarifies)! My solution is overly complicated in that case.

The single resistor between lugs 3 & 1 works great in this application... I've used it before on trimmers when I didn't have the correct value. The curve isn't quite linear though (a slight arc running above the linear curve, peaking at 50% rotation as Martin points out), but the endpoints are correct.

I don't know if the two resistor approach has any merit in Jazz mode, but now that I've wired it in I might just leave it as is for a while and see how it sounds.

[martin manning]

The two resistor approach in Rock mode has 65% of max resistance at 50% rotation, in Jazz mode you get an S-shaped curve as a voltage divider (still, it hits 50% at 50% rotation), and the resistance top-to-bottom (the input impedance) varies from 100% at either extreme of the rotation to 130% at 50% rotation. In PAB mode you see that same effect, depending on where the wiper is set, since the 1-3 resistance is all that's left in the circuit.

[ampdork]

It is true it may not matter in the most typical settings but I for one really like the jazz with mid boost and half bright on 183...I think it gets very fendery that way and I like it...

In any event I am not convinced a linear is the best taper for a bass pot.....again of no real relevance except that by saying so maybe it would allow and give permission not use an audio pot...

If one can accept that then just get a handful of 350K pots and I can almost guarantee you will get a value real close.

That what I did since like others I bought a ton of 250K and 500K and none of them were remotely close...

[David Root]

Aren't the old Allen Bradley RV4 2W "J" taper pots 30% audio taper?