i need a particular taper...how do i find it?

[daz]

For some reason, most 250k treble pots on my jcm style build don't work right. the amp's design is exactly like a jcm 800 from the athode follower thru the tone stack, but for whatever reason the treble control is way too sensitive giving me more high end than i'd normally use well before 12:00. that makes it rather sensitive to set. I tried a lot of pots, both linear and non, but they were all bad in that regard to one degree or another. then i found one that works right. It gives me the right amount of treble just about where it is on a JCM, at about 2:00. Why only this pot, i have no idea. The problem is it's one of dozens of old pots i had laying around, so i don't know who made it or anything about it. i tried researching the stamped numbers on google but got nothing. Maybe it's a reverse audio or something, i just don't know. So the question is, does anyone know of a way to find the same taper? Maybe by checking the resistance at various points and sending that info to mouser or some such thing? Theres got to be a way without actually spending a fortune to have a manufacturer copy it. I can't afford 10 grand for a few pots ! any ideas?

[ETR]

For some reason, most 250k treble pots on my jcm style build don't work right. the amp's design is exactly like a jcm 800 from the athode follower thru the tone stack, but for whatever reason the treble control is way too sensitive giving me more high end than i'd normally use well before 12:00. that makes it rather sensitive to set. I tried a lot of pots, both linear and non, but they were all bad in that regard to one degree or another. then i found one that works right. It gives me the right amount of treble just about where it is on a JCM, at about 2:00. Why only this pot, i have no idea. The problem is it's one of dozens of old pots i had laying around, so i don't know who made it or anything about it. i tried researching the stamped numbers on google but got nothing. Maybe it's a reverse audio or something, i just don't know. So the question is, does anyone know of a way to find the same taper? Maybe by checking the resistance at various points and sending that info to mouser or some such thing? Theres got to be a way without actually spending a fortune to have a manufacturer copy it. I can't afford 10 grand for a few pots ! any ideas?

Look on the pot and see if it says A on it anywhere if it does it's a probably a log taper pot which is probably what youre looking for. You could just set it to the middle of the range and read the resistance from the middle lug to the right lug. If it's a lot less than 100K it's probably a log taper. You can also mess with the taper of a pot by putting a resistor between the wiper and one side.

[soundmasterg]

Measure the pot and the halfway mark and whatever value it is should be close to what the taper is. I think for audio taper there are 10%, 20%, and 30%. The way they are made, they are almost always made with two tracks inside the pot so that when they are combined it is a mostly seamless transistion. To make a true audio taper with one track is too expensive. Many audio taper pots these days are closer to 30% and are not really suited to audio needs. Its getting hard to find good pots without special orders. RS Guitarworks makes some great taper audio pots, but they're spendy.

Greg

[Enzo]

Go over to RG's Geofex site, and read his article on pots and tapers. You can massage your own tapers out of pots.

[daz]

Go over to RG's Geofex site, and read his article on pots and tapers. You can massage your own tapers out of pots.

i know, i tried that after asking this once before but i didn't quite grasp it fully and couldn't get a pot to do what i wanted. Pathetic, i know.

[tubeswell]

Take a 1M linear taper pot and out a put a 330k resistor between the wiper and the ground lug - that will give you a pretty smooth 248k audio taper.

[daz]

Thanks TW, i'll give that a try.

[Bruce / Mission Amps]

Take a 1M linear taper pot and out a put a 330k resistor between the wiper and the ground lug - that will give you a pretty smooth 248k audio taper.

Hmmm ... I don't know about that.
What would the resistance above the wiper of a standard 250Ka pot be at 25% to 50% rotation compared to this 330K/1M combo with the wiper at about 25% to 50%?


I don't think this will be perfect either but I think you'll need two resistors... one above the wiper and one below.
And also Daz... besides the unknown one you like, who's name brand audio taper or log pots have you tired?

[daz]

All the pots i tried were unknown because they were old pots in my "junk drawer". Except for the first one which were the pots i ordered when i ordered all the parts for the amp, and those were alpha. I have tried both the 2 different alphas offered at turretboards.com.

[Bruce / Mission Amps]

All the pots i tried were unknown because they were old pots in my "junk drawer". Except for the first one which were the pots i ordered when i ordered all the parts for the amp, and those were alpha. I have tried both the 2 different alphas offered at turretboards.com.

OK... I'm not very happy with most all the current production audio taper pots either but I'm more satisfied with the recent 3/8" brass bushing CTS or newest 3/8" brass bushing Alpha pots from MOJO.... and both have solderable backs on them too.
>>>>>>>>>>>

I just went over to turretboards.com and it doesn't look like any of those are the newest ones from MOJO.

[Ptron]

I thought the orginal pots were linear. Aren't they are on almost all TMB tonestacks? Unless the manufacturer really messed up, I would think linear is linear. None of that is to say you wouldn't prefer an analog taper there. Maybe your amp is more treble-y than usual for some other reason and you just have to turn the treble down to get it where you like it? Just a thought.

[tubeswell]

Hmmm ... I don't know about that.
What would the resistance above the wiper of a standard 250Ka pot be at 25% to 50% rotation compared to this 330K/1M combo with the wiper at about 25% to 50%?


I don't think this will be perfect either but I think you'll need two resistors... one above the wiper and one below.

Hi Bruce

Plot attached. (Daz -If you use an off-the-shelf 1MA 'two-line' taper pot and a 330k resistor you will get more of a curve.)

If you put a resistor between the pot input and the pot wiper you will make an antilog taper, or if you have one 'above' and one 'below' the wiper you will make an antilog/log taper. (see article) The way to get a log taper is having the resistor between the wiper and the pot ground.

[daz]

Thats excellent, thanks so much TW !!! I read the secret life of pots at geofex, but this is clearer to me...thanks. can't wait to try this.

[daz]

Hmmm..however, unless i'm reading those charts wrong, it seems like a normal log (thats audio, right?) taper should work best since i want the amount of output to be LESS at greater rotation points. yet i've tried audio taper. Am i missing something?

[tubeswell]

Different tapers

Hi Daz

On the chart I posted, the amount you turn the pot affects the resistance less at the beginning of the turn (when the resistance is highest - i.e. the '1' position, and more when you are at the '12' position.

If you want the curve to be bulgier, (i.e. to increase the resistance even more slowly at from the '1' position) increase the pot resistance to say 2M and decrease the tapering resistor to e.g.; 270k - of course that will make the overall resistance around 237k.

If you want the taper to be smaller (i.e.; change faster from the '1' position) use a 500k pot and (say) a 390k resistor (which will be a maximum resistance of 219k)