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

[tubeswell]

Parallel pot & wiper resistor spreadsheet

This thing I did a year or so ago might be useful for helping work out the taper curves

Its pretty crude but basically you enter the numbers at the top of one of the spreadsheets (I think you will find it on Sheet 1) and then plot a graph/chart of that sheet's dial setting and total resistance values. If you can't figure it out get back to me and I'll post something simpler. Note that this is only useful for calculating tapers with linear pots. The plots of audio pot tapers in there were just stuff I measured manually out of curiosity.

Cheers

(PS - thanks to the impeccable tboy for making it possible to upload spreadsheets when I asked just now. What a great host you are my friend)

[ETR]

There are really better ways of taming the strident highs of a JCM800.

[Bruce / Mission Amps]

Hi Bruce...
(Would've posted this earlier but I had trouble hitting the reply button)

Can someone please tell me how to click on the attached pictures and not have to save it first?
For some reason over the last couple weeks my browser now opens a pop up asking me where to save this file... ??!!

[tubeswell]

Can someone please tell me how to click on the attached pictures and not have to save it first?
For some reason over the last couple weeks my browser now opens a pop up asking me where to save this file... ??!!

Check if the preferences/options in your browser are enabled to prompt you to always save it to disk (or something like that?)

[Chuck H]

Daz,
You have one pot salvaged from who knows what that is doing what you want, yes? Please measure the resistance on that pot from either leg to ctr at 25%, 50% and 75% rotation for us.

It is possible you have a pot that is proprietary to a specific MFG and design. But probably not. There are two "audio taper" log pots that are common. I may not remember the specs exactly but a proper audio taper in days of old, and common in most old amp and hi fi tube stuff, was the faster of the two at 20% to 25% pot value at 50% rotation. The other one is very slow giving only about 10% pot value at 50% rotation. Some MFGs mark log pots with the 10% figure as "10AVALUE" or just "10A". Mouser sells "guitar" pots that are "10A" and may be worth looking into.


http://www.mouser.com/catalog/636/680.pdf

All the "audio taper" pots I have from this page in the catalog are marked "10A VALUE" and have a very slow audio taper.

Chuck

[daz]

I'll measure it next week. I have asked several questions in the last week and said i would report back when i get the amp back here, but that will be next week.

[Bruce / Mission Amps]

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)

Not a big deal but, for what it's worth.... this still doesn't make a 1Ma pot act or work like a 250k audio pot with a custom taper .
If you actually set up two ohm meters, with each having their black lead connected to the wiper and one meter's red lead connected to the top lug of the pot with the other meter's red lead at the normally grounded bottom lug... you'll see that there is still 670,000 ohms (or equiv) above the wiper as the pot's wiper is rotated to audio low and a 250k pot simply does not do that.... the max it can be is 250K, not 670K.
That means when using the 1M pot with a 330K resistor from wiper to ground, and the pot set to a lower level there, is a very large series resistance (with some minor high frequency roll off) applied to the signal path before it can leave the pot's wiper.
That is what I was referring to in my original reply/comment about using two different value resistors... one above and one below wiper.

Yes, with respect to the wiper and ground, I can see where the total resistance and taper of the modded 1Ma pot is similar to a special taper 250K audio pot as seen by the following grid, etc.
Whether or not the modded pot sounds good set up like this wasn't the point I was making.
Somebody correct me if I'm wrong here... mildly >

[tubeswell]

Thanks Bruce - yep I had wondered about that series resistance but I wasn't sure whether it mattered too much if it was part of a voltage divider, where the series resistance is getting higher as the resistance on the ground side of the side of the wiper is getting lower. I guess you could bypass the 'series' side of the wiper with a small cap to keep the highs in there as the series resistance climbs.

[daz]

Dayum, you all sure know how to confuse a feller !

[Bruce / Mission Amps]

Thanks Bruce - yep I had wondered about that series resistance but I wasn't sure whether it mattered too much if it was part of a voltage divider, where the series resistance is getting higher as the resistance on the ground side of the side of the wiper is getting lower. I guess you could bypass the 'series' side of the wiper with a small cap to keep the highs in there as the series resistance climbs.

I frequently do put a 1M to 2M7 resistor to ground on the wiper of a 1Ma volume pot where I think it won't change the signal level much and the wiper goes directly to the grid of the following stage, but only to alter the taper and pot feel in a positive way.
I haven't tried this with those 1Ma and 330K values trying to get a 250Ka pot and taper reaction.
I can't comment on the actual effects of sound and it might sound just great with the 1M and 330K!

[tubeswell]

Dayum, you all sure know how to confuse a feller !

Well tarnations - this is what I meant (of course its out of tone-stack context - and sorry for the extremely small size - I haven't figured out how to zoom up the JSchems yet)

[Bruce / Mission Amps]

Well tarnations - this is what I meant (of course its out of tone-stack context - and sorry for the extremely small size - I haven't figured out how to zoom up the JSchems yet)

I have not been able to open any of your pdf artwork from the website... not sure what's up with that.

[tubeswell]

Try These .jpg files

Hi Bruce

(maybe these different formats will open more easily in your browser)

Did you get the spreadsheet? (it was in MS Excel)

[daz]

I think you uploaded a wrong jpg for that 2nd one. It's tiny and blurry.....can't see any of the values.

[tubeswell]

Dagnabit

I don't know how to blow it up (maybe I should try some TNT), so in words its the 1M linear pot with a 330k resistor from wiper to ground but with a small 47pF cap added across the input to the wiper to keep the highs in there (but maybe it wants something like 100pF?).

(Sorry about the attachments going off)