why does my gain pot do nothing till 12:00?

[daz]

That is, from full on gain to 12:00 theres no change. heres the deal....i have had a 200k (yes, it measures 200k, not 250k) pot as my gain pot for at least a year if not more without knowing it ! I thought it was a 1m till today when i measured it and found it measured 200k. (no, i didn't measure from the wiper....from outer leg to outer leg) So i slapped my forehead and eagerly installed a 1M audio taper pot, same exact pot brand as the 200k with a switch on it. NOS from a surplus elec store. Anyways, this one is a 1m, as i measured it to be sure. But when i tried it i found nothing happens between full up and 1/2 way down. I then measure it with the meter while turning it to see how it tapers and it goes down to about 7 or 800k at 1/2 way from full up which i assume is about right for audio taper. Can anyone look at the schematic below and explain to me why this is?

http://i24.photobucket.com/albums/c39/dazco/latest.jpg

[jrfrond]

You might want to try a linear taper 1M pot instead. Generally an audio taper pot is used in this Marshall circuit, but sometimes you must go with what DOES work rather than what SHOULD work.

[Steve Conner]

It's a musician... they rarely do anything before midday

[txstrat]

It's a musician... they rarely do anything before midday

Don't know if it makes any difference, but what if you interchange the positions of the 300pf cap and the 470 resistor (between the .022cap and the pots wiper). Now you have the 470K and the pot in parallel.

[yunger]

wow, it must not take much to pound that second stage!...and that's with a split load on the first stage too. i bet you're getting more bass outta that thing now that you have a 1M in there. sounds like a .1uF coupling cap and 250k pot would be more useful if you want the full range of travel from the pot.

edit: after looking again, maybe it's the third stage that's getting pounded. perhaps more attenuation before that stage would even things out a little more?

[daz]

I can't figure it out, but the 200k stays, as it works great and a 1M doesn't seem to sound as good. Theres no loss of bass or anything. Also, i tried a totally different brand of 1M and same thing. Wierd.

[J M Fahey]

Hi daz. Something does not add up there, so let's do a little measuring.
Rather than using clock positions, let's use a 0 to 10 scale, where pot fully counterclockwise is minimum or 0; fully clockwise is max or 10, and halfway is 5.
*With the pot disconnected* measure end to end resistance , minimum to wiper on 5 and wiper to max still on 5, for all 3 pots you have.
Thanks.

[Gaz]

I don't have an ideas about the initial question, but it doesn't surprise me that the 200k value, which you said you used for a year, sounds better. Since you've been designing with that value you in place - making mod after mod - then it doesn't seem strange that the 'correct' value doesn't seem right.

[daz]

I don't have an ideas about the initial question, but it doesn't surprise me that the 200k value, which you said you used for a year, sounds better. Since you've been designing with that value you in place - making mod after mod - then it doesn't seem strange that the 'correct' value doesn't seem right.

I know. I agree, and it's really hitting home now because i'm realizing the huge difference in a 1 M pot and wondering what might have been if it really was 1M. So now i'm trying a lot of things i've tried already to see what changes for the better with a 1M pot. there IS a certain smoothness and fullness that i think i it has over the 200k pot, but it also exhibits certain issues i have finally rectified and now have to find the fix for yet AGAIN. But i guess it may be worth it, and it also may be something that kept me from achieving what i wanted to from the start. really pi$$es me off, because the place i got those pots from had a box of them that said 1 M. And this was in that box along with the others i bough which WERE 1 M. but of course with my $hit luck i would of course use the 200k one as my gain pot ! Fruck me ! Can't win....possibly endless hours of tweaking because of this one mistake.

[daz]

Well, i have tried a 1M linear, and two different audio ones that are reverse of each other and the same thing happens ! I don't see how this is possible, but i measured the input to wiper of each and of the two that aren't liner, one barely reisters any change by the time it gets to 1/2 way on the sha\ft, and the other goes almost to 1M at the 1/2 way point. Yet both of those and the linear one all react the same. actually i should say the linear one does change before it gets to 1/2 way down, but barely. Weird....i keep trying to see if i'm missing something but i can't find anything odd.

[J M Fahey]

Hi daz.
It will be a guessing game forever, unless you post consistent resistance measurements.
Thanks.

[daz]

What i'm saying is that doesn't matter. One is linear, one is audio, and one seems to be reverse audio, yet they all do the same thing ! One does act a bit better, but for all of them theres little to no gain reduction between full up and 12:00. It's not the pot apparently. but i don't care....amp is great as is.

[J M Fahey]

Well,

One is linear, one is audio, and one seems to be reverse audio

you think so, you haven't checked yet.

anyway, if:

What i'm saying is that doesn't matter.

i don't care

so be it.
It's your amp.

[daz]

No, i DID check. Thats how i figured it was reverse because one changed a lot from full up on the meter while the other changed little. Which is why i'm so puzzled. If it makes little change from full to 1/2, i could see the one that shows little resistance change in that 1/2 and the linear acting similar, but not the one that shows all the change from full to 1/2. makes no sense unless something in the circuit is causing it.