Gain pot bypass caps

[iknowjohnny]

Theres one thing i've never quite been able to get exactly right. That would be the right value cap on the gain pot so that when turned down the tone doesn't change at all. Is this even possible or is it such that due to the way the pot itself changes the tone as it's turned down, theres no way to choose a cap value that would allow the tone to stay the same throughout the rotation?

I have tried both a cap alone, from 300 to 1000pf and cap with a resistor to lessen the effect of the cap. I've tried many values of resistor too, but so far it's never quite the same. I either get more high end then when at 10, or muddier. By the way, the resistor always goes to the wiper side if that matters. anyways, how do you go about this? Is there a formula or what?

The reason i ask is i like the tone of the amp when it has more gain than i like, then turning the gain knob down to where i want it. The amp just seems more alive when the max gain it's capable of with the knob at 10 is more gain than i ever want. (aside from super hot lead tones which i get w/a clean boost) This is why i want the tone to remain the same as far as freq response when the gain is turned down. If i can get it right i will drop the split load plates on V1 A and B and use the gain knob much lower.

[Structo]

You are talking about a bright cap and by design they influence the brightness less and less as the volume is turned up since the wiper is moving towards the CW lug on the pot taking the cap out of the circuit.

Most bright caps on amps are between 15-50pf or so and connect to the wiper and CW lug when viewing the pot from behind.

[iknowjohnny]

I know that, but the problem is that BECAUSE of that (that they are less influential as the gain is turned up and visa versa) i can't seem to get the tone to stay the same at every point in the rotation. So i'm wondering if theres a combo that perfectly interacts with the volume pot's own influence as it's turned. For example, you turn the pot to X position and the highs are cut by the amount of Y. So the cap must ADD the same amount of highs in the same freq range at position X to bring it back to the same as the knob is at 10. Now move the knob to another position and the cap then adds the correct amount there. see what i'm saying? Not i'm not naive enough to believe theres a value that will yield perfect results, but the question is whether theres a formula that will give you the closest values both in the cap and a resistor with which to allow the cap's influence to be as close to right as possible.

This all may seem anal, but it's been hard to be happy with it so far. By the way, marshall use a much bigger cap than 5-50 pf. In fact, i've seen many if not most at 1000pf. (.001uf)

[tubeswell]

Just put in a bigger cap. Experiment until you find the size that's sounds right for you.

[Jana]

Remove the split load and use all the gain. Send the signal to a cap and resistor in parallel and then to the pot. Reduce the value of the pot to about 250k to minimize the hf losses. Tweak with a small value bright cap.

[iknowjohnny]

Funny.....i just REMOVED a cap and resistor from in front of the pot ! (and put it elsewhere, as it sounded better) I didn't know a 250k pot would reduce HF....i thought it was the opposite. But i'll give it a shot. I've never verified it, but it just seems to me that split load plates don't sound as good. But it was the least detrimental way i found to dump gain. Voltage dividers like pots kill high end and in getting it back with a cap like on the gain control is never perfect.

[Structo]

She is saying to lower the pot value to 250K to keep the HF.

Just like a fixed resistor, the higher the value the more it attenuates the highs, but with a pot you also have the sweep to consider.

[selloutrr]

isn't there a software program for figuring this out?

ampcad?

[orrong65]

Isnt this the same issue with a volume pot on a guitar? The tone sounds great with vol on 10, as the signal is as close as its going to get to be straight through, but degrades as the vol is turned down, with treble loss.

Everybody thinks they have come up with a "treble bleed" solution across the vol pot; either a cap alone, a cap and resistor in parallel, or a cap and resistor in series. And a huge range of suggested cap values, from 47pf up to 0.002uF. But none of them appear to have a linear or similar effect over the whole vol range.

A perennial problem for guitarists.

[iknowjohnny]

I seem to have found the right one, tho only after i made some other changes. I'm finding certain values won't work right when the circuit is a certain way, but then can sound much more right when certain things are changed. I can't even remember what it was i changed, but now a 270pf which wasn't right when i first tried it is working quite well. Maybe not perfect but much closer then it's been so far. So i'm leaving it unless things change as i do the final tweaking. (final....yeah, right )