help....need to understand something

[iknowjohnny]

I realize how stupid this question is because it's pretty simple. But i can't grasp exactly what's going on when i do this. I have a 1M gain pot, and unless you are new here you probably know it's a high gain preamp marshal style homebrew. Anyways, when i put a 220k resistor across the input and output terminals of the pot the tone changes quite dramatically. The gain doesn't seem to change much at all, but the tone gets much chunkier and more rich or resonant. In trying to figure out whats going on i can see certain things that should be happening such as it must be reducing the resistance between the in and out lugs but the resistance between the signal and ground should stay the same. Am i on track here? So the way i see it the tonal change has something to do with less resistance between the in and out and therefore the ratio of resistance of those two at a given point in the rotation as they relate to the resistance from the ground lug has changed. How this changes the tone i'm totally clueless. But if i had to guess what would have happened before i tried it i would have guessed the tone wouldn't have changed much but the gain would have, at least at anything less than on 10. Can anyone shed some light on whats going on here to make for this tonal change? and would a smaller pot do the same thing? If so i imagine it would have to be much smaller because i tried a 250k in the past without such drastic results.

[vibratoking]

Without seeing a drawing of the circuit...this might help you.

http://www.elby-designs.com/documents/t ... meters.pdf

[passfan]

You are paralleling resistances thereby changing the value of the pot and changing the series inductance in the circuit. It doesn't effect volume so much as tone given that resistors in that regard are not so much a gain changing device. Now send that resistor to ground and start lowering the value and you will begin to effect the gain as well as the tone.

[iknowjohnny]

Thanks. Inductance eh? Well, i never had any understanding of that at all but it makes sense because i do know how it can change tone. Just not sure how what i did changes inductance.

It's funny because i don't typically see this used in other amps. In fact i can't think of any offhand. yet i did this when i first built this design gotta be 3 years ago by now i think. (tho it's a very different amp at this point) And every time i posted a schematic people would always tell me to remove that resistor, but every time i did it didn't sound as good. Eventually i took it to heart and removed it and i since forgot about it. So yesterday i was looking at a old schematic of my amp when that resistor was there and it hit me how i always liked it, and thats what prompted me to try it again. Boy, i'm glad i did. So i have no idea why it works so well for me but no one else seems to use it, but i'd be a lot of people would like it if they tried. I feel like the thing that first struck me about this amp's tone is back after all this time, but better than before because of the many changes since then.

[vibratoking]

Will you post the schematic showing the portion of the circuit that you are describing?

[iknowjohnny]

I don't have one thats remotely up to date, plus nothing at all here at work. But it's pretty basic. Up to the point of that gain control it's a pretty basic 12ax7 stage....10k input grid R, 100k plates, .0047uf coupler to a 500pf/470k network to the gain pot that i'm talking about. After than 2 more stages w/o the 500pf/470k network. First 2 stages have 220k grid stoppers. cathodes on first 2 stages are 1.5k with .68uf and 1 uf bypass.

[VacuumVoodoo]

You are paralleling resistances thereby changing the value of the pot and changing the series inductance in the circuit.

Would you please be so kind as to explain which inductance is changed by connecting a resistor in parallel with the potentiometer?

What happens is you change the load (to a heavier one) on the gain stage feeding the pot. This reduces its gain and distortion characteristics which is what causes a change in tone.

[Kagliostro]

This link explain the same

hope can be of some interest

http://www.geofex.com/Article_Folders/p ... tscret.htm

Kagliostro

[passfan]

You are paralleling resistances thereby changing the value of the pot and changing the series inductance in the circuit.

Would please you be so kind as to explain which inductance is changed by connecting a resistor in parallel with the potentiometer?

What happens is you change the load (to a heavier one) on the gain stage feeding the pot. This reduces its gain and distortion characteristics which is what causes a change in tone.

Well okay, perhaps I am wrong. I based it on my elementary knowledge of series and parallel resistance and capacitance in passive filter networks. We change coupling capacitors to affect tone. It occured to me series resistance would have the same effect.

[vibratoking]

A picture is worth a thousand words...Especially if you want the wording of the question and response to be precise.