YOUR thoughts on gain pot value

[iknowjohnny]

Looking at a lot of schematics of typical 3-4 gain stage preamps i notice that a lot of them use smaller value gain pots than marshall usually does. Marshall usually uses 1M gain pots with the exception of thier more complicated modern designs with lots of SS devices and clippers. While contemporary designs like badcat and soldono etc use 500 or even 250k. The obvious difference is gain, but what do you feel they choose smaller values for? Is there a simple design consideration that dictates what to use or is it 100% subjective. Or is it simply to tailor gain the way they want? I've been strapping resistors across my gain pot and the jury is still out, but so far it looks like with smaller values the distortion is clearer, more articulate even when turning the pot up to where the gain is the same as what i usually use with the 1M pot. Like theres less garbage behind the note. But it's one of those fairly subtle things so i'm asking to see what others who are more knowledgeable on the subject think.

[Gaz]

The value of the pot creates an RC filter with the preceding coupling cap, and so a smaller gain pot reduces bass, unless of course you make the coupling cap larger to compensate.

If you keep the RC time constant the same, then reducing the pot value reduces gain by reducing the load on the next stage. It can also have other tonal effects (you can read more about that elsewhere, of course).

The last effect I can think of bymaking a gain pot smaller is that it reduces series resistance, so that when the pot is turned down there is less treble loss. Go low enough, like 250K, and you may not need a bright bypass cap on the pot anymore.

On that note, remember that you must play the same RC balancing game with the aforementioned bright caps as well to keep the same frequency response (i.e. 470p across a 1M pot = 1000p across a 500K pot, etc.)

Another cool trick is to add a resistor from wiper to ground. This reduces the load as you turn up the pot so that higher gain settings sound tighter while lower gain settings are unaffected.

Oh, and keep in mind that putting a resistor across the pot changes the taper. You can observe this with you meter.

[Darkbluemurder]

Excellent post.

If you keep the RC time constant the same, then reducing the pot value reduces gain by reducing the load on the next stage. It can also have other tonal effects (you can read more about that elsewhere, of course).

I think you meant to say "reduces gain by increasing the load on the next stage".

It also reduces gain by increasing the load on the previous stage since it is in parallel with the plate resistor.

Cheers Stephan

[Gaz]

Excellent post.

If you keep the RC time constant the same, then reducing the pot value reduces gain by reducing the load on the next stage. It can also have other tonal effects (you can read more about that elsewhere, of course).

I think you meant to say "reduces gain by increasing the load on the next stage".

It also reduces gain by increasing the load on the previous stage since it is in parallel with the plate resistor.

Cheers Stephan

[iknowjohnny]

yes, great post ! I'm pretty noob to all this but i caught enough of that to be helpful, thanks. I did try a resistor from wiper to ground once long ago and seem to recall liking the effect. But i don't know why i removed it but after this i'll try it again. thanks.

[Firestorm]

I wonder if the design choices might also be influenced by the amount of thermal noise larger value pots add to the signal. In a high-gain circuit with pots early on, a 1M pot would be discernably hissier.

[iknowjohnny]

Hey Gaz, what approximately value resistor would you use from wiper to ground with a 1M pot and a 500k pot?

[Darkbluemurder]

Hey Gaz, what approximately value resistor would you use from wiper to ground with a 1M pot and a 500k pot?

I cannot speak for Gaz but I used 220k, 330k, 470k and even 1M with great success. The smaller the resistor, the stronger the effect - at the expense of a bit gain.

Cheers Stephan