1 meg vol pot on Fender BF Amp

[sbirkenstock]

Hi everybody,

I was wondering why Fender is using 1 Meg Vol pots on amplifiers?
Why not 500k or 250k?

Some amps (so does my built, something close to a BF Vibroverb) are very sensitive if you want to adjust for real low volume.
So I took a 250k vol pot and added an 220k resistor (measures 240k) on the grid side.
This makes the "ratio" easier to adjust.

I tried the 250k pot without the 220k resistor first and did not hear too much difference to the 1meg pot, if any.
It´s just a voltage divider, right?

Somebody told me it should be darker, referencing the AC30.
The AC30 uses a 500k pot and a 270K resistor kind of how I did it, but the other side of the pot goes somewhere else.
Guess the pot resistance matters for the AC30 but not for the BF Fenders.

Any info on that?

thanks a lot,

Stephan

[Leo_Gnardo]

"Tradition, tradition". You're free to use any value volume pot you please. Lower values yield darker tones, no problem there. And as you mentioned, you can pad down the volume control with a series resistor, to open up its 1-10 scale to useful volumes.

What I do find is an awful lot of pots that are supposedly "audio taper" are far from that, the volume "pops up" early and hard to control near the lower end of the control. Not just 1M pots either. It has more to do with the sloppy and convenient way current manufacturers construct the resistive strip on these pots. Heaven help 'em if they spent a penny more on each one and got the curve right.

[teemuk]

I assume you are talking about a tube amp. The triode gain stages tend to have very high-ish output impedance, 39k - 100 k isn't extraordinary.

It´s just a voltage divider, right?

It's that among other things, so you can see that too low resistance values may introduce too much attenuation. (Also think about all capacitances involved). Bigger issue is that the pot may be a significant load for a preceding gain stage, and too low resistance values may therefore also load the driving gain stage excessively, decreasing its headroom.

In essence, higher pot resistances are better for impedance bridging.

Somebody told me it should be darker, referencing the AC30.

Really depends on overall circuit. If the source impedance happens to be high at higher frequencies a lower resistance value will attenuate more.