A volume pot without high frequency loss ?

[Iplayloud]

Correct me if I'm wrong but we all have to choose one of these options:

1-Put a brightness cap on preamp and volume pots to compensate for the treble lost. Resulting in too much compensation at lower levels and not exactly the right frequency. Wether the cap is 10pf in a JCM900 or 5n in a plexi, it ain't exactly the correct frequency and when using distortion it sounds fuzzy at low levels.

2-Use no brightness cap and compensate with the treble or presence.


Now if I use a lower value pot like 250k versus a typical 1M i would reduce the losses, but by how much, enough to be worth it ?

And I would do it at the expense of some volume right, because the signal is at 250k from ground instead of 1M. Would it be a small volume loss or massive ?

If I play at bedroom volumes it is a great idea right, if I don't care losing some volume ?

[J M Fahey]

Yes, a compensation cap works very well ... at a fixed attenuation level , while pots excel at*varying* attenuation level.
That said, compensating at 10x loss (audio/log pot on "5" or "12AM") works reasonably well at other settings.
Real world is somewhat about compromise.
A lower value pot is more immune to parasitic loads, but must be driven from a low impedance source, usually meaning you need a buffer there.
Nothing is truly free.

[Iplayloud]

Well my problem is like I said, the excessive compensation at lower levels (bedroom volumes).
For example when you flip the bright switch on a Fender AB763 circuit amp it's WAY too bright. And most high gain amps that have a bright cap on the master volume they sound fuzzy at low volumes.
Would it be a good idea to use a 250k pot instead or like you say it would create an issue with the impedance or something ?

[Wombaticus]

Well my problem is like I said, the excessive compensation at lower levels (bedroom volumes).
For example when you flip the bright switch on a Fender AB763 circuit amp it's WAY too bright. And most high gain amps that have a bright cap on the master volume they sound fuzzy at low volumes.
Would it be a good idea to use a 250k pot instead or like you say it would create an issue with the impedance or something ?

Tell us more about the topology of the amp you are attempting to mod or design... I'll point out that if it's a new design, it may be possible to position the volume control immediately prior to a stage such as a pentode or cathode follower that doesn't suffer as much from excessive Miller capacitance. In the case of a pentode, the screen grid greatly reduces the inherent grid-plate capacitance. So if you keep the gain of the stage to something reasonable, you will see less high end roll-off than you would with a triode stage with equivalent gain. On the other hand, a unity-gain stage like a CF will have little roll-off, because the greatest contributor to Miller capacitance (and the consequent low pass filter that is formed with the series resistance from the top of the pot) depends on the product of the inherent grid-plate capacitance and the stage gain plus one. Therefore, if you kill the stage gain, there is a consequent reduction in the Miller capacitance, and the corner frequency of the low pass filter is greatly increased.

[J M Fahey]

OK, it's a problem of personal taste, so make thist test:
try the voume control with and without compensating cap.
If with it's too bright and without too dull, there's hope, it means some intermediate value o boost may do.
Leave he cap as-is, but put a resistor in series with it, from 1/2 to 1/4 the pot value.
This will not change the frequency above which it boosts, but the amount of boost.
Some of them should do.
Please post your results.

[loudthud]

Install a resistor between the center terminal of the pot and the following stage. With a 1Meg pot, something between 100K and 220K. Best to install this close to the tube.

[Iplayloud]

Leave the cap as-is, but put a resistor in series with it, from 1/2 to 1/4 the pot value.
This will not change the frequency above which it boosts, but the amount of boost.

Excellent, will try this.

Regarding which type of amp this is for, I change amps quite often but it's always around the same circuits:

-The classic Fender AB763 circuit (most blackface and silverface Fenders). The bright switch is not usable at low volumes.
-Marshall early Master volume amps, usually have brightness caps on the preamp pots and master volume pots.