Normally-closed SPST p-p volume pot for gain boost

[deci belle]

Does anybody know about such an animal?

I'm using a conventional normally-open 1M switching push-pull volume pot to engage a gain-boost circuit that takes two resistors (330k+180k), in and out of parallel as part of a grid leak to ground between two gain stages (I hope I said that correctly).

Anyway, the normally-open switch means I must have the pot's shaft pulled out (switch closed) for normal low-gain operation to engage the smaller switch-mounted resistor in a parallel path with the other resistor to ground. When pushed in, the higher-value of the two resistors (which is on the board) works alone to boost the gain.

Since I'm used to an effect being activated when a p-p pot is out, I forget!

It's a matter of convention and aesthetics…

Maybe there's a better way to use this p-p pot?

[Jazz P Bass]

You will have to go with a SPDT switch.

[deci belle]

ok, Jazz!

[loudthud]

Using that pot/switch would require that you use a resistive attenuator and shunt the series leg with the switch.

[deci belle]

I'd like to try that, loudthud, at least because I don't know how to do that yet.

loudthud wrote:

Using that pot/switch would require that you use a resistive attenuator and shunt the series leg with the switch.

Right now I have a lead from each end of the 330k R on the eyelet board to reach the p-p volume switch (5f8 chassis).

One lead terminates at one of the p-p switch's tabs. The switch's other tab gets one leg of the 180k R; the free end of the 180k R connects to the other lead returning to the resistor on the board.

Please describe your way in more specific terms for me, loudthud!

[loudthud]

The attached schematic shows values that I just pulled out of the air, you may need to change them to suit your circuit. The bad thing is that when the switch is closed in the Boost mode, there is a greater load on whatever is driving the attenuator. To remedy that, I chose a rather large dropping resistor, the 2.2 Meg. You may need to go even higher if you need more attenuation.

[deci belle]

Yay— I'll see how it works! Thank you, loudthud!!

Oh, what size cap is a good place to start?

[loudthud]

Oh, what size cap is a good place to start?

The cap might present a problem because it will pass more lows in the non-boost mode. If you need to limit lows in both modes, it best be done at a different stage. If you must limit lows in the non-boost mode at this point in the circuit, you will have to add another cap and resistor, let me know and I'll mod the schematic. In the Boost mode the cap limits lows in conjunction with the 470K so 0.022 to 0.047uF should give lots of lows, go smaller if you need to limit the mud.

This is a process called "Cut and Try It". Get the gain difference between the two modes right by selecting the resistors, then adjust the lows if need be.

[deci belle]

I've been doing just that lately, with some (surprising) success!!

I'll work on this one, loudthud~ thank you for the starting points!!