Fender Tone Stack With 1M Pots ?

[ncusack]

Hello,

I'm doing a scratch build and I'm limited to 1M lin and log pots. I'm looking to get a similar response to the normal fender stack which uses 250k log pots and fixed 6.8k mid resistor. I think I can get something close by using a 10k mid resistor, 1M lin for the treble and 1M log for the bass with a 100k slope, 100p treble cap and 22nF for the bass and treble caps. I used the duncan tone stack sim and these value seem to come close to the fender values. The mid notch seems to line up but the bass sweep seems to kick in more abruptly. I'm wondering if I might run into any other issues when using all 1M pots.

Regards,

Neill

[sluckey]

You will probably just have to build it then tinker it. I bet you can find a combination of resistors and caps that will please you.

Might be a lot of effort just to avoid spending $5 for the correct pots. Might be a lot of fun too. Let us know how this turns out.

[John]

Sluckey, would just hanging a 470k resistor between wiper and ground lug on those pots get him close enough to use stock values? A 330k || would be give him 250k pots if I'm thinking right?

[kagliostro]

A resistor in parallel with pot was used sometime by a friend of mine, he is satisfied of the result

but keep in mind that you can have some unespected effect rotating the shaft because of the non linearity of the resistence values


---

...... hanging a 470k resistor between wiper and ground lug ....

is a way to have a Log pot from a Linear pot



Franco

[tubeswell]

As a rule of thumb, if you scale up the resistance, then you scale down the capacitance in inverse proportion (or vice versa) in an R/C or C/R network, in order to get the same frequency response. (So with 4 x the resistance, you want 1/4 of the capacitance etc) but make sure all the resistors and caps in the tone stack are scaled using the same inverse proportion in order to get the same freq response.

[Joel]

Have you used this cool tone stack calculator? 

http://www.duncanamps.com/tsc/

[PRR]

> As a rule of thumb, if you scale up the resistance, then you scale down the capacitance in inverse proportion

Exactly.

The problem here (most tube tone networks) is that there's about 100pFd of input capacitance to the tube, and we can't easily reduce that. To get Treb Boost, Fender used 270pFd on top of the Treb pot. If we reduce that to 39pFd, while the next tube stays near 100pFd, we won't get much boost.

The choice to use 250K pots was well-reasoned. You want to be in that range to get "similar response".