Vox Tone Stack Weirdness

[Daver]

I've been fooling around with a Vox AC 30 type circuit in a cathode biased 4X EL84 amp. I've tried out different tone stacks and wanted to try the original Vox type but with a Midrange control. Modelling things in Duncan's Tone Stack Calculator showed that a typical Marshall tone stack with Vox values and a 25KL Mid pot was very, very close.
My issue is that the Bass control doesn't seem to do much of anything. I've been all over the wiring and pots. It worked fine before in other non-Vox arrangements. Is this typical of that tone stack? In TSC the Bass control works in either scenario. Ideas?
Dave

[Daver]

Well, I rewired the Bass pot so that the tone cap for it goes directly to the wiper of the pot and the "bottom" of the Treble pot. The "top" end of the Bass pot is unconnected. By doing this the Bass control now works. Odd. I'm not completely sure why the Bass pot wired as a rheostat didn't do anything. The TSC program has it wired that way in the Marshall simulation and it seems to work fine. Any more input is appreciated.
Dave

[robpietsch]

Hi Dave,
I've just come across the same problem - I think the answer is to wire the bass pot upside down (hot side to ground). I'm not 100% sure, as I haven't wired it up yet, but this works on a spice simulation.

Hope that helps,
Rob

[Daver]

Rob,
You're right that the stock Vox Bass pot is kind of wired backwards. Since I am using a modified Marshall tone stack to add a Mid control, that changes things. It's just odd that the Bass pot would seemingly do nothing when wired as a rheostat but work fine with the wiper connected to the cap. The TSC modelling showed it should work in the first instance.
Anyway I appreciate your help and input.
Dave

[robpietsch]

Dave,

Sorry, I misunderstood what you're doing. Essentially, you've got a normal marshall tone stack with different values, 50p treble cap, 1M treble pot, 100k slope resistor? And the only change you made was to disconnect the top of the bass pot from the .022 cap/bottom of treble pot? If so, that should make absolutely no difference at all, you're right, that is odd. the only thing i can think of is that the wiper wasn't properly connected before.

Or did you have the bass cap going to the bass wiper and the top of the bass pot connected to the bottom of the treble pot? If that's the case, maybe the bass pot needs to be upside-down, i'm not sure.

I hope that hasn't confused things too much,
Rob