Dual channelling a Turnabout...

[Willybomb]

Gudday all.  I've just boxed up a Turnabout that I've made dual channel (because that's what I do).

Works ok, not sure there's enough saturation for my liking unless it's set to the 2n7000s for clipping (but that's just me) and it doesn't seem to make unity unless it's clipping with the LEDs or Bats, but I'll work that out.  Probably with a post booster, or maybe I'll sandwich it between stages of a Katana like the 1969x.

No, my issue is that the gain pots are interacting with each other regardless of which channel is switched in.

I'm using the layout and schematic shown here:
https://prentisseffects.blogspot.com/2017/06/madbean-turnabout-jhs-mi-audio-crunchbox.html

I'm switching pin 2 with the volumes pots, with pins 1 and 3 connected to the other pots' pin 1 and 3 and that's working fine.

I'm switching out pin 3 on the gain pot as that leads to the rest of the circuit.  Should I be switching pin 2?  Looking at the schematic ideally I'd be switching out 1 and 2, I suppose, but I don't have the poles on the 4pdt for that!

Thanks,
Willy.

[sonnyboy27]

In this gain configuration the gain knob adjusts the gain of two stages simultaneously. The first is a non-inverting and the second is an inverting configuration (a lot like the marshall bluesbreaker). The gain pot acts as a voltage divider rather than a variable resistor. So as you increase the gain of one you decrease the gain of the other. I need to update this in the blog post so thanks for pointing it out.

All that to say you'll need to swap out the entire pot with a switch rather than just a single lug.

I'm pulling this info from Aion's analysis of the bluesbreaker (which this shares topology with). https://aionelectronics.com/project/cerulean-bluesbreaker-overdrive/

[Willybomb]

Yeah, thanks.

I went back and looked at the photos of my Ninja 2 channel crunchbox and found I was switching out pins 1 and 2.  I can probably do that in this instance, but I'll loose the diode selection on each channel....

Just to confirm - switching out pin 2 or 3 will just mean I have a pair of variable resistors in parallel, and the overall resistance resistance would be 1/r1 + 1/r2?

Why does switching pin 2 work for the volume pots and not the gain pots in this instance when they're both voltage dividers?

[Willybomb]

Think I've got it.

I'll run the diode clipping switching to the respective red/green LED ground lugs, and that will allow me another pole to switch out pins 1 and 2.

[Willybomb]

Ok, so I changed the gain pots so I was switching pins 1/2 out, with pin 3 as common.  That works.

But now the clipping switches interact with the channel switch LED.  Example:

Channel one is indicated by the bi colour LED being green.  I can actually make it go red by switching both channels to the 2n7000 or BAT clipping.  Which is weird because that leg on the channel LED isn't connected to ground...  It must have something to do with forward voltages but it still shouldn't change colour WHEN THAT LEG ISN'T CONNECTED TO GROUND!...