Vibro- no more. Adding a gain stage to the Champazoo.

[FXBDM 1832]

My Vibro-Champazoo does not vibrate anymore. I had a few problems integrating the tremolo, and then realized I never used it anyways, and decided to disconnect it from the circuit. This means I have this tube socket sitting empty, as well as three or four chassis openings.
So, as one does, I decided to not leave well enough alone and look into adding a gain stage and potentially a James tone stack, to give it more versatility.

I initially looked at implementing some kind of one wire cascade using one half of the available tube, but all the schematics I have seen have you disconnect the normal input, and I would like, if possible, to retain a normal champ channel.

So first, here is a diagram of what I understand are my options. The third line is what I would want to do (with two tone stack locations, I would use only one). The tube is there, so there is little extra cost to using the two halves (and I can maybe use one as a cold clipper or something).

So: questions:

A) is disconnecting the normal input required in a simple one wire mod?
B) what is the best tone stack location? Later in the chain would affect both channels, but it would be closer to a Marshall implementation if I understand correctly. Earlier in the chain would limit it to the gain channel and would give me my unaffected Champ on normal.

What are your opinions on the principle of the project?

 

[sds1]

I would encourage you to not worry about whatever this one-wire mod is exactly (I see it originated in the Marshall forums), because as you noted it doesn't meet your requirements.

Your third block diagram do-able. You can put the tone stack wherever you want! In the interest of your stated requirements I definitely think you should keep it in the "hi" side of the circuit so that the "lo" side remains stock.

You might look at restructuring your hi side to be V3A->TS->gain->V3B, this is kinda classic Fender-y topology so maybe that would appeal to you more. Again, doesn't really matter on paper, and nobody can really predict your outcomes here, you'll just have to build it and find out.

From here I would also encourage you to move your final block diagram into a schematic diagram, it will be much easier to validate how your switching will work as well as visualize the new topology.

PS: I kinda like where you have TS now, following V3B, this way you can drive V3B harder and generate some harmonics, then dump some signal in the TS before hitting the "lo" side of the circuit.

 

[sds1]

It occurs to me maybe I shouldn't leave you hanging on the switching part, I reckon that one-wire method you were looking at doesn't do any fancy switching.

You might copy the Dumble ODS, it has a transistor input jack that cascades in front of the normal jack when you plug into it. Exactly what you want to do here. Assuming you are using Switchcraft type parts, it simply requires you swap out the 12A on the Hi Input side for a 14B:

https://www.parts-express.com/Switchcraft-14B-1-4-Stereo-Jack-Female-Panel-Mount-Double-Closed-Circuit-093-1005

You still use a mono instrument cable obviously but this convention takes advantage of the extra jack switch:

 

[printer2]

Since you have a couple of holes in the chassis to use you can have a master volume, a gain control for the new channel. Keep the bulk of the amp as is but rather than the first stage having a 100k plate resistor change it to a 49k and connect one of the 'new' triode's plates to it. Use the other triode as the first stage of the gain channel and stick a volume pot between the triodes. The two channels mix in the 49k resistor and go through the tone stack. Change the 220k resistor on the 6V6 input with a pot. Put a 1-4.7M resistor on the wiper to ground (insurance if the wiper goes open).

 

[FXBDM 1832]

Looking into this more, it seems that making it a multi-channel amp would halve the gain of the stages up to the point where they are mixed. Is that self defeating, or are there ways to mitigate it? I guess switching would solve that?