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So I'm thinking about building the 18-watt kit, but like the idea of one input and then channel switching between the tmb and standard channel..
I decided to try modifying the schematic a bit to try an idea for a slightly different layout but the same channels.. I had to change the portion feeding the PI a bit, but I don't think that should matter too much.

How does this look? The switches would be each pole of a dpdt relay..
http://vwtweaked.ca/Trinity18_TMBchannel%20switching%20idea.jpg
Does this seem like an alright way to do this? Or is there some other implementation that's been done which would work better? I know relays can be noisy when they switch..

One of the great benefits of 18W TMB amps is how you can jumper both channels together, and then blend them with using their respective controls, to give an even wider tonal palette. IMO an ABY box on the floor is much more versatile than on-board channel-switching and also less likely to induce noise in the amp.

JMOHO

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Great tones come in small glass jars!

Yea, I was thinking that it would suck to miss out on that available interaction between the channels.

My only concern was extra noise if I was using an a/b/y box.. ie) I'm switched to the standard channel, and I have the TMB channel with the gain cranked way up.. Will I hear too much extra noise from the TMB preamp feeding the PI even though I'm playing through the standard channel..

Any comments on that?

If you're using a A/B/Y box you will need to use one that has a transformer in it to isolate the channels and possibly some phase control. There are kits to DIY and there are ones you can buy. I think once you get the amp built you will see how it sounds and then you will have lots of choices. There are a few switching options that are low noise, but lead dress is very important. On one amp I did for a guy we added an extra tube and just switched it in and out for a great lead boost, but there are also ways to simply switch a cap in and out for a good boost as well.

I agree that a good ABY should have isolation transformers inside. The channel that's switched off will also be shorted to ground inside the box. Phase changing however isn't required, since both inputs on an 18W TMB amp are in phase. In my experience, relay switching circuits can introduce more noise inside the amp, which can take some work to get rid of.

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Great tones come in small glass jars!