Signal splitting

[Airyk]

Hey I'm building an amp for a guy and he wants two parallel preamps that he can switch between.

The plan is to split the signal after the input jack, feeding both preamps, and have channel selection via a relay after each output.

My question is, should I be splitting the signal after a buffer stage? I have a free side left over on the first tube of one of the preamps that I can use...

Or can I achieve the split without any tonal degredation with just a couple of 68K resistors?

I hope I'm clear.... =P

Thanks in advance,
Eric

[Firestorm]

I assume he wants to set the two preamps to some level and switch between them: sort like rhythm and lead. That could be a problem if you leave the grid on the unused stage open. It will pick all sorts of stuff and amplify it. It's probably better to set the switching up so the grid of each stage is grounded out when not in use. That means you have to switch the input so its signal isn't loaded down by the other stage's grounding. It can be a little complex with relays: I think it would take three SPDT relays.

[brownnote]

Switching at the coupling stage (output) of each preamp would work by itself, but with 3 poles you could also switch the input signal and isolate them so that the preamp not in use would be grounded....kind of a belt and suspenders approach.

If you use relays, two DPDT's or one DPDT and one SPDT would work.

If you don't need this option foot-switchable, then you can use a toggle switch which would simplify things a bit.

I've used this arrangement and it works great...there's no stray signals or bleed...it's quiet.

It would help to know more about your preamp topology...are they separate, with dedicated volume and tone controls for each preamp or are they sharing all the controls?

[brownnote]

here's a pic that shows the switching [img529]http://www.brownnote.net/24amp/24switch.jpg[/img]