problem - two mic amps sharing input transformer ?

I fail to see the potential problem - why would switching be different from plugging and unplugging bantam jacks?

Jakob E.

In terms of impedances etc, I don't see any real problem. The practical difficulties may come in wiring and grounding. I assume you mean the switch to select which output of the preamps to use and not to switch the transformer secondary between the preamps.

That said, I'm a bit nervous of the wiring distances between the two preamp channels. You would have to be very careful not to be susceptible to hum and interference pickup, and maybe bandwidth problems, since the scheme you outline would mean connecting not only the grids but also the grounds of the two preamps together at their inputs.

I always treat the input transformer and first valve stage as a single indivisible unit in my valve preamp designs, both on paper and when designing the constructional layout. In this way, I have been able to get excellent noise figures coupled with a high bandwidth by minimising the effects of input capacitance. It's also important to keep the grid wiring well separated from components in the anode circuit, because even small capacitances between the two are multiplied by 1+the gain of the stage (Miller effect), and form a low-pass filter.