I have now conducted some initial tests on the prototype PCB using a CInemag CMMI-10-PCA mic input transformer (1:10) and a Canford VTB2291 output transformer wired 2K4:600. The results are encouraging.
Maximum gain is 70dB with a 600 ohm load (1.5dB more unloaded) which is plenty even for ribbon mics.
With gain set to 60dB, the output noise is -60dBu which implies the EIN is about -120dBu - not bad for a tube mic pre - there is a little hum in the output which appears to be due to the ac wires from the power transformer travelling across the PCB. I should be able to imrove the routing and eliminate the hum when the proper transformers arrives.
With the level control fully anti-clockwise, the output noise is -80dBu with a 600 ohm load - again a satisfactory result.
Distortion at +20dBu into 600 ohms is 1% at 1KHz. This is a lot higher than the Eurochannel design but not unexpected and allows you to create plenty of 'tube sound' by over driving if you wish. At 0dBu the distortion into 600 ohms was close to 0.1%. I did not measure it but the unloaded distortion should be a lot lower.
Nest steps are to fit the main PCB into the box, build a 4 toggle PCB and attach it to the front panel and connect it to the main PCB and retest.
Pic of bench test set up attached.
Cheers
Ian