mic testing question?

Hi,
I've built a power supply and a mic circuit on a piece of plywood as a bench test setup so I can plug in a few tubes and measure the PSU voltages with the "real life" load. The circuit is based on the AMI-Tab alternate tube "U-47" with a European pentode.

I'm just about done wiring it up and have been wondering if I need to include a capsule or some substitute capacitors to get useful measurements or if I can just leave the capsule out for now and still get B+ measurements that will be similar to when I have the whole thing assembled in a mic body.

Thank You for any tips or advice you can share.

You should hook up some capacitors in place of the capsule sides. Find out from your capsule manufacturer what the pF value is of the capsule you intend to use and get a couple ceramic's of the same value and voltage ratings.

You can then inject/measure external signals if you like and have the circuit be "the same" electrically without acoustic influence. You will need to shield the circuit since in open air on plywood it's going to produce a lot of noise.

But to answer your question WRT to B+ specifically, the capsule draws so little current that it's not likely to make significant differences in the B+ values with/without "dummy capacitors" in place of the real capsule.

Cheers,
jb

Thanks!

I have some of the China K67 capsules at the moment and hope to finish the mic off with a K47 or M7 type capsule.

How about 70pF for the front and 70pF for the back?

How about is I build a second test circuit for use with a 251/C12 type capsule?

Thank You.