On Thursday, September 5, 2013 12:52:20 PM UTC-4, n8zu wrote:
Thanks for all the great replies

Well I'm running the filament on DC so a doublier of 6.3vac with a tl431 programmable zener to get 12vdc only requires 3 cheap parts, with the doublier 7 cheap parts. 12sn7 was the cheapest I could find for a dual triode. Though I could have used a tl072 fet op amp, I like tubes and there's a tube socket that wanted a tube, instead of a tl072 ic mounted in a octal plug.

And the poster may be right about the plate and cathode circuits limiting the current a 12sn7 has like 3000umhos and the more current you run through a cathode circuit the more negative feedback you have, thus limiting the current.

When I used RCA VTVM's when I worked for Sears service back in the day. Sort of like band switching and you forget to change the band switch.
Alas I don't remember wrapping a needle around the movement post.
73
de n8zu