invitro-FETs

[jean-charles maillet]

>Y-ellow Y'all.
>        I'm wondering if anyone could give me a clue as to how to work out
>the bias arrangements for some Glow-FETs. In particular 12AX7s and 12AU7s. I
>want to bias them up to about 200 volts to overcome the clouding problem but
>I'm not sure how to calculate the bias resistors.

>I would assume that the bias for a 12AX7 would be identical to that for a
>12AU7. Whilst I'm not really up on invitro technology, I'm led to believe
>that the AU7 is just a lower noise, lower gain version of the AX7. So it
>will be those little suckers of which I'll more than likely be using.

... if you're biasing a punchless 12au7 as a common-cathode gain stage from
200v  then ground the grid with a 1Meg resistor, plate to 200v connected
with a 100k resistor, and cathode to gnd resistor anywhere between 820 to
2k7 depending on gain and headroom (a very typical circuit).  You can
adjust cathode resistance so that plate lies around 100v at idle to
maximise headroom (a 2k pot with 500 Ohm protection will do) .  Also you
can cathode bypass the cathode resistor with a cap for top end or overall
boost (25uF).  Don't forget DC blocking caps, the higher in value the more
low end you get.  ZoutDC for this circuit is anywhere between 10 and 50k
depending on tube type and individual tube.  Also, ZinDC is around 500k so
you may need to buffer plate with cathode follower circuit if driving lowZ
load.  That is, grid connected directly to previous plate, 100k cathode
resistor to gnd, plate directly to 200v, output at cathode which is at high
DC voltage.

jc

 http://www.lynx.bc.ca/~jc/    \\\\\\\\\\\\\\\\\\\\\\

     Vacuum tube modeling - Traynor amp schematics