[Milsurplus] BC-348's

[Mike Hanz]

J. Forster wrote:

>Can anyone explain how the grid(s) is/are physically structured in various tubes
>to make them high or low transconductace, remote cutoff, etc. I missed tube
>design by a year or two and just learned about them as voltage controlled
>current sources.
>

A good subjective description comes from my favorite glowbug engineering 
text - _Electronic Circuits and Tubes_, by the Cruft Electronics Staff 
of the Cruft Laboratory at Harvard.  Copyright is 1947, but it was 
developed for training engineers during the war, so there are no useless 
frills beloved by the educators of today.  For the remote cutoff tube, 
it says,

"The remote-cutoff characteristic is obtained commonly by spacing the 
control-grid wires more widely near the center than at the ends...
    As the grid potential is decreased from zero, becoming more and more 
negative, the electron stream first "cuts off" in the region near the 
ends of the spiral, where the spiral is tightly wound.  Higher negative 
grid potentials confine the current to the center, or loosely wound 
portion of the grid, where a complete and very gradual cutoff is 
achieved finally by a large negative grid potential.  Near cutoff the 
tube has a low gm.  For grid potentials near zero, the remote-cutoff 
tube has a gm which is intermediate between that for a tube with a 
tightly wound grid spiral and that for one with wider spacing."

There are probably other texts that go into it more deeply - this was in 
the first text I picked up.