Tube Challenge!! (2)

media at media at
Mon Nov 29 21:47:32 CET 1999

At 10:39 PM -0800 11/28/99, Harry Bissell wrote:
>"High impedance tube" is not an oxymoron (see jumbo shrimp, military
>High impedance tube is "redundant".

I guess it's all relative.  Compared to the op-amps used in instrumentation
sample and hold circuits, including those with FET inputs and picoamp bias
currents, they are low impedance.  I don't think much work has been done
with tube sampling -- by time there were analog to digital converters
looking for a voltage to be held, op-amps had already been invented.

Don't get me wrong.  If you want to design a tube S&H circuit, a thyratron
random voltage generator, or some sort of thermionic source of uncertainty,
knock yourself out!!

>Also: most oscillators are really "current controlled oscillators"

Certainly, most tube oscillators were adjusted by rheostats.  However I was
referring to the difference in approach used by Eric and Rene.  In the
Barbour thyratron VCO the frequency is controlled by the grid voltage.  In
the Schmitz neon VTVCO it is controlled by the cathode current.  Imho,
Rene's hybrid design with its linear response is much more useful.

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