Re: Max anode voltage?

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Shad <shenderson@xxxxxxxxxxxxxxxx>

On Wed, 2006-05-17 at 19:28 -0600, Tesla list wrote:
> Original poster: "Andrew Bonnell" <andrewbonnell@xxxxxxxxx>
>
> Out of curiosity, how fast to vacuum tubes switch?
>
> Andrew
>

The short answer to your question,

Tubes are fast.  Very fast.

Large power triodes run happily at maximum power up to usually 30(ish)
mhz, with reduced ratings at frequencies higher than that.  Tetrode and
pentodes operate at higher frequencies at maximum power, with similar
deratings beyond that.

In a nutshell, most of the tubes used for driving a VTTC tend to top out
at 30mhz or so (at maximum power), giving us *plenty* of headroom.


The long-winded answer to you question,

Switching in a vacuum tube is dependent on a few factors,

The ultimate limit is the electron transit speed.  The higher the plate
voltage, the faster the electrons travel from the cathode to the plate.
The smaller the distance between the cathode and plate, the faster the
tube can operate because the electrons don't have as far to go.

By imposing more control grids/screens in the electron path, it's
possible to get faster operation because it gives the tube more
"control" over the electron stream over most of the distance it
travels.

Generally, the higher the frequency, the more drive a tube needs because
of Miller Capacitance (between the plate and cathode, and various tube
elements).  The result is that for a set driving power, gain tends to
fall off at higher frequencies.

Microwave tubes run happily well up into the Ghz range, though they're
producing a different sort of power than we're wanting. The "acorn"
tubes used in the Voyager probes run at extremely high frequencies,
thanks to miniscule spacing between the plate and cathode.

While using my solid state driver to drive the grid of a vacuum tube in
pulse duty, it's worthwhile to mention that the vacuum tubes are capable
of operating *much* faster than the solid state parts driving them.
With my measuring equipment there's too much jitter to reliably see or
measure the delay between the signal being applied to the grid, and the
effect at the plate.