[ARC5] Stupid Question No.5036

[Jim Haynes]

On Thu, 24 Jul 2014, Robert  Eleazer wrote:

> Where in the usual tube data is the frequency response for the tube?
>
It's not a stupid question, but you have to know what question to ask.

The figure of merit for tubes (and other devices, for that matter) is
the gain-bandwidth product.  You don't find that in the tube manuals
either.  For tubes the usual limitation is the output capacitance.
If the tube is resistance loaded, as in an audio amplifier, you have
the R-C time constant of the load resistor and the output capacitance
of the tube.  You can't make that arbitrarily small by reducing R because
the tube needs a certain amount of plate voltage and can tolerate only
so much plate current.  So above the cutoff frequency imposed by the
R-C circuit you have to use a tuned plate circuit, making the output
capacitance part of the tuning capacitance.  There you can play with the
Q of the circuit to get bandwidth at the expense of gain, or vice versa.
So you can compare tubes by comparing the mutual conductance and the
output capacitance.

At VHF things get more complicated because of things like the inductance
of the leads to the tube, and performance of dielectric materials, and
ultimately by the limited speed of electrons through the tube elements.