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Re: Tube Coil Calculations
Interesting this should come up because at work we were just having a
similar discussion about high power microwave solid state amps. There is a
reason for the vagueness... high power amplifiers and oscillators, both
tube and transistor, are very tough to design in a cookbook way. Why is
this?
Cookbook procedures require a fairly simple, accurate model of the device.
For small signal circuits, this is fairly easy to come by, because the
device characteristics are fairly stable, and you can use a linear
approximation making the model simple. High power circuits, though, are
typically grossly nonlinear (operating in both saturation and cutoff), and
getting good device parameters is tricky, because they are circuit
dependent.
Another problem is that high power circuits are physically large, and for
most RF circuits, the parasitic capacitances and inductances from the
layout are significant, compared to the reactances of the active device.
For instance, there is a significant interelectrode capacitance in a
typical vacuum tube, but also capacitances on the same order in the socket
and surrouding wiring. The typical RF power amp or oscillator has lots of
adjustable components, whether as explicit variable devices (caps, etc.),
or inductors that can be squashed or stretched, or even, as we tend to
specify here at JPL, "value to be selected at test", i.e. you try a bunch
of different components and take the one that works the best.
Fortunately, for TC type applications, you aren't trying to eke out the
last percent of efficiency or noise figure, and you are probably using
surplus active components that someone else has used before, so you can
start with a pretty generic circuit that someone else has built and tested,
and then start modifying.
In general, for a tube oscillator (or amplifier) you will need:
A rougly adjustable HV power supply commensurate with the max plate voltage
rating of the tube (several kV) (so you can try your circuit at low power
before you put the pedal to the metal)
A big RF choke to feed the HV to the tube
A tank circuit resonant at the frequency of interest (i.e. your TC)
connected as the plate load
Some biasing circuit for the grid (typically a big resistor or resistors)
Some means to get feedback to the grid from the output, typically another
winding on the plate tank.
Some measuring tools, like a VOM
Some real respect for HV RF which will truly reach out and touch you in
amazing ways.
----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Tube Coil Calculations
> Date: Friday, August 20, 1999 5:38 AM
>
> Original Poster: "Benjamin W. Bouxsein" <bbouxse-at-comp.uark.edu>
>
>
> Hello,
>
> I need to find a good book that will give me the exact formulas for
vacuum
> tube coils. Also need a source that has this book available. If anyone
> could give me a step by step proceedure to follow over email that would
be
> great too. I would really like to build a VT coil but ARRL handbook if
> VERY vague on how to calculate feedback/grid coils and tank coil
> consciderations.
>
> Thank You,
> Ben Bouxsein
>
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krzr-at-comp.uark.edu
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