# Re: Tube Coil Calculation

```Tesla List wrote:
>
> Original Poster: DavidH8083-at-aol-dot-com
>
> I am curious about establishing the value of the primary capacitor for a
tube
> coil. I am aware that there is a simple formula for picking the optimal tank
> cap for a standard disruptive coil based on the power of the transformer
> feeding the tank circuit.
>
> Brent Turner's book suggests that this is not a good idea for tube coils and
> that the tank capacitance should be kept quite small.  He suggests .0015 to
> .004 µF for the primary tank cap.  But I really cannot find any guide
lines to
> picking the "right" capacitance.  I've looked at several schematics for tube
> coils using various power levels and tube configurations, but cannot seem to
> establish any sort of rule from these for a tank cap.
>
> Anyone have any experience with this?
>
> David Hutchison
> davidh8083-at-aol-dot-com

A rule of thumb used with tube RF amplifiers and oscillators would be
appropriate here.  From the plate voltage you intend to run you can
estimate the RMS RF primary voltage to be about 0.67 times the plate
supply voltage.  You can estimate the effective load impedance across
the primary to be the RF primary voltage squared, divided by the power
you expect to run.  (This will be around 70% of the plate input power,
if your design is reasonably efficient.)  Having gone this far, you now
need to know your operating frequency.  You want the primary circuit
reactance to be no more than 0.1 times the load resistance. You then
calculate the primary capacitance as the value which will give you this
reactance at the operating frequency.  You have a lot of freedom in your
choice of capacitance, this is just for starters.

For example, if the plate supply voltage is 2000 the rms primary
voltage will be about 1300.  If the power output is to be 1 kilowatt,
the effective resistance will be about (1300^2)/1000, or 1690 ohms.  The
desired primary reactance will then be one tenth of this, or 169 ohms.
At an operating frequency of 200 kHz this corresponds to a primary
capacitance of 0.009417 mfd.  Your value of 0.0015 mfd would be much too
low, the 0.005 mfd would work but is still on the low side.

If that sounds too complicated (and no one else gives you a better
answer) I'll formulate the equations and post them here.

Ed