# Re: Tube Coil Calculations

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Here's what you do.  You find the Cathode to the anode voltage, and find
the peak current.  Then divide the voltage by the current, to find the
impedance of the tube.  The primary of a transformer must be equal or
greater than that value to operate the tube properly.  If you don't want to
burn up the tube, then multiply the value of impedance by found for the tube
by 1.2, which will give you 80% of the tubes maximum output.  Then run a
signal through the primary of the coil you have chosen, short the secondary,
and match this value of impedance on the primary by increasing or decreasing
the signal voltage.  Now, if you have a potentiometer you can also measure
the resistance, and adjust the value of the pot to the value of impedance
you are looking for.  Then when you apply the signal you adjust the signal
generator to the frequency that places half of the applied signal voltage
across the resistor.

In a vacuum tube the element that you are most likely to burn up isn't
the properly biased filiment but, the cathode.  The cathode in the physical
vacuum tube is actually a thin piece of wire wrapped around to ceramic posts
around the heating element.  The cathode never touches the filiment, it just
uses the heat to boil off electrons.  Typically, vacuum tubes use a negative
ground, a postive supply voltage, and have a relatively high input impedance
but, not compared to a MOSFET The grid is negative 1 to and pending upon the
tube you are using 10 volts negative in respect to cathode/earth ground,
which is earth, or chassis ground.  It should be that earth is chassis in a
properly designed circuit because, there is a small chance of the anode
shorting to the grid in the triods, and the high impedance of the biasing
resistor used for the grid which can be up to 5 meg, isn't enough to keep
450 volts from reaching some-one who handles the chassis.  So, actually, the
power supply is split, 1 to 5 volts negative, ground, and 450 volts postive
using a metal chassis, and the metal chassis also is used for an AC, or
signal ground.

James.

>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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