[TCML] Tube question

[mddeming at aol.com]

Ralph

FB OM! That's the way I remember it, when I worked in our high school
station (W2CLE) 50 years ago this week. But going back on my notes to
2000, I could not find a single reference to design parameters/theory 
in
any of the VTTC threads. A lot of it seems to be cut and try or just 
copy
with far less attention given to how and why than is given to a spark 
gap.

73

Matt D.

-----Original Message-----
From: sparks at spectrotek.com
To: Tesla Coil Mailing List <tesla at pupman.com>
Sent: Sun, 21 Sep 2008 10:16 pm
Subject: RE: [TCML] Tube question



> Does anyone (maybe in the over 50 crowd)

That's me, for sure! <G>

> remember about
> tube characteristic curves and how to use them to determine
> the proper operating points for VTs? It seems that 45 years
> ago this was the right way to determine the proper bias values
> for tube circuits, but I've never heard anyone on the list ever
> discuss doing a proper analysis, so maybe it's a lost art that I
> only vaguely remember?

It still works just fine. I used the charts a few weeks ago to 
calculate the
operating point and design the plate tank circuit for my KW-level HF 
amplifier
using a large, gently-used TV transmitter tube.

In order to do this, you need to know the plate voltage you will use 
and the
average plate current the tube will handle during the conduction 
period. Then
you can look at the curves and determine the resting (no signal) bias 
voltage to
set the idle plate current (for linearity, if that is important) and 
the total
grid voltage swing to drive the tube from plate current cut off to the 
desired
plate current.

If you are driving the tube in pulse service, you will have to use the 
average
current during the ON time of the pulse for your calculations. 
Generally,
unless the tube was designed for pulse service, the curves will not go 
that
high. Simple extrapolation of the existing data curves may not work, as 
the
tube may saturate and "flat-top" before you reach the desired plate 
current.

Note that if the tube is a low-mu tube (generally triodes) you may not 
be able
to drive the grid sufficiently far enough positive to obtain the 
desired plate
current without damaging the control grid of the tube. Both the control 
grids
(G1) and the screen grid (G2) in triodes and tetrodes are much more 
delicate
than either the cathode or the anode, and may easily be destroyed by 
even a
brief application of excessive current.

It's best to keep the fireworks on the outside of the tube!

73,

Ralph W5JGV - WD2XSH/7

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