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Armstrong Configuration


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From:  Mark S. Rzeszotarski, Ph.D. [SMTP:msr7-at-po.cwru.edu]
Sent:  Friday, February 27, 1998 2:19 AM
To:  Tesla List
Subject:  Re: Armstrong Configuration

Hello All:
Greg said:
>I'm still trying to figure out the best way to employ an 833A triode
>and a 1KVA MOT in my next Tesla coil.  Right now I'm leaning toward
>the Armstrong configuration wherein plate voltage is supplied thru
>the tickler winding and the tuned tank is in the grid circuit. I know
>this isn't optimum, but it obviates the need for a HV variable cap in
>the plate tank (I don't have one).  <snip>

        I have built a number of tube coils using both the tuned grid and
tuned plate configurations.  You can have a look at a circuit and brief
discussion for a tiny tuned grid coil at Chip's web site: www.pupman-dot-com.  I
have found the tuned plate systems to put out longer sparks than the tuned
grid circuits, even after extensive experimentation with varying the
coupling between the plate, grid and secondary coils.  With the tuned grid
circuit, I use fairly tight coupling between the plate and secondary, and
loose coupling to the grid coil.  Either method will give you sparks, and
the tuned grid system is easy.  
        Bear in mind that the plate capacitor value in a tuned plate circuit
is usually quite small, on the order of .001 to .005 uF.  It is quite easy
to build a small poly cap of this size that only has to withstand a few
thousand volts, so don't let the fact you don't have a high voltage
capacitor stop you.

>BTW, can anyone give me a rule-of-thumb for selecting a grid leak
>resistor value?  I plan to run it class-C with 2250VAC unrectified
>on the plate
. 
        Using the RCA tube manual, the grid voltage should be -500 volts at
100 mA maximum grid current.  That works out to 5k, assuming you are using
4000 volts on the plate.  I would start out around 10k and plan to work
downwards from there.  If the grid resistance is too high, the tube works
deep in class C, and is not likely to be damaged.  The grid resistor power
rating is quite high, on the order of 50 watts for this tube.  I usually end
up with a final grid resistor value which gives me close to the maximum
rated grid current, if the plate transformer has enough power reserve to
drive it that hard.  After all, the maximum tube ratings are the starting
points for tesla coilers, right?

>Also, how much inductance do I need in the plate tickler coil to get good
>behavior from the 833A?  

        The reactance of the plate coil (2 x pi x Fres x L) in a tube system
is designed so that it is approximately equal to the tube load impedance,
for maximum power transfer.  For class C, the rule of thumb is to have the
reactance of the plate coil equal to about Vpeak/(2 x Iplate), where Vpeak
is the peak voltage your MOT can produce (not RMS), and Iplate is the plate
current.  The factor of 2 comes in from the fact that in class C operation
the tube is conducting only part of the time.  Depending on how tightly you
couple the secondary, your mileage may vary, but it should get you in the
ballpark.  If in doubt, use 15-20 turns.  For the grid coil, start with 20
turns.  It will most likely work in spite of what you wind!  Then adjust ad
nauseum until you get the longest sparks.
Regards,
Mark S. Rzeszotarski, Ph.D.