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Armstrong Configuration
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From: Edward V. Phillips [SMTP:ed-at-alumni.caltech.edu]
Sent: Friday, February 27, 1998 12:13 PM
To: tesla-at-pupman-dot-com
Subject: Re: Armstrong Configuration
"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). However, I do have a nice air
variable liberated from a dead Swan 240 SSB rig which should work OK
in the lower voltage grid circuit. "
I suspect you're going to need a larger variable capacitor \
than any ham rig uses. That depends on the frequency of you coil,
of course, but I would think that with any reasonable tank inductance
you will need a lot of capacitance. Sort of rule of thumb is that
you need a Q of greater than 20 under load, which says you need an
effective reactance of about 1/20 of the load resistance. Again,
rule of thumb would say that if you're planning on running 1 kw
input, at an effective voltage of around 2000, you will need a
reactance of less than (2000/20), or around a hundred ohms. At
300 kHz that works out to around 0.004 ufd, it I didn't slip
a decimal point. I would expect your Swan capacitor to be noe
no more than a few hundred uufd, depending on whether it was the
tank tuning capacitor or the output capacitor in a Pi matching
network.
As far as the grid resistor goes, it should be adjusted
to give the required grid curren t, which I would think would
be of the order of 100 ma or so.
Ed