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Armstrong Configuration
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From: Gregory R. Hunter [SMTP:ghunter-at-mail.enterprise-dot-net]
Sent: Wednesday, March 04, 1998 1:54 PM
To: Tesla List
Subject: Re: Armstrong Configuration
> To: "'Tesla List'" <tesla-at-pupman-dot-com>
> Subject: Armstrong Configuration
> Date: Mon, 2 Mar 1998 23:50:32 -0600
> From: Tesla List <tesla-at-pupman-dot-com>
>
> ----------
> From: D.C. Cox [SMTP:DR.RESONANCE-at-next-wave-dot-net]
> Sent: Sunday, March 01, 1998 5:48 PM
> To: Tesla List
> Subject: Re: Armstrong Configuration
>
> to: Ed
>
> Consider "borrowing" a variable cap for a short term from a physics
> research lab or a large local surplus house. If you befriend some of the
> researchers with your project you should be able to borrow one for short
> term. After testing and adjustment of your tuned circuit you can determine
> the exact value required and then substitute a fixed value unit. Perhaps
> slight retuning of the primary and feedback winding will be necessary but
> only for a few turns. This technique will help you determine the exact
> value you need and eliminates the cost of a large variable which is too
> cumbersome to leave in your final circuit anyway due to the high potentials
> on it.
>
> DR.RESONANCE-at-next-wave-dot-net
>
>
> ----------
> > From: Tesla List <tesla-at-pupman-dot-com>
> > To: 'Tesla List' <tesla-at-pupman-dot-com>
> > Subject: Armstrong Configuration
> > Date: Friday, February 27, 1998 9:42 PM
> >
> >
> > ----------
> > 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
> >
>
Ed,
You're right about that. The variable is too little to stand alone
in the grid tank. My calibrated eyeball says it's about 100 - 250pF
at perhaps 2500V. It's in the final section inside the PA tube
compartment. I've found a source for cheap .001uF 10KV ceramic caps
as well as .006uF 1500V mica caps. I plan to get in the ballpark
with some combination of fixed caps, and use the little air-variable
for tweaking. I talk big, but I actually have very limited experience with
vacuum tubes and zero experience with tube Tesla coils. If I get any
spark at all out of the finished product, I'll be thoroughly
satisfied.
Greg
Beck Row, UK