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Armstrong Configuration
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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
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> 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
>