[Amps] Size of Parasitic Chokes

[Bill, W6WRT]

ORIGINAL MESSAGE:

On Thu, 15 Jul 2010 20:21:11 -0400, Roger <sub1 at rogerhalstead.com>
wrote:

>
>How do you determine the size of a parasitic choke for a given tube 
>other than using "The Handbook" as a starting point and going with "cut 
>and try" from there?

REPLY:

The value is not particularly critical. One method I recall seeing is
to first determine the resonant frequency of the VHF parasitic tank
circuit with a grid dip meter and then make an inductor with a
reactance at that frequency equal to the resistance of the suppressor
resistor. 

For example, if you're going to use a 50 ohm resistor, and the VHF
parasitic tank resonates at 100 MHz, you would use an inductor of .08
uHy. Again, not critical. Just get close. 

Your initial grid dip measurement of the parasitic tank frequency
should show a very sharp, narrow dip. After installing the suppressor,
the dip should be very broad and shallow, so shallow as to be almost
no longer apparent. 

The tricky part is using enough inductor to be effective without
having so much that the resistor overheats when operating on ten
meters. A delicate balancing act, believe me. You can simplify the
balancing act somewhat by using a high wattage resistor, say 25 watts
or more. Typically, hams use a resistor of only a few watts or so and
that is where it gets dicey. The resistor must be non-inductive, of
course.

The best approach of all is to design the amp so no suppressor is
needed in the first place. This can be done by ensuring that the
cathode to ground input circuit has very low impedance at the VHF
parasitic frequency. If the impedance there is low enough, the tube
will not have enough gain to oscillate. Problem solved. 

73, Bill W6WRT