[HomeBrew] Plate Choke

[jeremy-ca]

Nothing wrong with the math but I doubt the tuned circuit part. A multiband choke is not a straight line reactance from 1.8-30 mHz. It varies considerably and some designs come very close to ham band edges yet they are no more rugged in construction than the best ones. All seem to rely on a bypass of .01 to .001pf depending on bands covered.

If I get time I'll try to research the development of the pi network. Probably something in the Collins "Fundamentals of SSB" if I can find mine.

Carl
KM1H



    ----- Original Message ----- 
    Sent: Wednesday, May 16, 2007 7:14 PM
    Subject: Re: [HomeBrew] Plate Choke


    In a message dated 5/16/07 12:13:18 PM Eastern Daylight Time, km1h writes:



      Alpha used the B&W in the 77 series and B&W used the 802 in their 
      own 1500W amp. Heath used 50uh on their SB series of 80-10M amps.


    I think what they're doing is making the choke part of the tuned circuit.

    Let's say we have a 50 uH plate choke and a pi-net that calls for the tuning capacitor to be 200 uuF on 80 meters (3.5 MHz) *IF* the choke reactance is infinite.

    If my math is right:

    The reactance of the plate tuning cap is 227 ohms (capacitive)

    The reactance of the RF choke is 1099 ohms (inductive)

    The resultant reactance of the two in parallel is 283 ohms (capacitive).

    So all you need to do to make it work is to add enough additional C to get back to 227 ohms, and you're all set.

    Of course there's a limit to how far this trick can be taken. If the choke reactance is such that it significantly affects the tank circuit, that means a considerable amount of RF current will flow through the choke itself, and through the bypass C at the cold end. That means choke and cap have to be able to handle the RF without burning up or ruining the Q.

    73 de Jim, N2EY


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