Hi Ed,
While the Doug Demaw article tells you how to wind a choke it stops short of
telling you how it works.
While discussing this very thing the other day with a friend is seems that
with a small choke (small inductance for the frequency) the choke is
actually tuned to resonance when its reactance is a low value compared to
load impedance.
As an example with a 90 uh choke at 1.8 MHz it provides a reactance of
around 1k. very low for a typical plate load of say 2000 ohms. So it
requires a little more plate tune capacitance to resonate the choke along
with the tank coil. In this case the choke would use around 85 pf (a
reactance of around 1k) to tune the choke to parallel resonance and provide
a high impedance to the circuit. In other words you would need 85 pf more
plate tune capacitance than you would if you used a high impedance choke.
A 200 uh choke would present around 2.2k and would need about 39 pf to
resonate.
The same 200 uh choke on 3.5 MHz would have a reactance of around 4.4k and
would require only about 10 pf to tune it.
As the frequency goes up the choke reactance goes up and becomes
insignificant to the plate circuit and doesn't have any play with the tank.
The same thing can be done with the input choke of a GG amplifier on 160
meters. If the choke is too small for that band add extra input capacitance
to the tuned input network on the tube side to resonate the choke. Rich
Measures talks about this in converting the SB220 to 160.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of Edward Swynar
> Sent: Thursday, February 19, 2009 3:01 PM
> To: amps@contesting.com
> Subject: [Amps] Minimum Value of RF Plate Chokes
>
> Good Day All,
>
> Many thanks to all who responded to me re. my recent query as to the
> "...black magic factor"(!) in determining the proper minimum value of
> plate chokes in linear amplifiers...
>
> I was fortunate in receiving some very FB tips & guidelines in this
> direction, and have come to the conclusion that such devices should,
> essentially, be "transparent" as to any possible interaction with other
> components in the tank circuit.
>
> In the case of 160-meters, my pea brain will still NOT allow me to see how
> a choke of some 90- to 200-uh., or so (published values, per different
> articles), could ever accomplish this!
>
> I dug-up a rather interesting piece on this very subject that interested
> parties might wish to follow-up on. It's entitled "UNDER CONSTRUCTION -
> PART 16: UNDERSTANDING AND CONSTRUCTING RF CHOKES" by Doug DeMaw (W1FB),
> and may be found in the February 1987 issue of "QST" magazine, pp. 16 -
> 22.
>
> Thanks again, fellas...
>
> ~73!~ Eddy VE3CUI - VE3XZ
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|