Merry meet you all:
Based on the past experience we had when working on high power transmitters
at Elisra :
The impedance of the choke should be about 5 to 10 times the design load
impedance of the stage:
That calls for about 15 kilo ohms for a typical 1kW amplifier
That would translate to about 1.3mH for 1.8MHz operation.
The main issue however is the propensity for parasitic series resonances in
such large chokes: not only is it getting lower in frequency as the choke
get higher in value, it also gets more difficult to place it between
operating bands, that got quite crowded with the WARC bands in.
Our solution in the end was to switch over to series feed: band switched
coils , parallel tuning and link coupling for the output.
It's not so bad as it sounds: once set up, they're set forever and quite
wide band. You might need only one or two coils to cover 21 or even 18 MHz
to 30 MHz.
It seems it's easier to design and build such a tank circuit than a really
foolproof plate choke.
Alex 4Z5KS
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Eddy Swynar
Sent: Thursday, February 05, 2009 2:35 PM
To: amps@contesting.com
Subject: [Amps] Plate Choke Values
Good Morning All,
Am I missing something here...?
Extensive research here into years & years of ARRL HANDBOOKS, Bill Orr
HANDBOOKS, and the internet have shed precious little light in the matter of
optimum / minimum values of inductance for plate chokes in the B+ leads of
our tube-type linear amplifiers.
Have a look-see yourself: in designs that feature amplifiers that only go as
low as 3.5-MHz, you'll see chokes that range in value anywhere & everywhere
from 200-uh, to 50-uh. On 160-meters, I've seen quoted values as high as
1.0-mh., and others as low as 200-uh.
Just what, exactly, is a "...minimum reactance" for a choke, on a given
frequency band, to do its job effectively, anyway...?
I know confusion can creep in in the form of the self-destruction of these
parts if the hapless home brewer happens upon a band where there's
self-resonance in the choke...but that issue aside, is this all some matter
of "...by gosh & by golly black magic", or are there very real minimum
standards & parameters that we should adhere to...? And if so, where are
said standards published...?
I certainly can't find them via "...the usual" routes --- but again, I must
be missing something here...
~73~ Eddy VE3CUI - VE3XZ
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