[HomeBrew] Plate Choke

[William E. Sabin]

The plate choke must also be tested for spurious resonances that can cause 
all kinds of problems such as "suckouts" and parasitic oscillations. An 
impedance bridge or a grid dip meter can be very helpful. Measurements 
should be made "in situ" as the lab folks say. RF choke design is an "art" 
as well as a science.

Bill W0IYH

----- Original Message ----- 
From: <N2EY at aol.com>
To: <rickbras at airmail.net>; <homebrew at mailman.qth.net>
Sent: Sunday, May 13, 2007 9:05 PM
Subject: Re: [HomeBrew] Plate Choke


** Please do NOT cross-post messages when posting to HOMEBREW **

In a message dated 5/13/07 8:38:42 PM Eastern Daylight Time,
rickbras at airmail.net writes:


> Generally speaking, am I correct in calculating a plate choke for a 160
> meter transmitter should be somewhere in the 5 - 6 mh range and about 10Ù 
> in
> resistance?

No. There are lots of factors to consider.

The first one is the output impedance of the tube (you did write "plate
choke").
The usual rule of thumb is that the choke reactance should be ten times the
output Z of the tube. A 5.0 millihenry choke, on 2.0 MHz, has a theoretical
impedance of 62,800 ohms, so it should be good up to an output Z of 6280 
ohms.
Say 6000 ohms in round numbers.

Depending on the tube(s) you are planning to use, and the mode of operation,
the output Z of a tube amp may be much lower than 6000 ohms, or much higher.

If it's much higher, 5 mH isn;t enough. If it's much lower, you could use a
lot smaller choke.

You have to know the output Z first.

As for the 10 ohms DC resistance, it depends on the plate current. More
important is the current rating of the choke.

What operating conditions are you planning on?

73 de Jim, N2EY


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