[Amps] Manganin RF choke?

Ian White GM3SEK gm3sek at ifwtech.co.uk
Sun Aug 9 11:50:10 PDT 2009

k7fm wrote:
>In a CQ publication, called "New Sideband Handbook", there is an article
>titled "The Little Linear".  It is the construction article for a small
>4CX1000A amplifier.  It was one of the many designs of Jo Jennings, W6EI, of
>Jennings Radio.
>What I was particularly interested in was the construction of the plate rf
>choke.  It is 100 uh and made of #28 manganin wire.  The article has a brief
>description on the use of this material.  "The output r-f choke for the
>plate circuit is made with manganin wire, wound on a form approximately one
>inch in diameter and five inches long.  The resistance of this material (#28
>d.c.c.) produces a very low "Q" and discourages oscillation which could
>occur in a resonant choke.  The solenoid-type choke has been found to work
>very well over the frequency range of the amplifier and does not contain
>"holes" that are characteristic of other chokes."
>The amplifier covers 10 - 80 meters.  I know there were a couple other hams,
>who were pretty sharp, who worked for Jennings, so this idea of using the
>manganin must have been tested to some degree with no adverse consequences.
>I wound one many years ago, but without adequate testing equipment at the
>time, never had the nerve to try it out in a live amplifier.  It had
>significant resistance, which may not have been bad, and would replace the
>fixed resistor in the supply recommended by Eimac.

Resistance in the choke would not entirely replace the surge limiting 
resistor in the power supply.

If the amp and PSU were two physically separate units, locating the 
surge limiting resistance in the amp would give no protection against 
shorts in the PSU or the HV connecting cable. (One of my customers made 
that mistake: BANG!)

Even in an integrated desktop unit, a resistive choke would still only 
protect against arcs and short-circuits in the tube itself or in the 
tank circuit. It gives no protection against any faults occurring 
upstream, closer to the PSU.

Since plate chokes that are free from in-band resonances are now readily 
available, it seems better to use  those, and continue to locate the 
surge limiting resistor in the PSU.


73 from Ian GM3SEK

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