[AMPS] Measuring plate choke series resonance

[km1h@juno.com]

On Thu, 29 Oct 1998 14:04:45 +1300 Bob Sutton <bob.sutton@xtra.co.nz>
writes:
>
>Greetings ...
>
>What is the usual method of measuring the series resonance of a plate 
>choke?  

A GDO is the most popular method for those without lab equipment. Many
GDO's also have the advantage of size so you can test while in-circuit.

>
>Probably a silly question, but could there be more than one series 
>resonant
>frequency?  

Yes, a typical choke will have at least one and usually two resonances in
the upper HF range. Older design series fed amps often used 1mh chokes
that had resonances all over the place but the feed method made them
unimportant.

>
>There is not much power out of my HB 8877/3CX1500a7 amp on 12M (300W) 
>and
>10M (zip!) that otherwise goes OK considering the small HT supply ... 
>1100W
>from 160M - 17M, and (a so-so) 800W on 15M.  The tube is OK as it puts 
>out
>about 2kW in my 2M RF deck with a decent HT supply.  


Too little info to go on but I would start with the input network and
then concentrate on the output to see if you are actually in resonance.
A truly bad choke design would go up in smoke at those power levels.


>
>I had never thought to measure the plate choke until the recent 
>threads on
>their series resonance, and had always blamed the anode circuit 
>layout.
>It's easy to see why from the picture at ... 


All things being equal, it is quite possible to have a choke design that
has a series resonance close to say 12M but still work OK and with only a
slight power loss.
At the 1500W level #28 wire ( US guage) is sufficient for most any
application. The larger wire sizes often contribute to more capacitance
and harder to eliminate resonances.


>
>http://www.qsl.net/zl1rs/dxpeditions/zl8rs/amptop.jpg
>
>When I get it back next month I will rebuild the anode circuit with 
>the
>copper tube high bands inductor next to the anode, the band switch 
>much
>nearer the tube, and the lower bands inductor between the band switch 
>and
>the front panel ... and hope the plate tune minimum + strays is not 
>too
>large for a better result on 10/12M.


Sounds like a plan to me Bob. Keep us up to date on your progress.


  Comfortable suggestions are 
>welcome!

A GDO and a digital LC meter are most useful tools and they dont cost a
fortune.

GL   Carl    KM1H


>73 ... Bob, ZL1RS
>
>
>
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