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[Amps] Shorted turns experiment

To: Carl <km1h@jeremy.mv.com>
Subject: [Amps] Shorted turns experiment
From: Larry Benko <xxw0qe@comcast.net>
Date: Thu, 05 Aug 2010 15:40:48 -0600
List-post: <amps@contesting.com">mailto:amps@contesting.com>
Carl and Denny,

Hope you don't mind me butting in here.  I had a private exchange with 
another guy and will post an experiment I did here as it might help.

This following reply was in reference to a particular amp that I was 
mistaken on concerning the output network which had some shorted turns 
in the output inductor which was wound on a powdered iron core:

Shorted turns must not cause enough heat to be a problem since this amp 
design has been around and is well respected.  As I indicated before the 
"designer" made a decision here given some set of constraints.  However, 
shorted turns ANYWHERE that are cut by flux lines will produce a current 
and result in loss.  An amplifier designer is in a predicament given the 
fact that band switches are becoming one of the biggest cost items in an 
amp after the tube and the transformer.  Some of the bizarre custom 
switches that we saw 40+ years ago will never be seen again unless you 
had switch volumes big enough to get the attention of the switch 
manufacturer so we use what is available.

Just for grins I just did a little experiment.  I found a #2 powdered 
iron core which happened to be already wound and center-tapped with 
about #18 wire.  I did a quick measurement and the inductance of 1/2 the 
winding was ~3uH.  I put this winding in parallel with a dummy load and 
preceded it with a large "L" network tuner to give the amp a 1:1 SWR.  I 
turned up the power until I got some moderate heating.  At about 600W on 
1.8MHz  the core temperature rise was about 10 deg. C in 2 minutes.  
Then I let it cool down a and shorted the other half of the coil (it was 
open in the 1st test).  The SWR was no longer 1:1 (as expected) so I 
re-touched the tuner and again applied 600W. This time the temperature 
rise was about 22 deg. C in 2 minutes.  Clearly the shorted turns caused 
the additional temperature rise.  However the 22 deg. C rise is still 
very much within the allowable temperature rise for the core.  So one 
person might say the shorted turns are bad and someone else say they are 
acceptable.  It is very possible that an amp designer might increase the 
core size a little and live with the shorted turns as the best 
compromise.  However if I was designing an amp I would try to avoid this 
scenario unless I did some REAL testing to understand what the effects 
will be.

Good designs are always compromises and if "my" legal limit amp was 20W 
more efficient than "your" amp but "your" amp cost $300 less than "my" 
amp you might sell a lot more amps than I would. :)  Thanks for the 
discussion.

Just a FYI data point guys and there is no reason to believe that 
another design would have the same relative temperature rise as my 
experiment.

73,
Larry, W0QE


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