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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