[Amps] Shorted turns experiment

[Gary Schafer]

I have an old TMC (technical materials company) amplifier that uses a pair
of 3-500s that has an auto (preset) tune tank circuit. The tank coil has a
tapped toroid core in it. I replaced part of that coil with a copper tube
air wound coil and the output power increased by 200-300 watts. Off hand I
don't remember exactly.  The efficiency of the amps was quite low. I posted
about it on hear around a year or so ago and had discussions about the
tapped core with W8JI as well. He also suggested that it is a bad idea to
short turns on a toroid. 

73
Gary  K4FMX

> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
> Behalf Of Larry Benko
> Sent: Thursday, August 05, 2010 4:41 PM
> To: Carl
> Cc: amps at contesting.com; Dennis OConnor
> Subject: [Amps] Shorted turns experiment
> 
> 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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