Posted by
Fred Townsend on
Aug 17, 2012; 11:28pm
URL: http://elecraft.85.s1.nabble.com/Elecraft-KPA500-vs-Tokyo-Hy-Power-1-1Kfx-tp7561226p7561231.html
Jim I'm not sure what you are looking for here. It is highly desirable to
use parts with a second source. That way if the factory burns down you are
not put out of business. That being said with RF parts the second source is
seldom identical hence the reference to the manufacturer. Manufacturing
processes differ from factory to factory. Sometimes the difference will be
noise or something you will have to look at a Smith chart to see. This makes
it difficult to interchange.
A major problem with using quads (two push-pull pairs) is current sharing.
If more current flows in one transistor than the others it will get hotter
and possibly be pushed beyond specifications leading to failure. Failures
may 'domino' leading to secondary failures. Fortunately FETs have a negative
coefficient of resistivity which makes them much easier to balance, hence
their popularity in newer amps.
73
Fred, AE6QL
-----Original Message-----
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Sent: Friday, August 17, 2012 3:21 PM
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Subject: [Elecraft] Elecraft KPA500 vs Tokyo Hy-Power 1.1Kfx
I am wondering if anyone has an explanation for why the Elecraft
KPA500 amplifier (500 watts) employs 2 VRF2933 FETs (Microsemi) in the final
PA where the THP uses 4 ST2933 FETs (STMicroelectronics) to generate very
similar power levels. The data sheets for each FET indicate that both
brands are rated at 300-400 watts, support similar mismatches and run at
similar voltage maximums. They have very similar specs all together.
Perhaps our amplifier gurus could weigh in here. Much appreciated.
73,
Jim Robbins
N1JR
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