[Collins] KWM-2A output

[Glen Zook]

The 10 ohm resistors that go from the cathodes of the
6146 tubes to ground often change value (especially go
high).  They form the "shunt" resistor for the "plate"
meter which actually reads cathode current.  When the
shunt resistance is higher than specifications your
meter will read higher than the actual current being
drawn by the finals.

I replace these with 1 watt (originals are 1/2 watt)
5% resistors that I buy from Mouser (any good source
is fine).  This gets the meter shunt back in
tolerance.

Then, since you are reading cathode current you have
to subtract between 25 mA and 30 mA from the reading
to get the "true" plate current.  This is because when
you read cathode current you are also reading the
screen current to the 6146 tubes.  As such, when you
are reading 225 mA cathode current you are only
running between 195 mA and 200 mA plate current.  That
gives a plate input between 160 and 164 watts.

On 80 meters I would expect between 90 and 100 watts
at 160 watts input with fairly new tubes. 
Occasionally you might do a little bit better, but on
average 100 watts or slightly less on 80 meters is
about "average" for a pair of 6146 tubes.  If your
final tubes have a "few" hours (read "many" instead of
"few") it is most certainly possible for your output
to be down.  Frankly, the guy on the other end isn't
going to be able to tell the difference between 75
watts and 100 watts.

What are you using for a wattmeter?  Has it been
calibrated (or verified) recently?  Remember that
wattmeters have an accuracy of +/- a certain
percentage of FULL scale and your wattmeter just might
be on the negative side of the scale.

Glen, K9STH


--- Bill Abate <wabate at dandy.net> wrote:

I was disappointed to find I only had 75 watts into a
dummy load.  With 225 ma and 820V (184 watts) on the
plate that only gives me 40% efficiency.  If I load to
275 ma and 800 volts (220 watts), I get 100 watts out
and 45%.  The spec in my manual is 160 watts input CW
and 100 watts out.  That's on 80, 40 and 20M.  I was
testing on 80 and 40M.

Glen, K9STH

Web sites

http://home.comcast.net/~k9sth
http://home.comcast.net/~zcomco


		
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