[Collins] KWM-2A output

[Robert & Linda McGraw K4TAX]

Good points Glen.

And do note that old carbon resistors tend to migrate up in value.  I've 
found several over the years and a few in my 32S-1 and 75S-3B that were more 
than 30% high.

73
Bob, K4TAX


----- Original Message ----- 
From: "Glen Zook" <gzook at yahoo.com>
To: <wabate at dandy.net>; "Collins CRA" <collins at mailman.qth.net>
Sent: Friday, March 11, 2005 3:27 PM
Subject: Re: [Collins] KWM-2A output


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