[AMRadio] Power Levels

[Donald Chester]

>From: rbethman at comcast.net

>...I've measured the output with at least three different RF power >meters 
>including a Bird 43.  My carrier on 40m using full line >voltage has always 
>shown 500W or better...

What kind of line voltage do you have there anyway?  What is the DC plate 
voltage and current to the final?  It is supposed to run 2000 volts on the 
plate at 250 mills, on AM.  On CW it runs 2500 wolts at somewhere around 300 
mills.  Are you sure someone hasn't changed the tap connections on the 
primary of the plate transformer to boost the power by running it on AM at 
the full CW plate voltage?  If so, have pity on the poor hard-to-replace 
250TH and restore it to normal operation.

If it is running properly at 500 watts input, the Eimac tube sheet says the 
carrier output should be 335 watts.  That assumes zero losses in the rf tank 
circuit and associated components.  In the real world, at 500 watts input, 
you are not going to get more than about 300 watts carrier output, and even 
that is being optimistic.  Because of the turns ratio of the modulation 
transformer, running the common power supply for modulator and final, you 
have a VERY good unit if it makes 100% modualtion on positive peaks without 
distortion.

Run the final at 2000 v and adjust antenna loading until the final plate 
draws 250 mills.  Make sure the grid current is at 60 mills, and adjust 
modulation for as close to 100% modulation as possible without negative 
overmodulation or flat-topping on positive modulation peaks as observed by 
the envelope pattern of an oscilloscope, and let her rip!

Are you sure that at the time those wattmeters were in line, your feedline 
was presenting the transmitter with a precise 50-ohm nonreactive load?  If 
otherwise, the wattmeter reading WILL be in error.  The best check is to 
tune into a 50-ohm dummy load made up of high wattage noninductive precision 
resistors, such as "glowbars".

Don K4KYV

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