[AMRadio] RF Amp

John Coleman ARS WA5BXO wa5bxo2006 at pctechref.com
Wed Dec 6 15:34:41 EST 2006

I ran a pair if 304tls in push pull.  I think the bias was around 500 volts
and the grid current was around 150ma.  I drove it with a pair of 812s in
push pull at about 110 watts drive.  The plate supply on the 304s was around
1600 volts and the plate current was 600ma.  Output power was 790 watts.
The modulator was four 813s in standard AB2 PPP configuration with 2700
volts on the plates.  The tank coil was shortened up and capacitance added
to get the Q up for the low load Z.  That was an ether heater but it used a
lot of power and it was large.  The 450s do require a lot of filament and
drive.  They can be run at lower voltages but as I did the 304s but you will
need to make special coils and have a lot of capacitance to 80 mtrs. Or the
efficiency will suffer.  Also lower voltages generally require a little more
drive.  These tubes are high current capable if driven hard.  And just as
point of reference the efficiency of a class C plate modulated amplifier
must remain the same for all plate voltages with in the modulation voltage
range or the modulation will not be linear.  So if the plate voltage is 2000
the efficiency must be the same from at least 1 to 4000 volts. 

John Coleman

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