[AMRadio] GG 813 Linear in GE Ham News

Donald Chester k4kyv at hotmail.com
Thu Sep 29 15:59:15 EDT 2005

>>>A pair of 813s in GG are only good for 150 watts carrier out on AM.

Actually, that figure is optimistic.  And p.e.p. has nothing to do with it.  
The limiting factor is plate dissipation of the tubes.

A properly adjusted AM linear runs about 33% carrier efficiency if it is 
adjusted for modulation capability of 100%  That means, with no modulation, 
about one third the DC input is converted to rf.  The other two thirds of 
the input power is radiated as heat.  So the maximum plate dissipation is 
twice the carrier output.  Conversely, the maximum carrier output power is 
one-half the rated  plate dissipation of the tubes.

The 813 is rated by RCA at 125 watts plate dissipation. (I believe that is 
the ICAS rating; the CCS rating would be even less).  So a pair of 813's has 
a total plate dissipation rating of 250 watts.  Therefore, the maximum 
carrier output in AM linear service would be 125 watts, not 150.  They would 
probably run at 150 watts, but if you maintained 100% modulation capability, 
they would run hot, and the tube life would be shortened.

At the maximum rated output of 125 watts carrier, the DC input is 375 watts. 
  Of that power, 125 watts is delivered as rf output, and the other 250 
watts is radiated as heat from the plates of the tubes.

Under modulation, the tubes will cool down a little.  With a properly 
adjusted amplifier, the DC input will not change as modulation is applied.  
Theoretically, the plate meter will remain perfectly still.  But as 
modulation is applied to the signal, the rf output increases because you now 
  have carrier power plus sideband power.  If the DC input remains the same, 
that extra power has to come from somewhere, so what happens is that less 
power is dissipated in the tubes and more power is delivered as output.

But since in AM voice sevice, there are substantial periods without 
modulation, the amplifier must be designed to safely run for extended 
periods in the unmodulated carrier mode.

You could increase the p.e.p. output of the tubes by increasing plate 
voltage.  It would work with SSB because of the light duty cycle of a 
suppressed carrier signal.  Depending on the peak-to-average ratio of the 
voice characteristic, you might be able to substantially increase the p.e.p. 
output without exceeding the plate dissipation  rating of the tubes.  But 
with AM, increasing the p.e.p. output necessarily means increasing the plate 
dissipation of the tubes, which are already running at max @ 125 watts 
output.  In the AM mode, the transmitter would be capable of only 500 watts 

Don k4kyv

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