[AMRadio] GG 813 Linear in GE Ham News

tim smith ve6pg at yahoo.com
Thu Sep 29 16:33:56 EDT 2005

.hey don,what are your thoughts of am transmission
with 4 811s?..i'm looking to get more output on 160

--- Donald Chester <k4kyv at hotmail.com> wrote:

> >>>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 
> p.e.p.
> Don k4kyv
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