Are 813's suitable for HF SSB?

Don Roden donroden at HIWAAY.NET
Wed Feb 25 23:35:19 EST 2009

Quoting kd4e <doc at>:
>  A source said:
> "Although a pair of 813s was perfect for legal limit operation on CW and AM,
> the tube was unable to supply the high peak inputs permitted in  
> single sideband operation, and thus the popularity of the tube  
> dropped as SSB > became the preferred  mode over AM."

Legal limit operation on AM in the era of the 813s was One KW carrier and
at 100% modulation, the tubes had to deliver 4KW peak.

Given those parameters, the tubes would have melted down quickly.

Each tube has a plate dissipation of 97 watts for a total plate  
dissipation of a pair around 200 watts.

Most legal limit AM transmitters used a pair of 4-400 tubes for a  
combined dissipation of 800 watts.

AM ( where the carrier is on all the time ) produces a lot more heat
than a SSB signal that drops to no power out between words.

So, for the same *peak* power output, SSB is much kinder to the tubes,  
and the tubes can be driven harder ( to higher peaks ) in the SSB mode  
than the AM mode.

Which is opposite of your quoted source.

The 813s will run cooler AND make more peak power by using SSB instead  
of AM, but you arent going to be in the SB-220 ( pair of 3-500Zs )  

You *will* be in the Collins 30L-1 output class and that's only a  
couple of "S" units lower than the expensive ceramic triodes.

Besides, the 813s look ( and feel ) great on a cold winter night if  
you put them behind a glass protective cover.


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