[AMRadio] Screen Modulated 813

Jim candela jcandela at prodigy.net
Sun Dec 10 22:34:31 EST 2006

>I understand the efficiency problems but isn't this sort of academic
>when talking about a max 300 w carrier?  Won't two 813's produce that
>much even when run inefficiently?

>Surely it would be an improvement over using the Heath SB200 and the
>Ranger... no?

>Any thoughts?



   Screen modulation is another form of "efficiency modulation" that will
behave a lot like an AM linear amplifier in terms of capability for a given
tube. Your SB-200 has 2 X 160 watts of dissipation available, and the dual
813 has 2 X 125 watts dissipation available. IMHO the 813 in ICAS rating is
more conservatively rated than the 572B, so I'd think this is a wash between
the two. I have a Central electronics 600L linear amplifier with a grid
driven single 813 in class Ab2. On AM the single 813 is happy at 70 watts RF
out, but it will do 100 watts AM and modulate upward 100%. It does blush a
little but as long as I talk the red stays away. The drive need is under 5
watts too with that 813.

   Screen modulation, although relatively simple, requires the screen audio
drive to stay linear when the screen current varies from zero to fairly high
level. This means the screen modulator needs to have a low source impedance,
and some inverse feedback is helpful. One approach is to use something like
a triode connected beam power tube as a cathode follower to drive the
screen. You can also use a low power ( 10 watts or so ) modulator circuit
with a small modulation transformer for the 813 screen.


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