[AMRadio] Screen Modulated 813

Jim Wilhite w5jo at brightok.net
Sun Dec 10 23:07:30 EST 2006

As an interesting sidelight to this, I read an article on 
Ted/KC3OL's website some time back about modulating an SB 
200 directly..  As I recall it was written by Chuck/WA0ZHH.

My memory fades on this, but as I recall, he drove it with a 
transmitter and constructed a modulator that worked the 
SB200 directly with audio.

I just checked and could not find the article, so you might 
email Ted to see if he has it in his archives.  It is a 
unique design and fun to read.


>>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?
> Jack,
>   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.
> Regards,
> Jim

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