|[AMRadio] Screen Modulated 813|
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
>>when talking about a max 300 w carrier? Won't two 813's
>>much even when run inefficiently?
>>Surely it would be an improvement over using the Heath
>>SB200 and the
> 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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