[AMRadio] Screen Modulated 813


Bry Carling bcarling at cfl.rr.com
Mon Dec 11 09:09:53 EST 2006


One major advantage with some of these schemes is 
to run AM with reduced carrier.

Heath does it with their DX60 and others.

You can run that screen mod scheme in the final
OR you can run it a lower level and follow that with 
a linear amplifier.

I would defer to the RF engineering experts here as to 
which is the better method. Yes, I would love to read an 
comparative assessment of the two methods.

DX-60 followed by an 813 RF linear or SB200.

Versus:

813 screen modulated final using the same method that sets 
the quiescent carrier level at about 10% of peak signal.

Which would be more efficient? 

I know it is just a personal preference, but I happen to think 
that the Heath screen mod method for reduced carrier 
sounds great and carries a lot of punch on the air.
I have read that the Heising choke modulation method 
will only get the signal to be 85% modulated, which is not
at all bad. It just means you have a nice fat carrier (unlike 
the reduced carrier methods) and a little more carrier than 
with plate modulation

What say the transmitter design aficionados out there?

73 de AF4K, Bry


 
> >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?
> 
> >js
> 
> 
> 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
> JKO
> 
> 
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