|[AMRadio] Screen Modulated 813|
w4bws at comcast.net
Mon Dec 11 18:09:24 EST 2006
You can use a series resistance in the lead from the heising modulator
reactor to the screen or plate bypassed with a capacitor to pass the audio
and you get reduced carries and more audio punch.
----- Original Message -----
From: "Bry Carling" <bcarling at cfl.rr.com>
To: "Discussion of AM Radio in the Amateur Service"
<amradio at mailman.qth.net>
Sent: Monday, December 11, 2006 8:09 AM
Subject: RE: [AMRadio] Screen Modulated 813
> 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.
> 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
> > behave a lot like an AM linear amplifier in terms of capability for a
> > tube. Your SB-200 has 2 X 160 watts of dissipation available, and the
> > 813 has 2 X 125 watts dissipation available. IMHO the 813 in ICAS rating
> > more conservatively rated than the 572B, so I'd think this is a wash
> > 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
> > out, but it will do 100 watts AM and modulate upward 100%. It does blush
> > little but as long as I talk the red stays away. The drive need is under
> > watts too with that 813.
> > Screen modulation, although relatively simple, requires the screen
> > drive to stay linear when the screen current varies from zero to fairly
> > level. This means the screen modulator needs to have a low source
> > and some inverse feedback is helpful. One approach is to use something
> > 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
> > with a small modulation transformer for the 813 screen.
> > Regards,
> > Jim
> > JKO
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