[AMRadio] Plate modulating a 807

Bob Bruhns bbruhns at erols.com
Thu Apr 8 02:52:29 EDT 2004

Hi Bob,

With the plate meter connection moved to R20, I think the 807 screen will
self-modulate reasonably well with a 15K screen resistor - but the 0.01uF
screen bypass is too big.  Probably it should be 1000pF to the 807 cathode,
with maybe about 300pF to modulated B+.  Maybe the screen bypass goes to the
cathode instead of ground, because it is an RF bypass for a wide frequency
range?  Actually, I don't know which ground is best, but whatever works is
OK with me.

A 300pF capacitor to modulated B+ and a 1000pF capacitor to the cathode
(effectively ground) would make a capacitive voltage divider.  If we have
(let's say) 55% modulation on the screen, that's +/-110V peak, or 220Vp-p on
the 200VDC on the screen  meanwhile there would be 100% modulation on the
plate, or +/-470V peak, or 940V p-p, on the 470VDC on the plate.  The ratio
of screen audio to plate audio is 220 to 940, or 0.234:1.  So the capacitive
divider needs to aim for 23.4% of the plate swing going to the screen, and
it will enforce this at very high audio frequencies.  300pF and 1000pF
divides to 0.231, or 23.1%, which is close enough.

If the self-modulation of the 807 screen produces less than the above amount
of voltage swing on the screen, then the 300pF capacitor can be reduced in
value to match it, or a more complex screen resistor setup can be arrranged
to increase the amount of 807 screen modulation.  However, the latter will
involve a resistor to modulated B+, and therefore might call for the use of
a clamp tube.  For now, take the simple route and reduce the 300pF cap value
as needed..

  Bacon, WA3WDR

----- Original Message ----- 
From: "RJ Mattson" <rjmattson at hvi.net>
To: <amradio at mailman.qth.net>
Cc: <bbruhns at erols.com>
Sent: Thursday, April 08, 2004 12:57 AM
Subject: Re: [AMRadio] Plate modulating a 807

> I agree, the 100ma meter should be across the shunt R20 (27ohm). I will
> check if that is a drafting error.
> Here is a marked up transmitter drawing showing the values in question:
>  http://www.hvinet.com/rjmattson/four20schematic+
> R24 screen is 15 K
> Screen voltage is 200V
> R24 node is 270V
> C24 screen bypass is .01mfd.  Why is bypass to cathode and not ground?
> Plate voltage is 470V
> The Hammarlund Four11 plate modulator is two 7C5s in PP (8K ohm xfmr sec)
> Should the screen self modulate as wired? (with plate meter corrected)
> Thanks,
> bob...w2ami
> www.qrz.com/callsign/w2ami
> ----- Original Message ----- 
> From: "Bob Bruhns" <bbruhns at erols.com>
> > Bob, the main problem I see with that circuit is that the plate current
> > meter will get blasted by the modulator output.  This will present an
> > unhappy load to the modulator as well.  The connection down to S1 from
> > should come from the R20 side of the MOD jumper,instead of the R17/C21
> side,
> > so that when S1 is switched to the plate position, M1 is across R20.
> >
> > If R24 is 10K or more, the 807 screen may self-modulate enough to work
> > reasonably on AM.  Series-resistor screen self-modulation occurs for the
> > same reason a choke provides modulation; screen current is inversely
> > affected by plate voltage.
> >
> > What is the value of R24, and what is the voltage on the bus being fed
> from
> > R17?  Also, what is the value of 807 screen bypass capacitor C24?  C24
> > should not be very large, or it will produce excessive rolloff in screen
> > modulation at higher audio frequencies.  (A small compensating capacitor
> can
> > be added between modulated B+ and the screen to improve screen
> at
> > the higher audio frequencies, but that would be a final touch.)
> >
> > If R24 is merely a fuse-resistor for the 807 screen, you will want to
> a
> > change for better quality modulation.  The simplest change would be a
> series
> > choke of several Henries between R24 and the 807 screen circuit.
> > Alternatively, you could add a larger dropping resistor from unmodulated
> B+,
> > or a divider resistor pair one from the 807 screen to modulated B+, and
> one
> > from the 807 screen to unmodulated B+.  That will give the best
> > linearity, if properly proportioned.  However, if you modulate the
> > from resistors going up to plate voltage, you will probably want to add
> > clamp tube to the 807 screen circuit as well.
> >
> >   Bacon, WA3WDR
> >
> >
> > ----- Original Message ----- 
> > > This url is to my Hammarlund Four20 AM transmitter schematic.
> > > http://www.hvinet.com/rjmattson/four20schematic
> > > The PA 807 screen does not seem to be modulated through a dropping
> > > resistor or self modulated with a choke. The Hammarlund Four11
> > > plate modulator is connected in place of jumper MOD on the
> > > schematic. Should I try it as is or rewire?
> > >
> > > bob...w2ami

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