[AMRadio] Plate modulating a 807

Bob Bruhns bbruhns at erols.com
Wed Apr 7 02:40:00 EDT 2004

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 B+
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 modulation 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 make 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 modulation
linearity, if properly proportioned.  However, if you modulate the screen
from resistors going up to plate voltage, you will probably want to add a
clamp tube to the 807 screen circuit as well.

  Bacon, WA3WDR

----- Original Message ----- 
From: "RJ Mattson" <rjmattson at hvi.net>
To: <amradio at mailman.qth.net>
Sent: Tuesday, April 06, 2004 11:19 PM
Subject: [AMRadio] Plate modulating a 807

> 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
> www.qrz.com/callsign/w2ami
> _______________________________________________
> AMRadio mailing list
> AMRadio at mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/amradio

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