[AMRadio] Plate modulating a 807

RJ Mattson rjmattson at hvi.net
Thu Apr 8 00:57:01 EDT 2004

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:

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)


----- 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 B+
> should come from the R20 side of the MOD jumper,instead of the R17/C21
> 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
> 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
> be added between modulated B+ and the screen to improve screen modulation
> 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
> change for better quality modulation.  The simplest change would be a
> choke of several Henries between R24 and the 807 screen circuit.
> Alternatively, you could add a larger dropping resistor from unmodulated
> or a divider resistor pair one from the 807 screen to modulated B+, and
> 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 ----- 
> > 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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