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" <[EMAIL PROTECTED]> To: <[email protected]> Cc: <[EMAIL PROTECTED]> 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" <[EMAIL PROTECTED]> > > > 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 ----- > > > 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 > >
