Patrick Turner wrote:
On Wednesday, 10 October 2012 10:39:54 UTC+11, Alejandro Lieber wrote:
I am thinking in building an AM transmitter for 7.1 Mhz (40 meters)
using a EL34 in class C as final. The idea is to modulate it by varying
the negative polarity of the suppressor grid (g3). Suppose I load the
EL34 plate with 500 volts DC and a current of 150 ma with 0 volts
suppressor, does anyone have any idea what negative voltage in g3 is
needed to reduce the anode current by half, needed for screen AM
modulation ?. Alejandro Lieber LU1FCR Rosario Argentina Real-Time
F2-Layer Critical Frequency Map: http://1fcr.com.ar ---
news://freenews.netfront.net/ - complaints: ---
Methinks an 807 would be a lot better because 807 has an anode top-cap
connection rather than to pin 3 of an octal socket, from where arcs to
pin 2 which is the filament usually at earthy potential.
The idea of using suppressor modulation is that no audio power is needed.
The 807 has no suppressor, and screen grid (g2) AM modulation is very
nonlinear in the 807. It works much better with a 6146 or 6DQ5 or 6DQ6.
AFAIK, suppressor grid 3 is a bit non linear for modulation on its own.
But in you post you wonder what's needed for "screen AM modulation" which
I should have written: "suppressor AM modulation"
I think is much more linear. You should br able to find the set of curves
for Ia versus Eg2amoung characteristic curves for EL34 or 807. Usually
you will need quite a high voltage swing of maybe 150Vrms for
considerable modulation %. And the screen draws DC current so its input
impedance is not as high as for a negatively biased grid for AF.
Presumably some RF current will be absorbed by the screen. There is
plenty info in old UK and USA amateur handbooks on screen modulation.
Maybe you will find you will need a another EL34 acting as an SET for
audio amp to drive the RF amp screen, i'd guess you'd need a 20H choke
feed to anode of audio EL34 and this anode could be directly connected to
RF screen with Eg2 supply to choke at say 350V. I'd guess you would need
a bypass cap from RF tube screen to 0V so that it operates as a pentode
at RF but at AF the bypass cap has no effect. Maybe 200pF might be OK,
its reactance at 7MHz = 113 ohms, but at 5kHz its 159k ohms.
Patrick Turner.
As you know, ham radio is about experimenting. Although many hams still use
valves in AM transmitters, no one uses suppressor modulation.
45 years ago, I did my military service, and was in charge of repairing all
the old valve transmitters. A lot used the 2E22 a 30 watt anode valve
specially built for AM suppressor modulation. I think a 3A4 was used as
final in the audio amplifier.
But now I discover that the 2E22 like the 803. both built for suppressor
modulation, are capable of very high screen dissipation, as very high screen
current appear when the suppressor is very negative polarizes.
--
Alejandro Lieber LU1FCR
Rosario Argentina
Real-Time F2-Layer Critical Frequency Map:
http://1fcr.com.ar
---
news://freenews.netfront.net/ - complaints:
---