Patrick Turner wrote:
On Wednesday, 10 October 2012 18:48:54 UTC+11, John L Stewart wrote:
'Alejandro Lieber[_3_ Wrote: ;963267']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: --- As
the G3 voltage is biased negative the G2 current will increase in
proportion to the decrease in anode current. That can result in a
dangerous increase in screen dissapation. To prevent that, the screen
needs to be supplied thru a limiting resistance. Refer to the
attachment lifted from FE Terman. The EL34 will easily handle your 500
volt supply, it is rated to 800 volts on the plate. But the socket is
something needs to be of good insulation. As Patrick T has said, an 807
would be a better choice. Many amatuer transmitters were built using TV
horizontal output tubes which are still common. That gets around the
socket insulation problem. And it better isolates the RF input circuits
from the output. Less likely to have parasitic oscillations. Cheers,
John
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-- John L Stewart
Amateurs I have known used to use 807 mainly for all their RF amps and AF
modulators because they were cheap from Army disposal stores and had
better insulation for anode connections. But one ham said that a pair of
EL36/6CM5 ( TV line output )could make 300Watts in class C. Other octal
line outputs with anode top caps were 6DQ6 and 6CD6. Then there was the
6146 which many hams used.
My first ham transmitter (1969) used only two valves: a EL36 from an old TV
set and a 6V6. The EL36 was a crystal controlled (3.640 Mhz) oscillator with
10 watts output, plate modulated by the 6V6 with a reactor. The microphone
was an old carbon telephone one with the DC current taken from the cathode
of the 6V6, and a 3.2 ohm to 10.000 ohm small transformer as voltage
amplifier. No power transformer, 300 V DC directly from the 220 V AC line.
I made hundreds of contacts up to 700 km. Much more fun than what I get now
with my commercial Single Side Band rig.
........
Patrick Turner.
--
Alejandro Lieber LU1FCR
Rosario Argentina
Real-Time F2-Layer Critical Frequency Map:
http://1fcr.com.ar
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