|[AMRadio] AM Amps|
W7QHO at aol.com
W7QHO at aol.com
Fri Jan 14 00:01:33 EST 2005
In a message dated 1/13/05 1:50:20 PM, garyschafer at comcast.net writes:
> How do you reduce the drive for AM? I take it that is how you operate CW?
> Gary K4FMX
> James M. Walker wrote:
> > Hum,
> > 4-1000A G.G. amplifier, 3000 VDC plate, 700 MA Plate I, 500 MA
> > Grid I, with 125 Watts out of DX-100 running apprx 2100 W input and
> > dissipating 670 watts plate = approximately 1430 watts output!
> > While the carrier level into 50 ohm dummy load is 1400 watts measured
> > and a really NICE orange glow! This is with a 200 cfm blower, air system
> > socket and chimney.
> > Jim
Good question. If I remember correctly, this thread originally started
around the question of reducing the output of an AM transmitter to a level
appropriate for driving a linear amplifier. Practically speaking, the process of
setting up a linear amplifier for AM is complicated by the requirement that the
amp be first adjusted for operation at the peak power level expected to be
encountered which, in turn requires an RF drive source capable of providing this.
This creates a problem if a small rig like a Ranger or an Elmac AF-68 is
used, neither of which is capable of providing sufficient CW power to properly
tune up the amplifier (presuming GG 3-1000Z, pair of 3-500Zs or equivalent
here). When I first ran my HB 3-1000Z with an AF-68 some years ago, I would
tune up the amp. using my Cubic Astro 103 which can provide any value of CW 0 -
100W. Worked just fine but switching rigs in and out was somewhat of a
pain. I have an Apache and a Viking II here but have never looked into a
modification to switch between the required tune and AM power levels (i.e., reduce
plate and/or screen voltage or whatever). These days I mostly use a T-368
for high power, but on occasion I drive the linear with a little Kenwood TS-50
which very conveniently cuts the carrier level back automatically when
switching from CW to AM.
I've considered building up a screen or suppressor modulated exciter
specifically for use with the linear. Grid modulated rigs, of course, have to be
tuned up just like a linear, i.e., loaded and tuned to the CW level and then cut
back to 1/4 of this for talking, thus also providing the tune-up and operate
drive values required by the linear.
Finally, however, it is possible to tune up a big linear with a small rig
like a Ranger or Elmac. The technique involves use of a loud, verbal utterance
containing a long string of repetitive voice peaks and tuning the linear to
maximize the output amplitude of these. Northeastern hams will remember the
YYYEEEEEEEEERRRRRRRRIIIIIIIIII tune-up wail frequently used by one well know AMer.
Dennis D. W7QHO
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