[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?
> 
> 73
> 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
> >
>  WB2FCN
> 


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
Glendale, CA



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