[Boatanchors] AM Strategy & 4-1000 & tube preamp & other dalliances

[Glen Zook]

The Collins 75A-1 is generally accepted as being one
of the better receivers for AM operation.  The
bandwidth is greater than the HQ-170A when the crystal
phasing is not activated and therefore AM "sounds"
better.  The HQ-170A has a bandwith of 6 kHz.  Also,
the 75A-1 is considerably more stable than the
HQ-170A.  However, the 75A-1 is designed for 300 ohm
antennas and generally has poor sensitivity when used
directly with an antenna fed with coaxial cable.  But,
there is a very simple "fix" that gets the sensitivity
where it is as good as any receiver and that is to use
a television balun.  Go to

http://k9sth.com/Page_2.html

and then scroll down the list of "Articles" to the one
entitled "Using television baluns to improve receiver
receiver sensitivity".  This is the article that was
in Electric Radio a while back and outlines the
reasons for using the balun.

The Apache VFO is more stable than that in the Ranger.
 If your particular Apache has a 6AU6 as the VFO tube
change it to a 6AH6.  This is a direct substitution
and the only thing that might happen is that the
calibration may shift a few kHz.  If this happens,
then "tweak" the trimmer capacitors.  The inductors
should not have to be adjusted since the 6AH6 has a
slightly different interelectrode capacitance than the
6AU6.  Heath made this change to stabilize the VFO in
the Apache for SSB operation with the SB-10 SSB
adapter.

There is a VERY simple (3 capacitors changed)
modification to the lower modulator stages that
improves the transmitted audio considerably.  Then
unplug the 6AL5 "clipper" tube (don't have to do
anything but unplug it).  For details go to the same
URL listed above

http://k9sth.com/Page_2.html

and the article is the 3rd down from the heading
"Articles" and the schematic is the 4th link down. 
Again, this article was in Electric Radio a while
back.

Remember that you are limited to 375 watts carrier
output on AM.  This is because 47 CFR Part 97 limits
PEAK power to 1500 watts and with 100% modulated AM
the peak output power is 4 times the carrier power. 
Therefore 375 watts of carrier equals 1500 watts peak.

Forget about using the audio preamplifier.  Both
transmitters have plenty of audio gain with the
"stock" circuitry.

Glen, K9STH


--- W8KZW <w8kzw at arrl.net> wrote:

I am considering putting together an AM station again,
and the options lying around the basement are:
 
TX:  Johnson Ranger, Heathkit Apache
RX:  Hammarlund HQ-170A, Collins 75A-1
 
I'm leaning toward the HQ-170A, but I'd like some
feedback from others.  I run an S-line on ssb/cw, so I
don't really care about the Collins panache with the
receiver.  I also got past all that when I ran a 32V-3
and 75A-3 combo on AM.
 
For transmitting, I'm guessing the frequency stability
and audio quality is better out of the chute on the
Ranger.  Right now I don't have the time for
heavy-duty audio mods, but please let me know if there
are any great articles out there regarding that for
either rig.
 
Also, how silly would it be to drive a homebrew 4X1
amp with the TX?  Any thoughts on the best amplifier
plate voltage / power out?  I'm sure purists will be
be up in arms on the architecture of not modulating
the final, but realistically ... this should provide a
respectable signal?  I doubt I'd want to run more than
about 400 watts out.
 
Finally, I have a tube-type Heathkit audio preamp. Any
value in inserting this between the microphone and the
audio-in connector on the rig?










Glen, K9STH

Website:  http://k9sth.com


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