|[AMRadio] Source Broadcast Xmtrs|
ranchorobbo at gmail.com
Sun Jun 12 10:58:14 EDT 2011
First, "tirade" may have been a bit of an exaggeration, sorry about that.
Those of use who have been around on ham AM on-line forums of one sort
or another can probably tell you that any time someone starts out by
writing some variation on:
"I am a professional broadcaster," or, "I am a chief engineer," of "X
years experience," we all know we are about to be lectured to by
someone who knows less then half what we do about ham AM but by virtue
of his career experience, thinks he has a thing or two, to teach us,
if we will just sit at his feet in silent awe. The same is true in
other cases like hams who go off and get a flex radio (like seeing
Paris for the first time), then come back and tell us poor hayseeds
that we should junk our boatanchors and move to the city (but when
asked how they repair their flex radio when it breaks they never seem
to have an answer). Don't get me wrong, it's not that broadcasters
have nothing to teach anyone, it's that broadcast AM and ham AM have
some differences. For one thing, any occupation a guy has to do all
day to earn a living is going to result in an attitude shift about
that activity. To earn a living, I spend all day doing things with
computers. So if someone says to me, "I like to tinker with computers
and software in my free time," I just cannot get away from that person
fast enough. A few years ago a friend of mine told me the CE of a
local class 1-A property (now retired) said to him, What kind of
whacko wants to see our transmitter site? Well, I wanted to see it
and would have thought it a really cool thing to see, but I understood
where he was coming from, since he had to be there any time day or
night whether he wanted to or not, in order to earn a living.
So, a professional might wonder, what kind of whacko _wants_ to put up
with a vintage vacuum tube broadcast rig? They are big. Hot. Very
heavy. They use a lot of power and are inefficient compared to a
solid state rig. They break down and are unreliable compared to s.s.
They are also usually dirty because their operating environments
(often an unattended shed with mice running around) are dirty. To
someone who had to keep one of these beasts on the air, and who
offered up burnt offerings of praise the day his station went solid
state, the idea of anyone voluntarily wanting to operate one, is
For full disclosure, I run a ricebox driving a linear amp. And I do
not have a home that could accommodate an old broadcast rig without
major remodeling. But, if I had a basement with a big door opening to
the outside with a concrete ramp to the driveway, I'd be actively
looking for a broadcast rig or three right now. Why?
Well for one thing, no one is making any money running a ham AM
station. We're not trying to squeeze every dime we can by making
sure everything is always running, never down, with no more then 10%
of the power company's product going up in heat. The point is not
profit but pleasure and fun.
If you understand and appreciate vacuum tube technology, anything with
vacuum tubes is a beautiful thing, but the sight of the PA and
modulator tubes in a transmitter showing color through a window is
lovely. The old rigs themselves were often works of art, sort of like
cars from the 40s and 50s--lots of chrome and glass, real jewels.
The cabinet size and build quality (continuous duty) is simply not
found in ham gear, even vintage ham gear (no vintage ham AM rig was
ever made with a modulation reactor as far as I know) and having one
of these rigs cleanly and quietly putting out a solid 300 - 400 watts
with a huge audio power reserve (no running out of mod power with
these rigs) and knowing it could do that with essentially no time
limit is a thing that is exciting to just about any ham AMer. Then
there is the restoration challenge--to you perhaps a PITA but to some
hams, a labor of love, like restoring a junk 55 Chevy. And if it
does break--it can be fixed. You may be able to repair your ricebox,
but I have doubts about my ability to repair mine. I think I can
generalize by saying that we want to be able to fix the gear we
have--and that that is one of the characteristics of a complete ham.
Throwaway plastic radios may be okay for consumer electronics users,
and hot switching plug-in RF modules that can be pulled out from a
DX50 and sent out for repair may be okay these days for broadcasters,
but at some level that doesn't cut it for us.
Sure, I could probably go along with my ricebox (modified by the way
because stock, it sounds like a traffic information station) and pair
3-500zg running 300 watts but the setup is at once boring and a pain
in the neck. It's boring because once you figure out how to modify
the amp to handle long AM transmissions at some decent carrier power,
the challenge is similar to running a CB rig with a "foot warmer" (or
whatever they call linear amps now). But it's a pain because at least
in my case, I have to always fiddle with the drive level, Z between
the exciter and amp input, and the amp's loading depending on what
band I'm on. Don't you think it would be a lot easier to dip and
load a class C PA and modulate it?
>> First I was a HAM wannabee when AM was king and the heterodynes were
>> both hideous and everywhere.
There are a few simple solutions to that today: Firstly, we tend to
spread out more most of the time. Secondly, it's possible to work up
a filter and wire it so it can be switched into the IF strip on a
vintage receiver. Mouser sells Murata ceramic filters that can be
used for this. I put together a 6 KHz ceramic filter circuit using
one for my 75A-3 because I'm too cheap to pay the big bucks for a
mechanical filter. Also, it is not as if everyone transmits 5 or 10
KHz of audio all the time. Sure 5 or 6 KHz sounds great. But every
ham I know of with that kind of frequency response capability also has
some kind of low pass filter arrangement to narrow his bandwidth when
necessary (usually at night on some bands like 75 meters).
>> Secondly the "big iron" that everyone covets limits the fidelity in ways
>> that a rice box followed
>> by a one kW linear amplifier doesn't.
It depends on the ricebox and whether or not it is run stock. Also,
if your amplifier is limited to 1 KW, it is really not adequate for AM
(in my opinion).
>> Thirdly, USA HAMS are limited to 375 Watts unmodulated carrier, (1.5 kW
>> P-P) a one kW AM
Oh boy, the mythical 375 watt power limit again. How do we measure
this 375 watts? What if my Bird 43 says 378 watts but I can only get
to 80% positive and neg.? Will I get a ONV from FCC? Yikes. Sorry
for the sarcasm, but the stupidity of the ludicrous imaginary 375 watt
power limit makes it hard to address with any seriousness.
>> transmitter can produce one thousand Watts of unmodulated carrier. (so
>> you need to cut it back)
What if you want to run it as one hell of a CW or RTTY rig? : )
Come to think of it, I'm a little surprised SSTV ops aren't running
around buying up old bc rigs so they can spend hours on end sending
photos at 1 KW with no worries like they'd have with their little
table top amps.
>> * everyone disappeared when SSB took over (what ever that is) as did the
>> heterodynes when I got a BFO
>> Early commercial and locally built AM transmitters were often a low
>> power modulated transmitter**
>> followed by a linear amplifier
>> ** frequently using Heising modulation
That's only true in a few cases. There was one design that followed
that scheme but it didn't sell, and was abandoned for all the reasons
any exciter driving an amp is not very good except maybe for brief
"two way radio" applications. Since we're not dispatching taxi cabs,
it's of little interest. The reasons were: For broadcasting, poor
reliability, heat, and inefficiency. The overwhelming preference in
broadcasting was high level plate modulated rigs. Honestly Ron, for
someone who says he has been in broadcasting 48 years, you seem to
have a selective memory. Let us take a look at one of these um, less
than popular rigs, the Gates Vanguard 1, made and sold in the
mid-1960s. It ran a 100 w. s.s. exciter driving a 4cx3000 as a linear
amplifier. A hot and inefficient 1 KW rig (and if you ask me, one of
the less attractive bc transmitters):
http://www.w8bac.com/vanguard.html I have never owned or operated
one however, and the ham who owns this one may not have any issues
with it, but I'd rather have an old Collins or RCA rig myself.
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