[AMRadio] Source Broadcast Xmtrs


W2XJ w2xj at nyc.rr.com
Sun Jun 12 13:48:28 EDT 2011



On 6/12/11 10:58 AM, Rob Atkinson wrote:
> 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).
There is nothing magical about the Flex Radio they are if anything more 
repairable than the average Rice Box. Ever open up an FT 2000? How many 
hams are prepared to deal with all the surface mount components in 
current gear. Don't get me wrong Flex is  not a panacea but an easy way 
to play with SDR which I personally think is a lot of fun and has 
increased the amount of experimentation in this hobby. SDR has caused 
new modes to be developed as well.

>   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.

Often that is correct but I have worked in broadcast 53 years and if you 
count years hanging out at transmitters and my pirate radio days as a 
kid - 56 but I still like visiting transmitter sites and still enjoy 
designing and building them. I am also very interested in the new 
designs and better ways to generate AM I am not alone there are a number 
of BC engineers who are the same way one even has a 3 letter call ending 
in AM. There is a lot that technically bonds AM as a business and AM as 
a hobby

> 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)
I know a number of pro audio people who feel exactly that way and have 
built some pretty elaborate AM stations. I am sure that some of you may 
know that Mike Dorrough the so called godfather of broadcast audio 
processing has been collecting vintage AM gear for decades and has done 
custom re-builds for some pro audio colleagues. I gave him a 1930s 
vintage Marconi 1 Kw back in the 80s it was a beautiful plate mod 833 
design. I also helped build a 3 tower in line directional 160M array 
with a local ham using parts and tower steel from an AM site he helped 
us demolish.

> 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).
There are easier ways depending on taste. The 455 kilohertz output of an 
R-390 ( I have bought 3 over the years) can be easily down converted to 
either 12 kHz or I/Q and then fed via sound car to an SDR program. Some 
of the SDR programs do things for AM reception that were only dreamed of 
previously. There are also some good digital front ends for SDR radios. 
An operator who is rock bound with a converted BC TX could even use a 
Softrock RX for between $20 to $60 as a front end for SDR

>>>    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.
The original poster is just flat wrong with that statement.

> 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).

There is a lot of 'stuff' that makes great SSB that is just bad for AM. 
There are so many things in the average rice box that work well with SSB 
but is just bad for AM and that starts with the pseudo modulation process.
>
>
>>> 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.
With a more modern surplus AM rig by use of EER it can do superior SSB 
as well.

>
>>> * 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.
In the very early days use of linears in AM broadcast was fairly common. 
By the mid 30s they were usually only found for powers above 5 KW. They 
were driven by a lower power TX often a plate mod unit. Two of the more 
interesting transmitters in broadcast history were both at WLW. One, the 
500 KW unit was unusual in that it consisted of 2 250 KW amplifiers with 
the outputs in series and driven by the station's 50 KW licensed main. 
What was unusual was that the high power amplifiers were plate modulated 
and the modulation cut to the 50KW 'driver' which only provided carrier. 
The other was the Crosly Cathanode 50 KW which was a high level 
modulation scheme but had no modulation transformers or reactors and was 
said to have response beyond 20 KHz at very low distortion.

>   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.
>
I installed and operated an early Vanguard and most who have will tell 
you it had all the problems one would expect operating an AM linear. 
Tuning was a nightmare if the transmitter was to modulate properly and 
sound decent. But there are other linear schemes Western Electric built 
Doherty Linears into the early 50s when Continental took over the line. 
Continental's 50KW offering to the mid 50s - the 317B was a screen 
modulated 10 KW followed by a 50 KW Doherty linear. This was followed by 
their modified Doherty design which was a high efficiency direct screen 
modulation scheme. That was the 317C which was the dominant 50KW BC TX 
from about 1956 until solid state took the market.  RCA built some of 
the most beautiful and heavy duty plate mod 50s in the 40s until they 
were replaced with the less successful Ampliphase. The most technically 
successful AM linear was the Continental 316F. It was designed in the 
early 70s (after the Vanguard disaster) and consisted of a direct 
coupled series modulated solid state exciter driving a 10 Kw Doherty 
amplifier. It had a frequency response to around 50 kilohertz and 
distortion around 1%. By using conservatively designed low Q tuning 
networks, tuning the Doherty became less difficult. The transmitter was 
extremely solid, sounded great and was quite reliable. I spent some time 
in Quincy Ill with the late Hilmer Swanson when he was developing the 
digitally modulated DX series transmitters. The prototype was a 10 Kw 
unit and he had me tire kick the box as the goal was to have better 
specs that the 316F which is probably the best TX ever built to that 
time based on audio specs.

There are many ways to make AM and even today it becomes more 
fascinating. Some of these technologies don't work all that well in 
Amateur service due to the complex tuning process. At the power level 
involved pretty well any 1KW tube TX will be plate mod. I prefer those 
running 833s. My favorite are the Collins pre 20V3 designs. My broadcast 
experience has long left a bad taste for 4-400s although if run at 
reduced power would not be as bad. Probably the highest power tube TX 
reasonable to convert to ham use is 5 KW. At that power level, there are 
surplus MW 5s which are PDM and would be a very interesting TX to use in 
amateur service as they could be modified to be a killer for digital, 
RTTY, SSTV and would blow away any rice box on SSB. The design is very 
basic and easy to service. The output stage is a triode (which I 
prefer). A Continental 315B would be interesting if there are still any 
around as the output stage was screen modulated PA tubes in parallel. 
Part of the power reduction would involve removing a PA tube. Any 5KW 
transmitter I can think of has a built in power reduction to under 1KW.  
Actually a 10KW 316B could be modified the same way. All for those who 
have the space and 3 phase power or are willing to mod the power supply 
and blower to single phase although at the reduced power it would be 
interesting to determine just how much air is still needed. These higher 
power transmitters are less in demand in the Ham market and a bigger 
pain for the broadcaster to remove so there are some good deals. I once 
saw an MW 50 sold on Ebay for about $2000 provided the buyer remove it.

The biggest difference between a modified MW rig and a real shortwave AM 
is the tuning.Dedicated SW designs had some interesting auto-tune 
schemes. It is interesting to note that the companies that designed and 
built AM transmitters with various interesting modulation schemes stuck 
with high level plate modulation in the shortwave designs due to the 
simplicity of tuning. Of course many of those transmitters have had the 
modulation scheme replaced with a digital modulator which is what most 
current designs use as well.



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