I had some tough choices to make. The 20kW transmitter at WPFW, Washington, was more than 10 years old and was showing signs of
increasing fragility. I had to purchase a new transmitter for WPFW, and
I had to do it soon.
|Performance at a
- Single-tube grounded-grid design
- No external low-pass filter
- Self-contained power supply
- Smooth motorized PA tuning
- Wide-band solid-state IPA
- Microprocessor control
- Optional AES-3 exciter input
I had been visiting the NAB for a couple of years, looking at all of
the usual choices and trying to develop an informed sense of what would
be the best transmitter choice for the long term. After looking over
all the equipment on display, an engineering friend suggested that I
look at the Armstrong booth. At the booth I found a cleverly designed,
grounded-grid amplifier that incorporated many of the features I
My first thought was that the transmitter looked like a Collins
830-D. I fondly remembered some of the Collins transmitters I had
installed during the past 30 years. Many of them are still operating
dependably. However, it appeared that Armstrong had taken the design
and improved it.
The transmitter uses a quarter-wave resonant final cavity. This
cavity closely resembles the classic design, but improves on it with a
motorized plate-tuning assembly and the use of a 4CX15000A7 tetrode
running as a grounded grid. This eliminates the need for tube
neutralization and allows easy tube changes with minimal fuss. It also
increases the gain of the grounded-grid design, allowing for a smaller
IPA section and improved overall efficiency. The tuning is smooth,
gradual and wideband. To reach 20kW, only 750W of drive are needed.
This is achieved with the solid-state IPA.
The other idea that caught my eye was the rollout internal
high-voltage power supply. Easy access to the high-voltage section, for
installation and maintenance, makes it possible to thoroughly clean
without the risk of accidental shock or backache. Everything is within
The transmitter is affordable. I am a firm believer in the saying
“you get what you pay for,” so I was a bit skeptical about
its quality, performance and longevity. Because Armstrong was unknown
to me, I checked with existing users of similar Armstrong transmitters.
Everyone I talked to liked the design and the operation, as well as the
price. I ran out of questions, and decided to make the choice. Because
the price was affordable, I changed our overall plan and bought two
20kW transmitters. The price for two was only a few thousand more than
buying one major brand transmitter of the same size and
Before delivery, I paid a visit to the Armstrong factory in
Marcellus, NY. Armstrong is a small company, but the staff was
competent, professional and happy to demonstrate the transmitters on
the factory floor. We put them through their paces, even simulating a
power failure and hard start from cold. The transmitters were rock
solid. I came away satisfied we made the right choice.
Putting it in place
We scheduled installation for the end of February 2001 at the WPFW
transmitter site. The Armstrong factory staff personally delivered the
transmitters, and despite incoming snowy weather, managed to deliver
both transmitters safely inside the transmitter building, put one on
the air and removed the old transmitter. The entire FM-20000T is in two
full-size racks, joined at the center, less than four feet wide. Both
cabinets, as well as the high-voltage transformer, are on heavy-duty
wheels, allowing for easy placement and installation.
The high-voltage power supply rolls out
for easy access.
After another full day of installation, remote control cut-over and
plumbing the new coax runs completed the job of installing both new
transmitters. Because the Armstrong design incorporates an internal
low-pass filter, the output plumbing installation was straightforward,
without the heavy lifting required for high-power external low-pass
filters on typical grounded-grid units. A remote control interfaces to
a hinged, rear-mounted rack panel that extends for easy access.
The transmitter uses microprocessor control of nearly all
parameters, and tracks power output, VSWR and temperature of various
points within the IPA and PA. In the event of a power failure, the
transmitter will soft-start and ramp up to the previous operating power
stored in non-volatile memory. The LCD is easy to read, and a wealth of
internal operating information is available within the easy-to-navigate
The FMX-100LCD is the 100W exciter we ordered with each unit. These
exciters can be upgraded to an AES/EBU input for direct connection to a
digital audio processing chain. We chose the analog input for the time
being, and the results are impressive. Measurements of noise, harmonic
distortion and frequency response revealed a quiet and clean exciter.
The wide-band IPA and final amplifier had virtually no effect on the
overall measurements. AM noise was well below what was possible with
the old transmitter. The difference between our old transmitter and the
new system was apparent even to the station staff, who could now hear
clearer definition and clarity in the on-air sound.
While there were some initial minor problems with a defective IPA
cable, Armstrong provided timely help and quick delivery of the needed
part. We realized the problem on a Saturday at 4 p.m., called the
emergency number and reached the factory service rep at a picnic. He
had a delivery scheduled within the hour. They have continued to be
friendly and available when questions and concerns have surfaced.
Overall, the installation of these new transmitters was painless and
easy. I now have a better choice available when I make my next
Mussell is a consulting engineer serving a nationwide client
base. He is based in Bonny Doon, CA.