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Field Report: Armstrong FM-20000T

Field Report: Armstrong FM-20000T

Aug 1, 2002 12:00 PM, By Don Mussell

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 glance

  • 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 needed.

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 easy reach.

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 configuration.

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 menu system.

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 transmitter purchase.

Mussell is a consulting engineer serving a nationwide client base. He is based in Bonny Doon, CA.

field report