AM Transmitters – Page 4 – Engineering Radio

The Nautel XR6 AM transmitter

I’ve been away working in Burlington, VT (WVMT, 620 KHz, Burlington) for the last coupla, installing this nifty Nautel transmitter:

Nautel XL6 transmitter, WVMT Burlington, VT — Nautel XR6 transmitter, WVMT Burlington, VT

I like the Nautel units, both AM and FM; they are well-designed, well-built, rugged transmitters. I have lost track of how many of these units we service in the field, partly because they are becoming pretty much standard equipment at all of our installations.

Continental 315R-1 AM transmitter, WVMT, Burlington, VT

The transmitter it is replacing is a Continental 315R-1, which is based on the Collins Power Rock design. It is a PWM transmitter with a 15,000 volt power supply. In their day, these were not terrible transmitters, however, like their Harris MW-5/10/50 PDM brethren, frequent thorough cleaning is required to keep the dirt/dust from arcing over. Unfortunately, it is becoming more and more difficult to obtain parts for these units. This transmitter was installed in October of 1983, thus, almost thirty years of service is quite enough. This unit we did not cut up and scrap, rather, it is sitting by the back door, waiting for any takers.

Continental 315R1 modulator/RF sections — Continental 315R-1 modulator/RF sections

The interior of the Continental 315-R1 transmitter. Modulator section is on the left, RF section is on the right.

The good news is, WVMT is another one of those “successful AM station” stories. You know, the kind of station that has local programming, local sports, news, community presence and most importantly, makes money. For all those diligently studying the “AM Problem” for the up and coming NAB conference this April, here is a clue: It’s the programming…

Nautel XR6 transmitter, wvmt Burlington, VT — Nautel XR6 transmitter, WVMT Burlington, VT

This is the Nautel XR-6 on the air. Positive peaks, anyone?

We turned that down a little bit. Also, the station does not run AM stereo, the AM stereo mod monitor is simply a usable relic of a bygone era.

WVMT is noted as the first radio station licensed to the state of Vermont, signing on on May 10, 1922. It has a three tower directional array located down in the swamp. For some idea of perspective, it is 1,150 feet (350 meters) from the transmitter building to the center tower, the towers are 411 feet (125 meters) tall spaced 405 feet (123 meters) apart.

WVMT three tower directional antenna array, Burlington, VT — WVMT three-tower directional antenna array, Burlington, VT

WVMT antenna system from back of transmitter/studio building. That is a long walk over rough terrain in the middle of the night or anytime really, but especially in the middle of the night.

The Continental D323C medium wave transmitter

I found a 1981 Continental Electronics equipment catalog at an old transmitter site. These finds are great if one is interested in history and looking at the way things used to be done. This particular transmitter is a 2,000 KW (2,000,000 watt) medium wave unit:

Continental Electronics D323C, Circa 1981

I believe most units like this were destined for use by government broadcasters in either the middle east or Western Europe. I know there were several 1,000 KW medium wave stations in West Germany at one time. The Continental transmitter is basically two 1,000 KW units (323C) combined. They used a modified version of Doherty modulation, that is called “Screen and Impedance,” which accurately describes how it works. More information from the Continental Catalog can be found here: Continental D323C. The tubes (or valves depending on where you are located) used in the D323C were 4CW25000A tetrodes as modulators and IPA the final was a pair of X2159, which is an impressive tube.

EIMAC X-2159 water cooled power tetrode — EIMAC X-2159 water-cooled power tetrode

The tube sat anode up. The filament, grid, and screen connections are underneath. Cooling water was pumped through the two connections on the top at about 130 gallons per minute depending on the plate dissipation. With a 30° C rise, that equals about 96,000 BTU per minute. The D323C had a dissipation of 400,000 watts for the carrier tube and 240,000 watts for the peak tube (640 KW total) under 100% modulation. That equals about 2 million BTU per hour. Notice the lifting hook, this tube weight in at 175 pounds. Tube date sheet here.

Continental no longer makes medium wave transmitters, their closest high powered broadcast product now is the 418/419 and 420 HF (shortwave) transmitters. The 420D does a wimpy 500 KW using a solid-state modulator section.

I remember in the early 1990’s when I was at the Harris plant in Quincy, they were working on a 1,000 KW solid state DX series AM transmitter for Saudi Arabia. It had to be liquid cooled, which added another layer of complexity to an already complex system.

I don’t know if there is much call for 2 MW medium wave transmitters anymore as there are more efficient ways to reach remote populations and I can’t even imagine what the electric bill would be like.

The Gates BC1H

In my never-ending fascination with broadcast transmitters, I bring you the Harris/Gates BC1H. This is an updated model of the BC1G, the main difference being the solid-state audio and oscillator sections in the later H model. This design uses the simple 833 parallel final and 833 push-pull modulator

Sales brochure, click to download the four-page .pdf

Schematic:

Harris/Gates BC1H Transmitter running at 1,000 watts into the antenna. Like many old tube transmitters, this sounds great on the air. The transmitter was made in 1975 and is in backup service. For a 37 year old transmitter, it runs like a champ and comes on consistently. Like the preceding Gates BC-1 models, this transmitter is rugged and reliable. My only comment is the transistors in the solid-state driver section are no longer available. If that were to become an issue, one can always look up the tube audio driver from previous versions (T and H models). It would be a shame to throw away a good transmitter for lack of a couple of transistors, but I know some who have done just that.

Australian Made Broadcast Equipment

Somebody working to preserve a record of past work:

Some of these have familiar looking cabinets and tube arrangements. They all look like classics to me and it is good that they are being saved. I noticed at the end of the video there is a Harris MW10A. As for the RCA Ampliphase transmitters; I maintained a BTA5J in Harrisburg PA on 580 KHz. It was reliable enough, but I could never keep it sounding good for more than a couple of days.

In any case, a worthwhile effort. More information at: AWA Transmitters.