A Look at a 1950s-era AM Radio Transmitter

For those who don't know, I work for a local group of radio stations as a broadcast engineer, and one of the stations I maintain is an AM with a facility dating back to 1946. This is a tour of the station's old RCA BTA-5G transmitter, which was installed in 1955 when the station (under its first owner) upgraded from 1kW to 5kW. I will admit I'm a little fuzzy on some of the details of this transmitter's operation (and this sort of old-school circuit design in general), but I explained it as best I could at the time. I probably screwed up some details, but such is life.

Something else I should have noted in the video is that this transmitter is powered from two separate circuits in the building's electrical system. One is the 3-phase 208V supply that I describe in the video, while the other is a single-phase 110V supply that only powers the crystal heaters, in-cabinet lights, and in-cabinet convenience outlets. The latter supply is supposed to always remain on so that the crystals are always held at operating temperature, even when the transmitter is off. So no, the circuitry that I reached in and pointed at was not energized.

[palmerjohnson]

Especially given the fact that this transmitter is in perfect operating condition, I certainly hope that you can take steps to prevent this from ever being dismantled and thrown away. I recently had been forced to dispose of a GATES BC-5B that could have easily been put back into service. Great job on keeping this alive and thanks for such a wonderful video.!

[SurnaturalM]

I made a 200w AM broadcast band transmitter for my neighbour and I, we have a huge collection of old am radio, but since there's no station where I live, I made one myself and I run the audio from a tube amplifier connected to my computer with a digital am compressor. It has 2 807 in the final rf amplifier, class C plate modulation. It's a smaller version of this one. It really sounds like a commercial radio. We don't have any neighbour around so I don't have to worry about bothering others people. The antenna is a radial half wave long wire. It runs very well. I used a all tube design because it's easier and I have plenty of parts.

[pedrodepacas]

You’re a good guy to show that beautiful old rig. Love the shag carpet trim.

[Turboy]

That RCA transmitter is a true American Machine. Beautifully made, simple, reliable, serviceable, and it might last as long as you can get tubes for it. Plus, being crystal controlled, zero phase noise! A super clean transmitter signal is the result.

[ctbcubed]

In the early '70s during college, I worked a 16 hour weekend shift as the transmitter engineer for a 50kw day/ 10kw night AM station. Our day pattern fed 2 towers and went to 9 towers at night. The daytime transmitter was an RCA Ampliphase BTA-50G. As I recall, the power supply used 6- 857B MV rectifiers that were almost 2 feet tall. It ran at about 16KV at 4 amps. The blowers on that thing were really loud. Unlike a typical plate modulated transmitter, this was essentially 2 phase modulated transmitters with a network on the output side that vector summed the phase shift of the 2 transmitters so as to create an amplitude modulated signal to the antenna. We needed to be sure not to exceed 100% modulation, or the result would be horribly distorted audio. The transmitter tubes were huge and required a lifting device to change them. Luckily I never had too. The doors on the power supply cabinet had spring loaded shorting bars that would discharge the capacitors if the door opened. Get ready to change your underwear if you weren't expecting a loud snap. The night transmitter was a plate modulated RCA 10D from the 1930s that was retired from use by the state police when they used to operate near the AM broadcast band. This thing was like a giant ham transmitter in that tune up and operation were the same. There was a huge modulation transformer that sat in a cage behind the transmitter. It also had 2 large brass balls to form a spark gap for over modulation. This transmitter had no forced air cooling and it was a dream to take that 50kw unit off line at night. You could hear the modulation transformer and it sounded like a tweeter. The inductors it the antenna tuning huts also acted like tweeters. I had a lot of fun working at this station and really enjoyed your video of the well preserved 5G.

[kenshinflyer]

As a little kid, I used to dream of working on radio transmitters, and that every visit to the local radio station and seeing the transmitters operate in full power was a thrill.

[douglashoff]

As a retired TV repair technician, I appreciate the care and craftsmanship put into older equipment. RCA was really RCA and not a name used by a Chinese company.

[davidm]

My dad was a chief engineer at a AM station in early 50's and I remember as a kid going out to the transmitter with him to check on things. Mainly remember the tremendous heat coming out of the equipment. Thanks for the video!

[RandyOnTheRadio]

Wow, this takes me back. I worked at a station in the late 70's, that was using a 1 KW Raytheon, (1947 vintage), at night, directional 4 tower array. Daytime, we used an RCA BTA-5, non-D. The RCA BTA-5 was only 2 units though. There wouldn't have been enough room at the transmitter site for that mega-monster 5KW that you showed here. I was one of the jocks at the station, but was at the x-mitter site on many occasions. It was nice to take a trip back to the fun old times of radio. I got paid crap, but I had fun.

[tonywestvirginia]

I wanted to be a radio engineer. At 13 I had my 3rd class FCC license. Then I was installing radios in cars. Cars became my passion. Retired an ASE master technician. Wish I stuck with radio, my back would be in better shape. Thanks for the tour.

[charlessmith]

As a young man, I used to help the station engineer at the remote transmitter site out in the sticks when lightning repairs etc. required some overnight work. He was a polio victim with leg braces and often needed some extra help. The 1KW transmitter used two 4-400 RF output tubes. I was a ham and he would let me have some of the older 4-400 tubes for my 1KW ham rig. Those were the days. I once volunteered to climb the tower half way and replace a burned out light. I will never do that again. My legs were like jello afterwards. Fond memories.

[kcscarecrow]

I enjoyed your tour of this transmitter. You voice and laughter conveys enthusiasm and positive tone. I worked for an AM station in Topeka Kansas. KEWI back in the early 70s which used a Collins 21E. We used mercury vapor rectifiers in the hi and Lo voltage power supplies. Modulation would really get those guys a blinking. They later went to solid state rectifiers.

[justicelut]

Thanks for that. Very interesting. I really enjoy watching quality old equipment like this still working. Unlike the throw away society today!

[SPboxcarl]

About 1979 I got hired as a transmitter engineer at WARM "The Mighty 590" in Avoca PA. Actually I didn't know anything, I was a college freshman and newbie Ham radio op. In fact I think maybe a Ham Op got me the job? The transmitter was basically 5 towers out in a farm field miles from the studio. I never saw the studio. Fun job while it lasted which wasn't long. The owners replaced most of us transmitter baby-sitters with automation. I wish I had taken some pictures of the facility but I didn't have a camera (and of course that was before the era of smartphones with cameras- or even 'dumb phones")
Thanks for the transmitter tour.

[redsquirrelftw]

That was really cool. One of these things most people will never get to see in person or work on. Something so cool about tubes and analog electronics though, how you can actually see and hear it work.

[dfpytwa]

About 20 years ago I got the unofficial B tour of a long time popular AM radio station's backup transmitter while working on a cell site nearby and chatting up the engineer for the station through the fence during a smoke break. Similar equipment and vintage. They kept it operational rather than installing more modern equipment since it was there, still worked, parts were still available and it was easy to maintain. A few months later a small plane crashed into their new tower at their new location and they had to fire up the old one and operate from it for months. Their newer site had more power but their old site had high elevation. During their back up operation I was working on another cell site a good 900 miles away and I was able to pick them up on my truck radio at night no problem. I live about 400 miles from there now and I can tell when their new site is under maintenance because I can pick them up here day or night while they are on their backup.

[BobOnTheJob]

I started in radio in 1967 (on FM) and by the time I 'worked up' to AM, these were no longer around. I often wondered what they looked like and you showed in great detail what I have long wondered. I bet I would have gotten along well with that old girl. I was in the room with one 30 years ago and still remember hearing it 'singing' the Children's Marching Song by Mitch Miller. It was an honor to be in the same room. Thanks very much for taking the time to walk me through the innards of this majestic transmitter.

[secretlab]

Marvelous transmitter. KUGN (Eugene) ran the very similar BTA-10, operating at 5kW, thru 1978 when it was replaced by a Continental. Amazingly solid, good sounding, and reliable. Also easily capable of 110% to 120% positive given the oversized components.

[marzsit]

very cool to see another old rca transmitter operating. in my state we also have a functional old rca transmitter, a bta-50f from 1945 installed on vashon island in washington and operated by komo am 1000. it's so big it has man doors on it and you walk around inside of it to service it.. it's a massive beast.

[thejoneseys]

Thank you for showing us this awesome grandaddy of a transmitter it was absolutely fascinating to see you start it up! I used to listen to AM radio a lot as a child here in the UK (we had a few pirates) and have always been fascinated by AM transmitters and how they operate. The running hours is staggering when you do the math. 507125 hours is 21130 days or a staggering 57.89 years 😳 Hearing the audio resonance from all the components within has a real charm to it and it deserves to run forever.