This is the RCA Section of
The Broadcast Archive

Maintained by:
Barry Mishkind - The Eclectic Engineer
Last Update 4/8/11

Radio Corporation of America

Camden, NJ.

RCA 50 kW Transmitters

From the time RCA was formed (using the combined engineering of Westinghouse, AT&T, Marconi and GE), the company was a formidable source of broadcast gear. The last 50 kW unit was shipped in 1977.  In 1985, GE bought RCA and dissolved it.

The RCA 50 kW Series:

50A (1927)

The first 50 kW unit built by GE for RCA was the 50A. Reportedly, only one was made in 1927, for WEAF. 

From an old magazine. Notice the tubes are exposed, and the railing is to keep the operator from the high voltage.
The 50A and 50B used motor generators to run the transmitters on DC, the later transmitters ran on AC.

All together the transmitter used 20 UV207 tubes for audio and RF. Modulation was high level Heising modulation.


50B (1928) 

The 50B was also built by GE, starting in about 1928. This was Heising-modulated, linear-amplified, and water-cooled.  One of the earliest practical 50 kW transmitters, 50Bs were in daily use for over 30 years.

The KFI RCA 50B, installed in 1931, and featured in the RCA Broadcast News


The 50B had a 250 Watt exciter, followed by a 5 kW IPA to drive the UV862 finals. 


The 50C seems to have been an upgrade from the 50B, and the first design changes by RCA. The filaments were changed from DC to AC and the final tubes were changed out.  

50D (1937)

The 50D was produced starting in 1937. It was the first truly RCA design: a Doherty-style modulated, water-cooled unit. However, since David Sarnoff did not want to pay royalties to Western Electric (for a 90-degree lagging network between the tubes), his company developed the "RCA Class B-C High Efficiency Linear Amplifier."  This was a 270-degree network, to get past the legal niceties.

Nevertheless, legal hassles continued. A total of only six 50-D's were made. They were installed at KNX, Los Angeles, WLS, Chicago, WWL, New Orleans, WCKY, Covington, KY (Cincinnati), WWVA, Wheeling, WV, and CBA, Sackville, New Brunswick, Canada. The last of these were said to have been scrapped in the 1970s.


The 50D was available in two layouts, a "flat" layout and a "U" shape.

Being the operator for this transmitter must have been as close to operating a spaceship as you could get in those days!


 BTA-50E (1941)

The 50E was introduced in 1941. It was a plate-modulated transmitter. The first 50E delivered is said to have gone to KOB in Albuquerque, NM. Reports are that only four "E"s were built.

Now on display in Tommy Bolack's Electromechanical Museum in Farmington, NM, KOB used this transmitter from 1941 until its removal in 198x.

 This 5C was the exciter/driver for the 50E.
It also lives at the Bolack Museum.


BTA-50F  (1947)

The BTA-50F was produced starting in 1947 (it was after WWII that RCA added the BTA ... presumably to avoid confusion with the new BTF FM series). This was also a plate modulated transmitter. The upgrade to the "F1" series was the following year, using a thoriated tungsten filament 5671 tube for higher efficiency. (The "F"s were upgraded to "F1".)

KOMO in Seattle used the 50F

The 50F Power Amplifier Tube compartment

The 50F even had a door that you could walk through to get into the transmitter from the front. Here, the door to the power supply section is open. 

KRMG in Tulsa installed a BTA-50F1

BTA-50G  (1955)  (and the McClatchey 50 (1948))

The 50G was the first Ampliphase transmitter. (Ampliphase was RCA's trademark for outphasing  modulation, and was based on the 1935 design of Frenchman H. Chireix.) The first version was developed and built by engineers at McClatchey's KFBK in Sacramento in 1948, RCA bought the rights. Production began in 1955. According to one source, about 30 were built.

Some folks have used the pejorative term "Amplifuzz" for the displeasing audio that sometimes came from the transmitter. However, rather than a failing of the transmitter, it was far more often the failure of the operating engineers, who failed to understand the modulation technique. A properly maintained Ampliphase was capable of a very clean, wide-banded signal.


The front of the 50G, from the RCA catalog

 A 50G was at WLKW, Providence, RI


BTA-50H  (1961) 

The 50H was a second generation Ampliphase, starting in 1961. A major change was to use solid state rectifiers in the power supply.
The 50H's front, from an RCA spec sheet. Cosmetically very similar to the 50G.

The 50H used lighter, cheaper tubes. Although one person could change them, they didn't last as long.

This is a view of the 50H installed on Radio Caroline, a pirate station in the British Channel.


more info is at where the Ampliphase is discussed at length.


The 50H1 was "two-toned"

The 50H1S at KFBK (pictured) had a solid state exciter


BTA-50J (1970)

The 50J was the last of the Ampliphase transmitters. Built starting in 1970. Production ended in 1978.

The exciter was solid state.


The 50J had no windows or meters on the outside


KFQD in Anchorage, AK had a BTA-50J in use until 2001

Thanks to Jack Sellmeyer and Nat Kayle, among others, for assistance in putting this page together.

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