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*** The Ampliphase Ancestry ***

The BTH-100B 100Kw AM Ampliphase



The 100B was more or less a beefed up BTA-50H, it used the same modulator, drive regulator and driver stages, but with four power amplifier cabinets. Indeed, during factory production, the units were normally treated as a pair of 50H's, though only a single exciter cabinet was built. The units designation was an interesting departure for RCA as previous standard AM transmitters were prefixed "BTA", meaning "Broadcast Transmitter AM", as opposed to "BTF" for FM sets. Logically one would expect this transmitter to be a BTA-100, but for some reason RCA called it a "BTH-100", not to be consfused with the shortwave "BHF-100". Perhaps the H stood for "High" power.

The four PA cabinets all used the 6697 tube, in the same arrangement as the 50H, with two amplifiers in parallel for each channel of modulation. The driver stage was the same 4-250 and 4CX5000 as the 50H, though it appears the driver supply had been increased from 5KV to 6KV to upgrade the amount of drive power available to the PA's. The most significant difference from the 50H was in the power supply area. The high voltage remained at 15Kv, but the filter reactor and capacitors were removed from the transmitter cabinets and placed alongside the high voltage transformers in the transformer vault. As the silicon rectifiers remained in the cabinet, this must have involved additional highly insulated wiring to take the high voltage DC to the capacitors. The reactor sits in the negative return rail and would require less insulation. As the reactor normally sat below the RF filter and combner section in the 50H, I assume this vacated space was used to house upgraded vacuum capacitors and coils required to handle the higher RF power of the 100 kilowatts. The output could be configured for either 50 or 230 ohm transmission lines.


Very few of the 100B's appear to have been made, possibly as few as 4 or 5 with the first three being shipped in early 1961 to an overseas customer and all seem to be regarded as prototypes. Possibly the most well knwon example of 100B was onboard the Radio North Sea International ship, MV Mebo 2 from 1970 to 1974 as shown in the above picture. This unit was Serial, number 2, which suggests it was part of the first batch of three units - thus the original owner must have sold it after just 7 or so years. After RNI closed the MEBO 2 was sold to Lybia and it was used for a while as an offshore station but the transmitter was apparently removed from the ship, and put into storage in a landbased transmitter hall. It's eventual fate is unknown. The above picture taken from the Gerry Bishop book "Offshore Radio" (photo credit: Martin Stevens) shows a sideways view of the six cabinets, on the right side is a BTA-10H. The RNI transmitter was often known to suffer from overheating when running on full power. Other 100B's may have been shipped to Finland and Nigeria. If you know of any more, or know more about the RNI unit, then please contact me. Thanks.