People who have been in radio a long while will know the term “plate voltage” and “plate current.” Transmitters of the past (and some present day FM transmitters) utilized vacuum tubes. At one time, the FCC required us to log those numbers every few hours. By multiplying those two numbers and an efficiency factor, the operating power of the station could be calculated using the “indirect method.”
You had to know that even to pass your FCC Third Phone test with a “Broadcast” endorsement back in the 1970s. By the 1980s the test was eliminated, and the “Restricted” license was required. During the 1990s, one had to pay a $30 fee for your “license. ” This gave you the authority to be “on the log,” to make minor adjustments to the transmitter, to turn it off an on and in the case of AM directional stations, to change the power and patterns according to the license. You could not be a “DJ” at a station without this license, unless someone on staff was licensed and in charge while you were on air.
At one time, this type of knowledge was also taught at Specs Howard School. With updated equipment, and equipment that would automatically record such things once manually kept, the FCC also relaxed the Rules somewhat. Some stations continued the practice, however, as the role of the full time Chief Engineer became less prominent, or at least one or two persons became responsible for several signals.
The FCC’s Restricted licensed was discontinued many years ago, along with the requirements of the Chief Engineer to have a General or “First Phone” license (which actually went away by the 1990s).
Returning to techie stuff, in tube technology, the “plate” referred to the element in the tube that acted as the output of the tube, and any changes had a direct impact on the power output.
Another switch on transmitters was the “Filament” switch. Higher powered transmitters typically required a little warm-up, and the filament was as its name implies: the part right down the middle of the tube that illuminated orangey or yellow depending on the design of the tube. By allowing warm-up time, the life of the tube is extended, by not exposing a cold tube to the shock of thousands of volts.
Tube technology remains popular today in the music world. Many guitar players prefer the sound of a tube amp, and the equipment is still manufactured. Most have a “Stand-By” switch that does the same thing as the “Filament” switch in broadcast transmitters.
The differences were broadcast transmitters were designed to operate well within the manufacturer’s specifications, as the tubes could cost as much as thousands of dollars each, depending on power level.
Guitar amplifier manufacturers, however, tend to design equipment BEYOND the range of the tube, as this helps them to achieve a particular tonal quality.
Many songs you’d recognize with famous guitar riffs with a particular sound probably came from a amplifier operated on the “edge.” The drawback is of course, shorter tube life.
There would never be an advantage to doing this with radio transmitters. The few watts of extra power would not equal an increased coverage area, and in fact, could push the station beyond its legal limit.
