After graduating from engineering school in the early 1960s, I worked for a large radio communications company, which is now out of business. My assigned specialty was the design and development of various linear power RF amplifiers to complement the company's line of transceivers. We catalogued amplifiers from 400W PEP output to 10Kwatts PEP output -- in a total frequency range of 1.6 MHz to 400 MHz. Of course, you must understand that not every design covered the total frequency range mentioned above. Some were band-specific, covering the HF, VHF, or UHF band.
Some of these amplifiers were fairly straightforward designs, not deviating significantly from what one would find exampled in a (then) current edition of the ARRL Handbook, serving HAM radio enthusiasts. Others were far more sophisticated, using self-tuning and self-adjusting circuits. Some of these amplifiers worked into antenna tuners, and some worked directly into antennas designed for the purpose.
But the one thing in common for all these was that they were tube type amplifiers -- the higher power output designs used EIMAC tubes, while the lower power ones used AMPEREX and/or RCA transmitting tubes. All of them had lethal PLATE voltage power supplies.
Aside from the DC side of the circuit, there was another important consideration. When you're designing these types of amplifiers, it is imperative to understand the circulating currents of the tuned circuits, as well as the instantaneous RF voltage levels that appear across the circuit components. Many of these designs, of necessity, required the use of vacuum variable capacitors in the tank circuit to minimize the probability of arc overs. Furthermore, working on one of these amplifiers, one also had to be very cognizant of the possibility of getting bit by a flash over. Putting one's fingers too close to a circuit component could initiate an arc over. There's nothing like the smell of burning flesh being instituted by a parasitic current. Moving your finger or other body part away from the source of the arc had to be swift and definite. Sometimes the arc would continue to travel for upwards of one inch or more, depending, of course, on the output of the amplifier.
Was I careful? Of course. As a young engineer, I observed very carefully the procedure of the elder engineer to whom I was destined to replace as he moved on to greener pastures. Whenever we worked on these amplifiers, he always carried a long (about two feet) Teflon rod that was approximately 1/4 inch in diameter, to which he had taped an NE-51H neon bulb. With the power on, we would test the cabinetry and periphery of the system using "the wand." If the bulb lit up brightly, we knew there was reason beyond doubt to exercise inordinate amounts of caution. If it glowed dimly, then we could relax our vigilance a tad. That's the way it was, and that's the lesson I learned early on, which I carried throughout my RF design days.
Robert Johnson grew up in a family of builders, technicians, and trades people, and he has always been fascinated by mechanical and electrical devices. He went to college to formalize his knowledge, and 50 years later, it is all history.
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