[Amps] Nichrome in VHF amplifier

[John Lyles]

i the early 1980s I had a cavity amplifier project (88-108 MHz 5 kW) that came up with a particularly difficult parasite. It was the first use of a new Eimac tetrode, 4CX3500A, so there was a lot of concern about the hot VHF tube too. The bug wouldn't happen if I inserted a 4CX5000A. Eventually a scheme was found that not only killed the parasite but also took care of second harmonic. Patent 4,333,203 had been issued to a coworker at the company a few years earlier, and it used a small paddle and copper strap to force a short for the second harmonic, at the right location along the transmission line circuit. This had been applied in the 30 kW models before. I was able to change the copper strap to a small nichrome coil, that was tunable by pressing it tighter or looser, over the coarse FM BC band. It was de-Q'd enough that the setting wasn't so critical (copper strap didn;t work, too touchy) and it eliminated the bugger. I think they sold many hundreds of these cavity amplifiers over
the subsequent decade before everything went to silicon. So I have to say that the use of nichrome in this VHF circuit, at high power, was effective. Later in life, I made the plate RF choke in a large industrial oscillator (27.12 MHz, 30 kW, 3CW30,000H3 triode) using solid nichrome to de-Q it, with good results. I spent a lot of time sweeping chokes with a vector Z meter, observing the changes in Q near self-resonance, varying the diameter, length, and wire composition. This was all fun work, and it convinced me that every situation has good and bad approaches. Its up to RF engineers to figure out the good ones and not stumble too badly. 
73
John 
K5PRO