Amps
[Top] [All Lists]

[Amps] Nichrome in VHF amplifier

To: amps@contesting.com
Subject: [Amps] Nichrome in VHF amplifier
From: "John Lyles" <jtml@losalamos.com>
Reply-to: jtml@vla.com
Date: Wed, 11 Feb 2009 19:41:59 -0700
List-post: <amps@contesting.com">mailto:amps@contesting.com>
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 amplifi
 ers 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


_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps

<Prev in Thread] Current Thread [Next in Thread>
  • [Amps] Nichrome in VHF amplifier, John Lyles <=