There are 'suppressors' in VHF transmitters (FM broadcast) using Nichrome
alloy wire
directly in the plate choke for the tetrode plate DC feed. This is in a
cavity amplifier, where shunt feed is used for DC to the plate, except it
is connected at the fundamental voltage minumum on the structure, which is
the second harmonic voltage maximum. We used a 10 Ohm series resistor on
the outside of the cavity, for arc protection - it was a Carborundum Globar
type 887AS I believe. At Broadcast Electronics, Nichrome was used in the 35
KW rig, at 100 MHz. This little coil was mounted on Mycalex blocks, to
stand the heat due mainly to the DC current. I used Nichrome wire in a
special harmonic trap/parasitic killer that I designed there, in the
smaller cylindrical cavity amplifiers for 5 KW. It 'broadbanded' an
otherwise narrow notch suppressor design. Also took the heat. It was
dissipating a minor amount of fundamental power, but the overall PA
efficiency was still 78% But it also stopped an oscillation which would
otherwise eat harmonic filters for lunch. Nichrome really helped there.
Collins Radio (the line was later bought by Continental Electronics
Manufacturing Co) used a Globar resistor in the back of some amplfier
cavities, connected to a probe, to suppress unwanted parasitic modes. These
are the plague of VHF cavity amplifiers, where an unlucky choice of
dimensions can excite undesired modes where the tubes still have power
gain. I thought I remember seeing similar tricks in RCA amplfiers. If you
left these strange devices out, you could cause lots of smoke and bad magic
in the amplifiers.
Globar is actually a product of Carborundum, which is now the Cesiwid
company. So, both are successful HF and VHF resistance materials, where you
want high R with respect to the amount of L. I agree that silver plated
suppressors always looked nice....
R. Measures might remember me from a lettter I sent him in 1988 about my
experiences using Nichrome wire in amplifiers. I was WB4PRO then.
See the classic paper from GE on "Parasites and Instability in Radio
Transmitters" by G. Fyler. It's in Proceeding of the IRE, September, 1935.
They built the WLW 500 KW MW transmitter, and had some phenominal arcs when
paralleling 100 KW tubes!
John
K5PRO
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