[AMPS] Nichrome or Globar
km1h @ juno.com
km1h@juno.com
Mon, 10 Nov 1997 19:26:58 EST
John and others....
I have mentioned the use of alternative suppresor materials on this
reflector for almost a year. The "experts" refuse to even acknowledge
them. That includes Rich.
I will continue anyway even tho some appear brain dead or hung up in
their own world for whatever reason.
Back in 1963 or so, while I was at National Radio, we had a major problem
trying to tame parasitics in a 5KW military tetrode amp.
The engineers...I was just a tech... finally wound up using a suppressor
coil made from a material similar to the Mu-Metal used in CRT shields.
Fast foreward to 1997.... I use that material in my own suppressors and
find it the best ever material to tame even 572B's on 6M.
I have offered the "experts" the info...no strings attached...but you
know what? Not one has asked for the info......I wonder why ?? Ego
problems IMO...also known as " not invented here".
73 Carl KM1H
On Mon, 10 Nov 1997 16:07:05 -0700 John Lyles <jtml@lanl.gov> writes:
>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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