[AMPS] Nichrome or Globar

Rich Measures measures@vc.net
Mon, 10 Nov 97 22:32:50 -0800


>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.
>
 
-  The subject of the debate was VHF suppression in HF amplifiers, Carl.  
6M amplifiers is a different subject because the anode-resonant circuit 
IS the tank circuit.  In an HF amplifier, this is not the case, and 
therein lies the problem.  "Best ever" means what?  Is 'best ever' better 
than 'totally rad'? 

>I have offered the "experts" the info...no strings attached...but you
>know what?  Not one has asked for the info......I wonder why ??  
... ... ...
Because a VHF amplifier is not a HF amplifier.  //  Nobody asks for info, 
Carl.  If you think you have something useful, you lay it on the table 
for everyone to have a look at.  

>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 35KW 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.
>>
I received hundreds of letters during the last three months of 1988.  I 
can't put the callsign with one of the letters, John, but there were 
several letters along these lines.  In addition to Collins and RCA, 
Hughes seemed to have a handle on the problem.  An ex-Hughes employee who 
currently works for NASA, said there were a number of such devices being 
used in a Hughes 100kW amplifier design.  
Rich---

>>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!

I wonder if G. Fyler knew F. E. Handy?
>>
>>John
>>K5PRO


R. L. Measures, 805-386-3734, AG6K   


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