>> To: amps@contesting.com
>> From: John Lyles <jtml@lanl.gov>
>> Subject: [AMPS] Nichrome or Globar
To: <amps@contesting.com>
>> Date: Mon, 10 Nov 97 23:07:05 +0000
>
>> 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.
>
>Hi John,
>
>There's nothing magic or "special" about nichrome. A resistive
>material is a resistive material.
>
Agreed. However the combination of inductance with built-in resistance
can be useful in certain applications. IMO, you quite likely came to
this conclusion in the first week of December, 1996 -- which is seemingly
why you cancelled your post of 28 November, 1996, sans-comment - and have
stonewalled the matter ever since.
>Properly implemented, nichrome has it's place...just as carbon, steel
>and other resistive materials do.
>
>The problem occurs when someone who does not fully understand
>theory or physics suddenly tries to "copy" something he
>saw or read. In that case, a misapplication can occur. Just like a
>conventional resistor can be misused if all the important parameters
>are not fully understood.
>
>Let me give a specific example of this.
>
>In a conventional HF PA, the resonant path is from the anode to the
>plate tuning capacitor. The undesired resonance is one that places a
>high operating impedance from the anode to chassis at the SAME
>frequency where the grid(s) have a high impedance.
>
This is pretty much true. However, some parasitic oscillations occur
above the grounded grid's resonant frequency. The TL-922, SB-220 and
AL-80 are examples.
>What we want to do is add a large series resistance to the anode
>to chassis path at the frequency where the grid is also parallel
>resonant. What we DON'T want to do is add series resistance at the
>operating frequency.
>
The output Z of a 3-500Z is typically more than 3000 ohms. According to
Wes' measurements, the resistance-wire parasitic suppressor had 4.4 ohms
of ESR at 30MHz.
>Sometimes the operating frequency is close to the frequency of
>instability,
Not in any HF G-G amplifier I am familiar with.
>
>The problem we should all be aware of is that some kits being sold
>actually INCREASE the overall Q of the VHF system, because they
>DECREASE the resistance of the series suppressor. ...snip...
According to Wes' measurements, the opposite is true. As VHF Q
decreases, VHF-ESR increases, and VHF-Rp decreases. . . Less VHF-Rp
means less VHF voltage gain.
- Mr. Rauch appears to somehow be confusing parallel equivalent
resistance, Rp, with Q. In a VHF suppressor, VHF-Q can be decreased by
increasing Rs and/or by increasing Ls. The danger is that going too far
will result in the incineration of Rs during operation 28MHz band.
- All things being semi-equal, a resistance-wire suppressor provides
roughly a 40% improvement over a conventional suppressor.
>
>In order to sell these money making and "fame" establishing kits, the
>vendor has to denigrate all other systems and all commercial and
>homebrew projects NOT using his device.
>
The idea appeared in print in the 1926 *Handbook*. No one applied for a
U. S. patent on the 'device', or, under current law, can apply for a
patent on the idea..
>No matter what the failure...be it a grid short, open filament, tube
>gas, glass melting, capacitors arcing, relays arcing, bandswitch
>arcing, or choke damage.... it ALWAYS has to be a parasitic.
>
References?
Rich---
R. L. Measures, 805-386-3734, AG6K
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