> 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.
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.
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.
Sometimes the operating frequency is close to the frequency of
instability, and in that case we need a resistive tank system. We
trade efficiency for stability, but ONLY after all other means of
stabilizing the PA are tried and unsuccessful.
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. They do this at
the same time they lower HF Q slightly.
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.
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.
The tecnical hyperbole that parasitics are the root cause of
all or even most problems misleads people with problems, and the
technical falsehoods and misstatements damage our general knowledge
about how things really work. But then that's what salesmen do, they
convince us they have the only cure.
73, Tom W8JI
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