The Broadcast Electronics tetrode-based FM broadcast transmitters
almost all used Nichrome wire in various parasite killers. There was
a patent issued in early 1980s to them for a particular arrangement.
What we found was that you could build tuned parasite suck out
devices (traps) in the big cavities but that as tubes were replaced,
or the cavity was tuned around the band, they would become
ineffective. So using the lossy wire de-Q'd them enough to cover
either the entire 88-108 MHz range, or a portion of it, depending on
the model. In the big 30 kW rig using the 8990/4CX20,000A variety of
tubes, the plate choke itself fed a small nichrome coil which then
connected to the half wave output line. In some versions, there was a
separate LC trap in the wall, which had nichrome in the L part.
In another company i worked on RF dielectric heating equipment which
used the 3CW30,000H3, and we changed the plate choke to nichrome, to
lower the Q, since we needed some impedance over a range (free
running oscillator from 26-29 MHz). It ran hot from the DC, but
eliminated having a series R anode transient resistor entirely. I
believe in Nichrome where you can get away with it.
Now I am making BIG parasitic suppressors for a 3 MW 200 MHz
amplifier using double ended THALES TH628 tetrode (Diacrode is what
is is called by the mfr). Since it is a large cavity line circuit,
cannot use LC circuits that HF systems might use. Since the worse
parasites come from TE11,TE21 and TE31 modes (400, 800, 1200 MHz
ranges), the suppressors are high pass absorptive filters, loaded
waveguides located in the appropiate spots in the cavity. Nichrome no
good for this. Someday i will publish it and post a note here. Back
to work....
K5PRO
>
To: <amps@contesting.com>
>Date: Sun, 14 Jul 2002 12:25:33 -0400
>Subject: [Amps] suppressors
>
> Are there any commercially built amps currently using nichrome or stainless
>steel wire for their parasitic suppressors??? Just curious !!!!!thanks!!
> carl / kz5ca
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