But now we are getting far away from the typical ham amp tube where the
damping resistance is simply part of the parasitic suppressor and that
resistors true purpose is to stop a parasitic from even forming. The coil is
self resonant hopefully near the known parasitic frequency and the resistor
is the load of a very low Q circuit. Ideally it should not absorb any
parasitic power since it wont even start.
In some amps, such as the SB-220 family, the so called VHF choke RFC-2 is
self resonant close to the 3-500Z parasitic and invites fireworks. Replacing
it with a true wirewound "glitch" resistor serves a double purpose....snubs
the VHF energy on the B+ line and limits discharge current during a tube gas
arc.
With HF and low VHF SS a ferrite bead and 1-2 turns at the input serves as
the damper. At microwave/millimeterwave a small piece of ferrite sheet is
placed over the over the input microstrip for the same purpose, plus as a
brute force impedance matcher. I spent years of R&D lab time getting a
circuit stable enough that it could be manufactured and production tech
tuned with a minimum amount of time. There is still a bit of black magic
involved at those frequencies; Ive been as high as 300 GHz with DoD aviation
electronics.
Taming a glass tube on HF or 6M is childs play (-:
Carl
KM1H
----- Original Message -----
From: <donroden@hiwaay.net>
To: <amps@contesting.com>
Sent: Saturday, July 19, 2014 4:17 PM
Subject: [Amps] damping resistances
Quoting John Lyles <jtml@losalamos.com>:
In cavity circuits, we can use damping resistances instead of parasitic
suppressors
I installed an RCA 20KW FM transmitter in 1976 that included a couple of
ceramic insulators that supported a 50 ohm /100 watt film resistor in the
cavity. There were brass dipole elements on each end of the resistor.
I looked on the schematic to see which wires connected to the
resistor.....
I couldn't even find the resistor on the schematic.
There was a brief mention of the resistor and it's function in the
instruction manual.
Don R W4DNR
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
-----
No virus found in this message.
Checked by AVG - www.avg.com
Version: 2014.0.4716 / Virus Database: 3986/7883 - Release Date: 07/19/14
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|