Subject: [Amps] suppressors
> Here is a point I've never seen discussed , anywhere, and I've often
> wondered about it !!
> what reason is there for a specific value of resistor in a
> parasitic suppressor. ?? I have seen 50 ohms, 100 ohms, and two 50 ohms in
> parallel.. ! Is there a valid reason for using any particular value ?? over
> the years it seems that 50 ohms was the usual value, but today I found in
> the amp I'm working on , a 100 ohm, two watt unit.
> Any answers out there ??? thanks !
> carl / kz5ca
Rich, Tom, et al will probably jump on this topic with both feet.
They will furnish the technical side, I'm sure, but the practical,
layman's side is this:
In the early days of glass tubes, the internal elements, leads length
to the pins, etc. created an "oscillation condition" as Eimac calls it.
Each choice of tube, layout, and operating parameters required a
different design of the parasitic suppressor to tame each particular
beast. Most parasitic oscillations took place back then far below 100 mhz,
as the old tubes had very little gain above that region. The "cut-and-try"
idea of the early amp builder, I suppose, was to build a device that had
enough ohms in the resistor to do the job, but not so many as to self-
destruct when operating in the 20-30 mhz area. As tube envelopes became
shorter and shorter, and vhf/uhf gain came into the picture, suppressors
were designed with less R and less L so that they would do their job at
a higher frequency. The modern ceramic tubes took it a step further,
with some amp designs getting away with just copper straps or no
suppression at all.
The suppressor that Eimac sold and used in their 4-1000A amps
(yes-Varian made a line of 2X 4-1000A RF generators and other
products) consisted of a 25 watt 120 ohm Globar resistor with 2 turns
of 1/8" silver plated copper tubing around it. It always made me chuckle
when 4-1000A construction articles appeared in ham publications with
2 watt carbon resistors and #12 wire; these would crispy-critter on 10 meters
in short order. (been there, done that!)
The bottom line is that one cannot look under the hood on an amp
at the local radio emporium, or copy an article in a magazine and
expect 100% good results. There are many variables involved. That
is why we have never seen formulas and charts published for making
suppressors like we do with tank circuits. There will never be a "one size
fits all," or a plug-and-play suppressor for all occasions.