It is sad that in most cases of "fixes" reported here on the reflector, when
a parasitic suppressor is replaced, no one measures the value of the old
resistor compared to what its original value was. Operation on 10-12 meters
will cause heating of L-sup on these bands if it is properly designed and
contribute to its demise faster than those of us who don't operate there. If
you notice on most modern amps, L-sup has been beefed up to 2-3 resistors in
parallel, or better yet, a 15 or 25 watt Globar unit of the proper value.
Several have reported in past threads of old carbon resistors changing value
just sitting on the shelf over the years.
Sage advice has been given to measure the value of R-sup as a regularly
scheduled preventative maintenance procedure. I conduct an annual "spring
cleaning" on my amps where all interior dust/debris is removed, the blower
blades are cleaned and its motor lubed, the condition and alignment of all
switches are checked, and R-sup is measured for any change in value.
I always do an autopsy of old removed parts to get an idea of whey they
failed, or why they were no longer doing the job.
The bottom line is that if a parasitic suppressor is replaced with one of
"stock design," nichrome, or a "beefed-up" version, and it solves the
problem, we learn nothing unless the values of the old parts are measured
and reported. All three of the examples above have a proven track record of
working just fine if none of the component values have changed.
The 8877 is an example often discussed; whether a suppressor is needed or
not. When this tube appeared on the market, construction projects
immediately appeared in the Orr and ARRL Handbooks. I thought it novel at
the time that no suppressors were called for in these early applications. I
have never heard anyone report that they had to add a suppressor due to
problems with these early circuits. Then
industrial/commercial/amateur/home-brew models came on the market. Most had
suppressors, some didn't. (Ameritron, RF Power Products) I have to assume
that the majority of these amps are still working fine. I also have to
assume that folks who have added a suppressor (copper or nichrome) to an
8877 amp certainly have done no harm. It is sort of a win/win situation; if
none was needed in the first place, you have changed nothing, and if it was
needed, you have made an improvement. If adding one makes you sleep better
at night, knock yourself out! My guess is that some lay-outs require them
and a few don't. That makes both camps on this issue neither right nor
wrong; there is no blanket rule to cover all applications.
(((73)))
Phil Clements, K5PC
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