> 1. This amp has had every known mod on the Rich
> Measures site done to it already. Don, KI6SZ, did
> the modifications. Thanks to Don and Rich for their
> help/contributions.
Since parasitics don't cause tube problems, as a general rule, and
especially since the stock 922 is stable, that change should make no
difference.
> 2. After the initial mods were done, I suffered another
> grid short on one of the tubes. These tubes are the
> 3-500ZG's from RF parts. After getting and re-installing
> another set of tubes Every thing has been FB for about
> four months.
There are an abnormal amount of grid -filaments occurring in tubes.
It's getting more and more difficult to find good tubes.
In tubes I have examined, the filament has moved over into the grid
about half-way up the filament. This happens even in tubes that never
have HV applied, and seems to be occurring in certain manufacturing
batches more than others.
> about 500 watts. Again, OK. But, when I upped the
> power to full, about 1200 watts or so, I heard a POP!
> Followed by the sound of my voice coming from the
> amp and accompanied by a bright white light.
> Not good... Upon examination, I found that the two 1-ohm
> resistors that are part of the new parasitic suppressor had burned.
> This suppressor is formed from nichrome wire and forms a
> series-parallel network.
>
> I replaced the resistors, and re-installed the suppressor.
> Apart from an initial sput or zap sound from the amp, all
> seems normal. After the initial zap sound, I did remove and
> inspect the suppressor, suspecting that I might have burned
> another resistor, but it looks normal. I also installed another
> 3-500ZG to check against the first. This was done to see if
> the initial tube had developed a fault. So far so good. I then
> re-installed tube number one and tuned the amp on 10,15 and
> 20 meters. Power out, plate and grid current, HV, input SWR,
> all appear OK.
> So now to the 64 dollar question: Was this just some kind of glitch,
> or was I visited by the dreaded VHF parasitic demon?
Parasitic damage is greatly exaggerated. You are seeing the same
simple problems with tubes that everyone else sees.
The first tubes had a grid-filament short and that is a very common
failure. The second was a momentary arc probably due to outgassing in
the tubes. This is more common with graphite anode tubes than metal
anode tubes, although many tubes have this problem. It, like the grid-
cathode problem, is NOT oscillation related.
All you did was finally reach the peak breakdown voltage of the tube,
and it flashed over.
When the tube arcs, it often clears the fault. If the fault is a
small amount of gas the gas will sometimes getter from the arc and be
rendered harmless. If the fault is from a metallic whisker or debris
in the anode/grid space the arc can clear the fault.
The fault might return or might not. You just have to wait and see.
One thing that helps is to always make sure you have the amp loaded
heavy enough to not create excessive and needless peak anode voltage.
Also be sure the antenna system has no problems like loose
connections or lightning arrestors or things that cause momentary SWR
problems.
Another helpful check is to be sure the antenna relay in the amp
closes the output contact before the input contact physically closes.
These are the common problems I have seen many times. If you are
worried about stability, put the original suppressors back in (but
leave the glitch resistors in, or better yet add the correct type of
HV glitch resistor). They have lower Q at VHF.73, Tom W8JI
W8JI@contesting.com
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