>> 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.
>
? Can parasitics cause tube problems? In "Parasitics Revisited",
*QST*, September/October, 1990, there is a photograph of a fil./grid
shorted 3-500Z that apparently shorted when the amplifier sustained a
"big-bang" anomaly. There's also a photograph of a toasted TL-922
bandswitch that burned up from what I believe was intermittent bursts of
energy at c. 120MHz.
- When sending dits, even when a 922 is not oscillating, 120MHz
damped-wave ringing can be detected near the anodes with a spectrum
analyzer. The reason is that there is a 120MHz resonant circuit at the
anodes. This resonance is formed by the 9.5pF of anode-C, the Tune-C,
and c. 160nH between them. If 3-500Zs have internal feedback-C and they
can amplify at 120MHz, is is possible for them to occasionally oscillate?
>> 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.
>
Perhaps the problem is that amplifiers which don't use low enough vhf-Q
parasitic suppressors allow enough vhf-gain to sustain vhf oscillation,
which in turn puts stress on the cathode, thereby bending the hot
filament toward the grid? However, the drawback is that using low vhf-Q
suppressors reduces power output by c. 2% at 29MHz.
>In tubes I have examined, the filament has moved over into the grid
>about half-way up the filament.
Agreed. This can be seen in the autopsy photo of a fil/grid shorted
3-500Z in "Parasitics Revisited" in the Sept., 1990 *QST*.
>This happens even in tubes that never
>have HV applied, and seems to be occurring in certain manufacturing
>batches more than others.
>
I have not seen this in a new tube. A typical, new 3-500Z or 3-400Z has
a filament/grid withstanding potential of c. 8kV. After a few big-bangs,
the withstanding ability can be under 1kV. When such a tube is hot, it
shorts.
- The amount of force need to produce a short in a 3-500Z appears to be
considerable. In a centrifuge, it takes 11G for 40-seconds to change the
fil/grid withstanding voltage c. 5kV -- and that's with 5.7v on the 5v
filament to make it more bendable.
>> 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.
I have never had a tube fil/grid short in my TL-922 or SB-220.
>
>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.
>
the lifelong trip on the wide river in Egypt continues.
>All you did was finally reach the peak breakdown voltage of the tube,
>and it flashed over.
>
A good 3-500Z does not flash over with 30kV between the anode and grid.
>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.
>
Good points. Tuner arcs tend to create tank arcs.
>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,
In N7WS' tests of copper-wire vs. resistance-wire vhf suppressors [on my
Web site], this was not the case. Does it make sense that a better
conductor produces a lower Q ?
>Tom W8JI
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
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