>
>I'm sure using a sledge-hammer to drive paneling nails works, but
>honestly, it's better to do the job right to begin with.
>
>If a parasitic can be determined and suppressed, without the need for
>brute force automatic shutdown of a circuit, then that is
>IMHO, the correct choice (at least until someone shows me that tree 8877's
>grow on).
>
// Chortle. As I understand the phenomenon, stopping a parasite
with a grid-I auto-shutdown is probably not as easy as it first looks.
An 8877's cathode is capable of producing over 50A of peak electron
emission. During a parasitic osc., the peak anode potential drops to a
low value due to an unloaded condition produced by the lo-pass HF tank.
Whenever the anode potential is not substantially more positive {more
attractive} than the grid, most of the cathode current will go to the
grid. Thus, the transistor that that is used to stop the flow of grid-I
during a parasite should have a substantial current rating. However, the
transistor the designer typically selects has a 1 - 2 ampere rating.
Thus, when the wolf comes to the door with 40 or so amperes, the
transistor shorts and does not interrupt the flow of grid current in
time. Autopsy of kaput 8877s shows that the result can be
gold-sputtering.
/Curious? - see Fig 24 on my ?Web site/
It is my opinion that the safest way to deal with the VHF parasite
problem is to reduce VHF gain by reducing the Q of the Unavoidable
VHF-resonant anode-circuit -- plus install glitch protection.
cheers
Postscript -- I would like to buy another kaput 8877 from an
out-of-warranty AL-1500 so that I can make a close-up photograph of the
gold meltballs with my New Macro Lens!! - come on down!! Please name
your price.
>alex wrote:
>
>> Hmmmm...
>>
>> That does not seem to answer my friends question ...
>>
>> However, the parasitic oscillations should be taken care by the grid current
>> measuring system that will shut down the amp if excessive grid current will
>> flow ( I did some homework, as you see) ... This should protect the tube
>> and tell you that something is wrong with the unit. Operating it further
>> under this conditions are not advisable.
>>
>> As I said, my friend needed some other info than looking at your site :)
>>
>> Alex, N2NNU
>
>jim
cheers
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
end
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