>
>> Tom,
>>
>> So what you are saying is that a 3-500Z will oscillate at 1 ghz just as
>> well as a 2c39?
>
>No, if I wanted to say that I would have said that.
>
>What I said was the fact the tube has useable gain at a high
>frequency has no direct relationship to instability. As a matter of
>fact the most stable tubes (in HF amplifiers) are those useable to
>UHF frequencies.
>
>That's because the frequency where the tube tends to oscillate
>would be so far removed from the operating frequency, making
>suppression (if required at all) very very easy.
>
>> Seems that whatever tube has a tendency to oscillate at VHF would also
>> stand to sustain a parasitic oscillation at VHF when used in the HF
>> range... Or do the laws of physics and common sense not prevail in RF?
>
>The laws of physics always apply, and precisely why all of this
>parasitic paranoia is mostly nonsense. Common sense sometimes
>does not seem to prevail.
>
>> On a side note, I noticed in quite a few of the "10 meter" amplifiers of
>> yesteryear, (with multiple sweep tubes) manufacturers placed ferrite beads
>> at the anode clip. This tamed many a amp from eating tubes (with the
>> subsequent "big bang")... Seems that the ferrite would also act as a
>> reatance at VHF, too.
>
>Sweep tubes "bang" because the anodes warp and hit the screen,
>or because the anode or grids melt from excessive current and
>voltage. That's because the rated anode dissipation is 30 or 35
>watts, and the grids are much much less.
>
>Bangs are caused by internal tube arcs, not by parasitics.
Arcs that leave no internal marks. . . . Should a big-bang in a
vacuum be heard?
>If parasitics caused arcs, so would excitation at the operating
>frequency.
>
An amplifier is normally heavily loaded at the operating frequency. If
an amplifier has an intermittent VHF parasitic oscillation that is Near
one of the resonances in the Tune Capacitor, the amplifier is lightly
loaded and large potentials can exist across the Tune-C. . For example,
in a TL-922 (nom. 2900v supply) the parasite at 120MHz can intermittently
arc the 6000v Tune-C. The laugher is that the man who discovered Tune-C
resonances is Mr. Rauch. Without this discovery, there would be no
logical explanation for bandswitch VHF arcing. congrats, Tom. .
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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