>
>On Tue, 13 May 97 11:56:36 -0700 Rich Measures <measures@vc.net> writes:
>
>>>Rich, your test seems to infer that VHF oscillation is the only thing
>>>that could cause a substantial increase in Rs. Is that the case? If
>>>so , is there a discussion somewhere that could be read?
>>>
>>>cheers, Paul
>>>ve7cqk
>
>>external heating and a large change in resistance. This happens fast.
>> A
>>'big-bang' is often heard. I have seen 100 ohm 5% carbon-comp. Rs
>>resistors that measured over 400 ohms--yet appeared to be in good
>>condition. OTOH, Rs can be burned out completely by VHF energy.
>> Since Rs can be completely burned out by either VHF or HF energy, it is
>>technoblatherous to state that only VHF damage can cause a substantial
>>increase in R. . However, a substantial increase in R with no
>>external signs of damage is probably VHF-related.
>>Rich---
>
>
>You and Tom are still failing to follow this thru Rich. The same
>resistance increase, but due to skin effect and chemical changes, is a
>primary cause of blown filaments in 811A/572B's .
Chemical change does not appear to explain two 811As or 572Bs failing at
the same instant with broken filaments, accompanied by the infamous
'big-bang'.
>The VHF parasitic is a primary player in this instance...I will have to
>agree with your premise with the above tubes.
>
>As I have said to Tom, I will say to you and others. The voltage in a VHF
>parasitic is very much dependent upon the unloaded Q of the tank circuit
>since the load IS NOT absorbing the energy. Even in a basic 811A circuit
>this can exceed the tubes breakdown voltage by many times....20+ KV is
>common. Just run the math.
The math is one thing, however, the 811A or 572B can not see a higher
voltage than the breakdown voltage of C1 (tune) plus the anode supply
voltage. In a 30L-1, this total is probably around 4000V.
>Any BS to the contrary and the refusal to admit to the destructive energy
>in a parasitic is irresponsible IMO.
However, BS appears to be a common defence of "expert" status. IMO, the
vanishing gas theory is a primary example of such feculence: The
operator hears a big bang. The filaments simultaneously break in two or
more 811As. Subsequent testing of the kaput tubes indicates that a good
vacuum is present. The "expert" explanation: Gas. Give us a break, Mr.
Bandini. Gas that mysteriously vanished AFTER the glitch? And for those
who didn't buy the vanishing gas theory, the "barnacle" story was
proffered. Talk about your 'sea stories'.
In an SB-220 (2, 3-500Zs), a 'big-bang' event can burn open one or both
1A pi-wound grid to ground chokes. I have seen the pi sections in these
chokes collapsed together by magnetic force that apparently existed
during the big-bang. Without a large current flow, this would not have
been possible. If 1A pi-wound chokes can be collapsed by magnetic force,
keeping in mind that 100% of the grid current originates in the
filament/cathode, why should it not be possible that the filament would
be affected by the same current? IMO, a hot tungsten filament can be
bent in a 3-500Z during a big-bang. [A photo of a bent 3-500Z filament
can be found on page 15 in the 9/90 issue of QST.]
> Granted, parasitics can take many
>forms and many are not destructive; ...
agreed
>...it IS ALL dependendent upon how much
>of a real load it sees. We see them as anywhere from an occassional
>spitting to a catastrophic BANG.
Verily. At 100MHz, an HF tank pretty much isolates the load from the
generator.
>73....Carl KM1H
Rich---
R. L. Measures, 805-386-3734, AG6K
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