[AMPS] Filament breakage

[Rich Measures]

>
>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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