[Amps] Parasitics & Filament Sag

[R L Measures]

On Aug 26, 2006, at 12:00 PM, Harold Mandel wrote:

> Gentlemen,
>
> A previous post mentions a phenomenon wherein is written
> "parasitic oscillation" and what I deduce as filament sag.
>
> Is it possible that these two conditions are related?
>
> It is understood how a strong magnetic field within the tube envelope
> can perhaps alter the filament geometry, but can this happen as a  
> result
> of a VHF "excursion?"

Hal -- Parasitic suppressor resistors are normally paralleled by 60  
to 100nH of L.  Thus, to develop enough V across a suppressor to  
damage the resistor, VHF is required.  After observing a number of  
cases in SB-220s and TL-922s where a large increase in VHF suppressor  
resistance accompanied a blown grid RFC, I concluded that VHF energy  
was the common denominator.
    RE"  Sag:  In cases where a grid-filament short occurs in a  
3-500Z during an intermittent parasitic oscillation, the filament  
does not sag, rather it is pushed sideways toward the grid--  
presumably by EMF from the large flow of grid current during the  
parasitic.  The amount of this current is anybody's guess, but it's  
enough to do damage
>
> Respectfully,
>
>   Hal Mandel
> W4HBM
>
>
> [SNIP]
>> Hundreds of empirical observations from all makes of amplifiers  
>> must be
>> wrong. Clearly, the problems must be the mysterious parasitic
>> oscillation, which obviously is pushing the filament sideways.
> [SNIP]
>
>
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>

R L MEASURES, AG6K. 805-386-3734
r at somis.org