[Amps] Silly idea?

[rlm]

>
>> Here's the question: Would it be worthwhile to place a small resistor (say
>> something between 0.05 and 0.1 ohm) in series with the filament while in
>> standby?  Or is this a no-win scenario, because the small amount of
>thermal
>> cycling would stress the filament enough to send it to an early grave?
>>
> Hi Kim,
>
>In all my conversations with engineers at Eimac, they always assured me any
>reduction in filament voltage below the specified range would "be lost in
>the noise" of other failures.
>
>My own experience over the past 20 years with production tubes where data
>was tracked seems to bear that out. The vast majority of failures I've seen
>have been broken welds on anodes and gas problems. Only recently have
>filament to grid shorts appeared, and only then with certain batches of
>Amperex tubes. 

**  Early Heath SB-220s had numerous fil-grid shorts in Eimac 3-500Zs.

>There isn't anything that has shown those failures to be
>avoidable by anything like reduced voltage.
>
>I've seen a few low-emission tubes, but most of them have been imports that
>failed very early.
>
>It is always a good idea to run the rated filament voltage on the tube no
>matter what service. I can't imagine you would see any change in tube life
>by reducing standby voltage, unless you have some really high operating
>hours! In high operating hours commercial service it is often a good idea to
>reduce the voltage of thoriated tungsten tubes to extend emission life. In
>amateur service it is probably not useful.
>
>MOX cathodes are another thing. NEVER reduce the voltage in a MOX cathode
>below minimum voltage ratings, unless you are sure anything near the peak
>emission will never be reached.
>
Sound advice.

-  R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K, 
www.vcnet.com/measures.  
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