[ARC5] Power Tube Life Question

[J. Forster]

Hi Ken,

Thanks VERY much. I'll look for the Eimac book. I have it in HC, but not
to hand.

It sounds like a Sola CV transformer might be a good idea in the filament
line.

Best,
-John

==================


> On 16 Mar 2010 at 17:43, J. Forster wrote:
>
>> The amp uses 4 type 838 tubes, push-pull and parallel connected, with
>> directly heated cathodes (flaments) presumably Thoriated Tungsten,
>> although the RCA data sheet does not say specifically.
>
> Actually, both of the RCA data sheets I have on this tube,
> and the Amperex version, all state quite clearly that the
> filaments are indeed  thoriated-tungsten.
>
>> Since the tubes are somewhat precious, I'd like to run them at reduced
>> filament voltage (maybe 5-10% less) to extend their life. However, I'm
>> concerned about loss of Gm and/or emission from prolonged operation
>> with the reduced voltage.
>
> According to what I have seen, Gm and/or emission are not
> effected at all. Furthermore, it appears that when the tube
> is severely "starved" of filament voltage, the distortion in an
> audio amp drops drastically. In one case, from about 10%
> to 0.03%.
>
> Furthermore, according to Eimac, each 3% decrease in
> filament voltage from the "normal" voltage in a thoriated-
> tungsten filamented tube results in a doubling of tube life.
> Conversely, each increase of 3% results in cutting the tube
> life by 50%, or 1/2.
>
> I.e., if you reduce the filament voltage by 3% you double
> the tube life: if you reduce it an additional 3%, you double it
> again, etc.
>
> You may wish to get and read Eimac's latest version of
> their "Care and Feeding of Power Grid Tubes". It is
> available on the web in a 6-part PDF and it explains this
> and much more of interest.
>
> Just do a search of "Eimac Care and Feeding of Power
> Grid Tubes" and it will show up in multiple places.
>
> Eimac suggests that in the interests of conserving TT tube
> life, the operator should reduce the filament voltage to a
> point at which the plate current just begins to drop. Then
> the tube(s) should be run in that condition for at least 24
> hours to determine if the tube(s) are stable at that voltage.
> If not, raise it a tad, and repeat the test until you arrive at a
> filament voltage that maintains needed emission
> (determined by the constant plate current).
>
> Now keep in mind that these methods ONLY work for NEW
> tubes, or ones which are known to be good. Weak tubes
> will show plate current reduction under practically any level
> of reduction of filament voltage.
>
> Also, it appears that you have to DRASTICALLY reduce the
> filament voltage in order to make them suffer from "cathode
> poisoning". In your case, I think that would never, ever, be
> a problem and I would not worry about it in the least.
>
> Lastly, I think you should know that there are at least three
> methods of "reactivating" flat thoriated-tungsten filamented
> tubes. I have personally used all three methods and can
> attest to their working extremely well in certain cases, and
> not at all in others, the results depending on several
> factors, the most important of which is the ruggedness of
> the filament.
>
> I am writing an article for Electric Radio Magazine on this
> subject and it will be published some time this year.
>
> Ken Gordon W7EKB
>
>