SET Asylum

Single Ended Triodes (SETs), the ultimate tube lovers dream.

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RE: Tube life over bias rule of thumb?

Posted by Paul Joppa on November 10, 2016 at 13:05:39:
I have a 1939 data sheet for the 300B, which lists a wide variety of operating conditions, with all of those at 450v plate voltage being "Maximum Operating Conditions" and notes that such operation will give a shorter lifetime; the conditions at 400v or less are called "Recommended Operating Conditions." I also have a 1950 data sheet which lists the maximum plate voltage as 400v; it also says 36 watts max dissipation (was 40 watts in the 1939 sheet.) No mention in 1950 of lifetime effects.

Because that is only a voltage difference (not current), it seems that ion bombardment (stripping) of the cathode is the primary life-shortening cause. This is in contrast to popular opinion which says the bulb temperature is the main effect.

Filament voltage errors are another limit - too much voltage raises the filament temperature which has a strong effect on filament lifetime - much like incandescent light bulbs. Reducing the filament voltage below the spec will extend its mechanical lifetime, but it will reduce emission and thus the protective effect of the electron cloud. You can do it (I have in fact done it) safely if you reduce the plate current, but it must be a substantial reduction to match the emission reduction.