Quote:
Originally Posted by PJL
EF80's are indestructible and EF91's may not have even made it to the end of warranty. Hardly surprising Philips were not promoting it.
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Assuming that assertion to be statistically supported, one may wonder whether the valve makers put any effort into obtaining longer life. The EF91 was used quite extensively in industrial equipment, where durability would have been important. For example, the Marconi HR24 dual-diversity point-to-point SSB receiver valve complement included 25 EF91 out of 70 total. The EF91 was used in most oscillator positions, where one would think that retention of high gm was critical. (The 70 valves included two octals (both voltage stabilizers) and 4 novals (AFC motor drivers); the rest were all of the B7G type.)
Receivers in the HR24 class had built-in valve checking equipment, so that failing valves could be detected before they caused any interruptions. Thus the short life of the EF91 would have been quite visible, and one might have expected Marconi to either ask for better from its suppliers or change to a more durable valve type. Or maybe the EF91 was heavily discounted and users were encouraged to keep a large stock of spares?
Cheers,