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kalee20 · 2nd Oct 2019, 8:37 am

Two AF uses for both valves:

The Soundmirror tape recorder uses two EF91's, one as second amplifying valve (playback) or record output stage to drive the head; the other is used as a 28kHz bias oscillator on record only.

Philips have an amplifier circuit which, rather oddly to me, uses two EF80's as phase-splitter to drive the output valves! I can't recall the circuit in full detail (hopefully someone may); but given that they could have used one of their own double-triodes, why they would use two high-slope RF pentodes beats me.

EF91's and their equivalents - I can't comment on inherently shorter life. PJL's remark about barely making it to end of warranty period might be less a reflection on expected-life-in-service, and more a comment that infant mortality was higher, maybe quality control of the new-fangled miniature all-glass valves was not fully developed, and the later EF80 reaped the benefits? In which case, burn-in screening for industrial valves would have helped: those that 'died' within the first 500 hours never left the factory; those that passed were defect-free and were shipped, secure in the knowledge than many tens of thousands of hours of performance could be expected.

2nd Oct 2019, 8:37 am	#55
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,081	Re: EF91 to EF80 Two AF uses for both valves: The Soundmirror tape recorder uses two EF91's, one as second amplifying valve (playback) or record output stage to drive the head; the other is used as a 28kHz bias oscillator on record only. Philips have an amplifier circuit which, rather oddly to me, uses two EF80's as phase-splitter to drive the output valves! I can't recall the circuit in full detail (hopefully someone may); but given that they could have used one of their own double-triodes, why they would use two high-slope RF pentodes beats me. EF91's and their equivalents - I can't comment on inherently shorter life. PJL's remark about barely making it to end of warranty period might be less a reflection on expected-life-in-service, and more a comment that infant mortality was higher, maybe quality control of the new-fangled miniature all-glass valves was not fully developed, and the later EF80 reaped the benefits? In which case, burn-in screening for industrial valves would have helped: those that 'died' within the first 500 hours never left the factory; those that passed were defect-free and were shipped, secure in the knowledge than many tens of thousands of hours of performance could be expected.