Quote:
Originally Posted by kalee20
I'm a bit surprised by this. The EF91 is smaller so would be expected to have smaller capacitances. But I wonder if the comparisons are 'real, for instance, was the EF91 Cout measured with or without a screening can? The EF80 is stuck with its internal screen, whereas the EF91 does have some flexibility in that respect, and if push came to shove a large-diameter screening enclosure could be used, to increase distance from anode and reduce Cout. (Of course, if you need a 2" screen around your valve, some of the benefits of miniaturisation disappear anyway!).
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As Mullard didn’t state the conditions under which the measurements were made, we can but guess. Also, the statement about the EF91 being the best along this vector was made before the EF80 was released.
The Philips data sheet for the EF91 quoted the capacitances
with a screening can as Cg1 7.3 pF, Ca 3.4 pF and Cag1 <0.01 pF. That does suggest that the Mullard numbers, Cin 7.0 pF, Cout 2.0 pF and Cag1 < 0.008 pF were measured
without the screening can; the Mullard datasheet is mute in this regard.
A perspective on the EF80 vs. EF91 performance was provided by the quote from Jason in post #39:
Quote:
Originally Posted by Synchrodyne
An interesting commentary on the EF80 as compared with the EF91 for the FM case was provided by Jason in respect of its Jasonkit FMT2 of 1958. The FMT2 was essentially a self-powered version of the FMT1 in the same case as used for the then-new FMT3. The FMT1 was in 1958 the new name for the original Jason/Jasonkit local-area FM tuner circuit that used four Z77 (EF91) valves, by then available only in Jasonkit form. The FMT2 had essentially the same circuit but used four EF80 valves. (The Jasonkit FMT3 also of 1958 used five EF80 and one ECC81; it was different to the Jason (built version) FMT3.)
[I]“The valves have been changed to EF80, and this results in an improved sensitivity. In the RF and FC stages the fact that there are two cathode leads reduces the feedback which is experienced at this point and gives a slight increase in gain. In the IF stage advantage has been taken of the slightly lower anode-to-grid capacity which is found when the valveholder capacities are also included. Therefore less damping may be used across the coils and more gain results.”
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This I think better aligns with your expectations.
Cheers,