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RE: Mac 1500 vs Fisher 500c.

Full pentode mode "finals" are easy enough to drive. It's the "fixed" bias grid to ground resistance limit of the 7591 that's the sticking point. Fisher used 330 Kohm parts that have drifted upwards, over time. The data sheet gives 300 Kohms as max. From the beginning, Fisher was "out on a limb". :>( Current production 7591s are intolerant of the liberties Fisher took.

Mikey is on the right track in thinking 12DW7/7247. AAMOF, Fisher used the 7247 in the X-100. Then, "bean counting" set in. Reduce inventorying costs by stocking "only" 12AX7s. Never mind the fact that the high RP 'X7 triode must work into a high net AC impedance.

I'll upload the relevant X-100 schematic fragment, later today.


Eli D.


Edits: 11/29/16
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