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RE: Which tube integrated to keep forever? Pilot or Sherwood?

"With the standard 5/20 style EF86 driving a EC83/ 12AX7 cathode-coupled phase splitter, do you think the is enough juice to drive a pair of 6550/ KT-88's in PP, class AB ultra-linear mode?"

The Mullard topology is very good. Mullard style circuitry is less dependent on O/P "iron" quality than Williamson style circuitry. OTOH, the small signal complement in 5-20 leaves (IMO) MUCH to be desired. Never forget Mullard published their designs in order to sell the tubes they manufactured. Both the EF86 and 12AX7 are low gm types. High gm is your "friend" in designs with long loop NFB, as it provides resistance against HF error correction signal induced slew limiting. IMO, the best vintage example of Mullard style topology is the Harman/Kardon Citation V. The Cit. 5 contains a 12BY7 as the voltage amplifier and a 6CG7 as the LTP splitter. Compare the gm of those 2 to the pair in the 5-20.

If you want PP UL mode KT88 "finals", Mullard style circuitry generally following the Cit, 5 example is fine. A 6922 in cascode would provide a high gm voltage amplifier in a current production tube. A current production ECC99 twin triode, with a mu of 22 and HIGH gm, is my choice for the LTP phase splitter. As cascodes exhibit poor PSRR, regulate 6922 B+ to put 180 V. on the upper triode's plate. Force symmetry in the differential gain block by using a 10M45S CCS in the tail, instead of a resistor. A modest negative rail under the CCS rates to be beneficial.


Eli D.


Edits: 12/03/14
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