As per a prior thread about the Mullard 5-20, I was looking for a suitable circuit to build and use a pair of Dynaco 6.6K PP OPTs I recently acquired.
I listed the weaknesses of the Mullard 5-20 amp that Morgan Jones cited in his book, "Valve Amplifiers." However, I made an error when I posted a Hafler modified version of the circuit; Jones was referring to the original (unmodified) version.
Nonetheless, I just posted a .pdf schematic on alt.binaries.schematics.electronic (entitled Dynaco A420 Project) that was designed by Hafler & Keroes in 1951. It is a justifiably famous circuit, for the use ultralinear and a nicely balanced topology.
Apparently, V1 is a differential phase splitter with gain? I don't understand the operation of V1b . . . Why would the grid be connected to the junction of R6 & R7? It looks to me like the output of either V1a or V1b would appear at the grid of V1b, and why doesn't it oscillate?
I like the concept of the mu follower, and amps that I have built using it sound better than ones that don't. I could see how one could be used with V1a, but I'm totally clueless about V1b. If a mu follower could be employed, might a 6SN7 be used for V1 instead of the higher mu 6SL7?
Maybe I should just leave well enough alone and implement the original circuit . . . .
Sorry to return once again to the subject of tubes, but I think this circuit is interesting and would love to know more about how it works.
Tia,
Jon
(a.k.a. "the dishonest garage trader, to which Andre shall add, the "ignorant" dishonest garage trader . . . . ;-)