Dyna A420 Project continued

[Jon Yaeger]

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 . . . . ;-)

 

[John Woodgate]

I read in sci.electronics.design that Jon Yaeger <jono_1@bellsouth.net>
wrote (in <BE608F5A.1DE71%jono_1@bellsouth.net&gt about 'Dyna A420
Project continued', on Fri, 18 Mar 2005:

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?

Not optimally. 6SN7 has more gm but less mu.

The original concept of the 5-20 (and 5-10) phase splitter was written
up in 'Wireless World' August 1947 and I'll put it on a.b.s.e when I am
a little less under the affluence of incohol.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk