SET Asylum

Posted by Thomas Martens (A) on January 04, 2008 at 18:08:59

In Reply to: RE: Seeking opinions on this migration path from push-pull to SET posted by Paul Joppa on January 04, 2008 at 17:40:47

a proper way. I build MANY different Push-pull EL84 amp circuits on
breadboard both ultralinear and triode. I also experimented with
various biasing schemes and invertor types and variations. The one
which ended being head and shoulders above the rest was:
Triode mode with separate individually bypassed cath bias resistors
for each tube, class-A running around 25 mA per tube, fed with a
classic 12AX7 long-tailed pair ( a la Leak stereo-20) and a basic
1/2 12AU7 common cathode input stage. 12AX7 invertor could be fed
from the source directly no prob, but harmonic complementarity of the
12AU7 stage transformed the sonics for the better --in spite of
the ridiculously high input sensitivity that resulted. The input
stage is carefully isolated from rest of circuit with an 0A2-0B2
VR tube stack (250V). The long tailed invertor has good PSRR by
itself, and so extreme PS filtering efforts are not needed for it.
Oh yeah, and it makes a very clean (conservative) four watts with
no loop feedback used.

Result: sounds better than most SETs I have heard. Far better than
all around than any higher powered ultralinear connection I tried.
Only gives up a bit in terms of ultimate bass extension, organic
midrange and absolute dynamics/SPL as compared with my bigger
(ridiculously large and heavy)8W no-holds-barred SET. It's a sleeper.

Unfortunately, there are lots of SETs out there which do fall down on
dynamics and bass performance. It's too bad, because the problem is
so easy to fix with SET amps --which I still consider the ultimate
in sonic quality if done right.

-T.M.

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