JE LABS 300B SE AMP
VOLTAGES FOUND INSIDE TUBE AMPLFIERS CAN KILL
YOU! IF YOU DON'T UNDERSTAND PROPER ELECTRICAL SAFETY PRACTICE, DON'T BUILD
THIS AMP!
Realizing that I have exploited all the possibilities out of
my push-pull 2A3 amp, for my next
project, I decided to go single-ended. I have serviced and built quite
a few Kit Is for Steve while
he was busy building his WE 91 clone. Although sonically quite different
from each other, they
are both fine sounding amps. My instinct sensed that I can combine the
virtues I admired from
those two amps using my favorite octal based twin triode, the venerable
6SN7. Since I need at -
least 7-8 watts of clean power to enjoy my music, 300Bs were a natural
choice.
Off I went, borrowing back-issues of MJ from the Angela archives, searching
the internet and
talking to friends. Funds are a bit tight this year but sourcing transformers
and parts that are
beyond the realm of my "junk box" is not a problem; they are
available from Steve (many thanks
and also to the gentleman in Japan who bought my PP2A3, you actually helped
fund this project)
along with his liberal terms policy. Only to me folks, I am privileged!
My circuit choice narrowed down to the Reichert, Fi Primer 3008, and the
Audio Note Conquest
series. All these amps use the 6SN7 as an input/driver tube so no drastic
alterations were
necessary to try these circuits on my pair of chassis.
For two weeks, I listened in mono; one side Reichert, on the other, Fi
Primer/AN Conquest (thus
called because my choice of values for load resistors are somewhere between
the two). I
treasure my Acro 20/20 amp because it has that dark and austere sound quality
that I find
haunting. The Fi Primer/AN Conquest circuit had these qualities but added
a hint of glare or
sheen in the upper midrange which bothered me. Herb's amp was very smooth
and full bodied in
the mids but comparatively lacking the impact and dynamics of the latter.
At this point I was
itching to listen in stereo, so a decision had to be made...... DC coupling
won by a narrow
margin! This circuit was also a bit too sensitive for my system so I removed
the cathode bypass
cap on the first stage. The glare was gone and I got the sensitivity needed.
Nothing is unique about this design; I am merely a "triode chef' mixing
a "concoction of
ingredients" learned from trying the aforementioned circuits. The
first two triodes are direct-
coupled in cascade, then, capacitor coupled to the grid of a 3008. 1 prefer
the "taste" of 300Bs
running at or close to 80 ma. with approximately 425 volts on the plates.
This may elicit some
controversy in some circles...... it is sufficient to say that I tried
and listened both ways and voted
with my "earbuds", so no arguments please. Anyway the AN 3K OPTs
are rated at 140 ma. If you
are worried about burning up NOS WE300Bs, be a cheapskate like me and use
Chinese tubes.
Believe me, they do not sound 10 times worse considering the insane prices
being asked for
NOS WE, that is.... if you can find them. Chinese 300Bs are rugged and
take it with a smile while
playing the gig.
To the more seasoned and discerning palette, may I suggest, going all the
way with Black Gates
sprinkled with Rikens and/or Tantalums served through copper foil caps.......
Joseph Esmilla, April 1997
Derek Walton of Ireland has an excellent website devoted to this project with some very detailed and specific pointers and tips. We recommend you check it out. http://indigo.ie/~walton/300b.html
