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The wonderfully simple
300B directly heated triode is virtually
Lee De Forest's very first evolutionary 3-element piece,
comprising only filament, input grid and anode output.
Brought to reliable perfection by Western Electric USA and
later by Standard Telephones UK as the 4300B, the 300B was
very much intended and widely used for telephony
amplification. But audio enthusiasts of older times knew a
good thing when they saw (heard!) it and the great love
affair between the 300B and music began... and dwindled many
years later as the American Western Electric 300B was taken
out of production in the late 1980's making the genuine ones
extremely valuable and, accordingly, very highly prized as
collectors' pieces. With the Asian-driven passion and
resultant revival of SINGLE ENDED topology the call for the
300B to be put back into production was too overwhelming for
the Chinese factories to resist. The Russian and the
re-start of USA manufacture of the genuine Western Electric
original have further expanded the selection. Basic
thermionic simplicity... directly-heated triodes... it is a
drug for some, a religion for others...
With its decidedly (and studiedly) 'retro' physical
proportions- very deep front-to-back dimensions with a
narrow side-to-side width, this layout closely follows
concepts of a bygone era, where the well-conceived intention
was to keep the power supply as far as possible from the hum
and noise susceptible octal input tubes. The output tubes
themselves require careful distancing from the mains
transformer and, of course, require pure DC on their
filaments. Again, reverence to past times begs the use of
vacuum tube rectification with its very gentle voltage
rise-time. But to bring the performance up to modern
demands, we use two rectifiers to enable the use of very
substantial filter / reservoir capacitors... over 1300
microfarads in the B+ rail- much improved from the 4 or 8
microfarads utilized in the 1930's.
When several other manufacturers started selling
single-ended amps with 5 to 9 watts of power, we understood
subjectively and objectively the good part of the story: the
superb mid range, the delicate detail, the inherent
simplicity. The bad news was that there were almost no
speakers efficient enough to help form a dynamically viable
system. The worst news was that either these single-ended
amps cost more than the condo and or they had specs that
could only be read as a joke. As the triode fans increased,
more efficient speakers became available and several
fanatics began to dismiss push-pull as a bad thing to be
avoided at all costs. We objected to such blanket
generalities and stressed that the push-pull done right with
truly phase-balanced transformers, quality parts throughout
and well designed circuitry has proven to be a superb
technique. We also enjoy single-ended topology but only when
it is done right, mostly because we expect flat frequency
response and low distortion out of an amplifier. We knew
that we could achieve this in a properly conceived
single-ended design. We chose to rise to the challenge of
designing special circuitry that could be switched "on the
fly" From 12 watts in parallel SINGLE-ENDED to 25 watts in
PUSH-PULL which, we believe is unique in the industry.
The Output transformer (hand-wound in our own factory) is
of complex design employing inordinately high primary
inductance to compensate for the mandatory magnetic "gap"
required for the uni-direction current and voltage flow of
the single-ended output stage. Another factor in the single
ended triode saga (currently enjoying a wide spread
popularity) is demand for little (or sometimes zero)
negative feedback. We at Manley Labs believe in the
judicious or sensible amount of feedback to refine and
maximize performance. Our Neo-Classic SE/PP300B gives you the
choice in precision switched 1 dB steps to go from ZERO dB's
to 10 dB's of negative feedback. Even with zero negative
feedback, this amp is cleaner and flatter than any
single-ended amp we know of. For more information about
variable feedback read
"Notes on Negative Feedback".
One interesting aspect of negative feedback is it gives a
higher damping factor. Conventional wisdom would have us
think that the higher the damping factor, the better. Our
experience is that the optimum damping factor is not
infinity and that it depends on the speaker, the room. and
the taste of the listener. In other words, the optimum is
variability, and this we provide. This amp satisfies us as
long time amplifier builders, amazes the critics in several
well known magazines and is bound to satisfy you with
the ability to fit into your system and your expectations.
BUT REMEMBER PLEASE the overall facts of power ratings
versus loudspeaker sensitivity and room size when electing
to venture into lower power amplifiers (less than 50 watts)
and especially when contemplating single-ended topology.
Please read "Considerations of Amplifier Power"
for more info.
As highly regarded as the Retros were by
dealers ,audiophiles and reviewers, the
Neos take the design and the sonics to a new level. In 2000, Mitch Margolis completely overhauled the previous
"Retro" model to create the Neo-Classic Version you see
above.. So what's new with this Neo-Classic version you ask?
Now... onto the techno mumble jumble... here's
all the guts improvements we
made in 2001. Mitch spent six months doing all this:
The driver stage was completely re-engineered. Peak output volts of driver
increased
by 60V p-p in PP mode. Clever new driver circuit delivers way better
drive,
with less punishing plate volts, and less THD. This offers enough drive
even for more exotic tubes such as VV30B, etc...
The NEO's have WAY improved power bandwidth. NEO's deliver
much more power at higher and lower freq's than the ol' Retro's did. We did rescale the B+ and reduced idle plate dissipation
on the NEO's 300Bs for better tube reliability.
New power max ratings are 12 watts in SE mode and 25 watts in PP mode.
BUT because of better bandwidth delivery and better driver "drive" into 300B's these NEO's
sound louder and "seem" to be more powerful. They distort much less early when
pushing the power envelope. The power supply sags-on-demand at a much
high point than before for more headroom. It is all beefier 'n' heftier.
The heater (300B "cathode" which is directly heated triode
design) now has
40dB less ripple. NEO design has MORE capacity in heater supply and LOTS of PI filtering! So
the result is a quieter amplifier. Big time.
Now in the NEO version, the B+ rectification is all-real-tube AND
Full Wave Bridege vacuum tube
rectification. Even more better. Especially sonically for you vacuum tube rectifier
fans...
The NEO'S chassis was redesigned to accommodate a much bigger power tranny. Huger VA. Next frame size up. Cooler running.
Much more able to deliver-on-demand. Less PSU sag. Kick-ass. And now,
finally QUIET! REALLY! There wasn't enough room on the old
Retros for next frame size up to achieve a nice'n'quiet power
transformer design. Now the NEO's chassis is big enough to have a big
enough power transformer that is really silent and running on holiday.
Bias supply is now cleverly cross-coupled so that changing one tube's
bias
doesn't whack the other one out. This is very cool.
We even isolated the input B+ for more stable input B+ supply. Again,
so
when pushed, the whole amp doesn't drag everybody down with it.
We added a MUTE switch on the front panel. SE-PP switch on the other
side of
front panel.
Power switch at the IEC so all those nasty AC volts don't come up and
infect
the input stages.
Discarded the transformer-balanced XLR input. Because WHAT'S THE
POINT???
At gunpoint, we will make a balanced XLR transformer coupled input
available "upon special request"... but you need to have a
bigger gun than EveAnna does...
Still has those nice sounding MultiCap® coupling caps and even more of 'em....
Now NEO has nice beautiful WBT binding posts that everybody loves.
Still has
impedance load switch as before.
Service is now a piece of cake in the NEO design. The main PCB can now
be
lifted out for service without a soldering iron... if switches get
banged or
broken they can be user replaced with a phillips screwdriver to remove
the
bottom stainless-steel (now) cover and then remove faceplate with little
allen key, then unscrew the switch with pliers or (better) correct nut
driver, then unplugged out old switch then new one plugged in with
switch
assembly from us. Easy as pie!
But, just in case, we did invent newer stronger foams and packaging so
hopefully no more breakies....
And bottom line, in all our listening tests, this NEO is very much
improved. Now with this six months of
re-engineering of the Neo-Classic SE/PP 300Bs, "THD BE-GONE!!!!"
The Electro-Harmonix Russian 300B tubes are being shipped as standard.
That's 300B-EH.
And finally, sorry to say, we had to raise the price to PAY for all
this R&D and nice improvements and so we can start making some
money on this product for a change... (insert trite capitalist
expression here). Sorry folks, that's the way it goes....
So guys, WAIT UNTIL YOU HEAR THEM AND SEE THEM....
then of course, as always, we welcome your comments!
There is reason for our madness....
Cheers, EveAnna
Manley
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