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Test & Technology Valve power amplifier
virtually all inherent problems immune to soundinfluencing
Notes on the 300 B in single-ended mode of the singleen ded technology hum and HF interferences via
at one go! The effort is accor the heating.
The uncrowned queen of au- dingly massive, even monst Finally in the “basement“ of
dio triodes, the Western Elec- rous, and we’re now going to the two mono biggies we have
tric 300 B, is a directly heat- roll up this exceptional ampli the power supply: two mighty,
ed power triode from the fier from behind: other than the low stray field transformers with
1930s with an extremely standard Octave output trans almost one kilowatt of overall
linear characteristic curve formers, this newly developed output take care of the anode
field. It was designed ex- model, which sits quite on top high voltage, all auxiliary volt
plicitly for audio ampli- in the housing right below the ages and also the supply of the
fication, and with its
direct 5-volt AC valve “floor“, has now four heating generator. Of course,
heating (the fila- and eightohm speaker taps. the current supply of the power
ment doubles as The core laminates have no amp including the anode volt
cathode), it con- air gap; instead an extra, ages is electronically stabilised
forms to the valve quasi inversely applied win all the way and even shortcir
technology of that ding compensates the core cuit proof. Instabilities due a
time, of course. magnetisation by the anode fluctuating mains voltage are
Despite a balanced current of the three power hence no longer an issue. An
heating via two resis- valves. This winding is fed by dreas Hofmann’s dry remark on
tors, the 50-hertz hum an elaborate current source the this: “Voltage fluctuations are
issue associated with performance of which must be current fluctuations“, a stabilisa
AC heating is inaccept- equivalent to the added currents tion is “mandatory“ with triodes.
able in a modern audio
amplifier. Therefore a DC of the three valves; since this
heating is mostly used compensation winding lies on No compromise
today which, beside other the transformer core, it must The audio circuitry, of course,
problems, leads to an un- not induce any interference vol follows a similarly uncompro
even cathode wear due to tages and therefore be absolu mising road as well. Since (also
the potential gradient over the tely “clean“. with regard to different 300 B
filament length. Smartly de- types) three quiescent current
signed amplifiers lessen this Seven hertz – with power! levels can be adjusted for the
effect by reversing the heat- The heating of the 300 Bs is 300 Bs, the power stage uses a
ing voltage polarity during effected by means of an AC vol combination of active grid bias
power-up, but will not really tage, more precisely a distorti and a socalled automatic grid
solve the problem, either. on factor optimised, pure sine bias via cathode resistor. Need
In the (soundwise most pop-
ular) single-ended mode of wave with a frequency of seven less to say, the active grid bias
the triode, the preload (mag- the power demand of modern hertz. Since the heating of the must be stabilised and extreme
netisation) of the output loudspeakers – is the man- 300 B also acts as its cathode, ly lownoise because it lies right
transformer by the anode DC ageable output of a realistic it must be galvanically isolated. beside the control grids. By the
current requires transformers six to seven watts by a single This is why an analogue gene way, the 300 Bs are not simply
with an air gap and a gener- 300 B (performance specs of rator, actually an amplitude wired in parallel, but have their
ously sized core, whereby the ten or twelve watts are “opti- stabilised 7hertz power stage, own external wiring which is
air gap will significantly down- mistic“). The output could be generates this AC voltage which referred to as “in direct parallel
grade the qualities of the boosted by switching several is fed into a heating transformer operation“. The designer con
transfomer (shunt induct- power valves in parallel (sin- on the primary side. Three se tinues to comment: “Our goal
ance). gle-ended parallel mode), but condary windings supply the was not power for power’s sake,
Empirically concepts with that way the magnetisation 300 Bs in a longtime stable but to give the 300 B a perfect
(phase-shifting) capacitors problem of the output trans-
between a power valve and a former would sort of be mul- manner with exactly five volts environment to show the sound
transformer, this time without tiplied by the now summed at 1.2 amperes each. The men qualities of the valve to their
air gap (single-ended para- up anode currents of two or tioned transformer sits in the best advantage“.
feed), often sound better than even three power valves, middle compartment of the Ju
conventionally designed SE which is why such amplifiers bilee 300 B and contributes si Heavenly
amps. Another factor – given are very rarely built. gnificantly to the overall weight. Only very rarely we are itching
This technology is absolutely to heap praise on an amplifier
14 09/19 stereoplay.de
