THÖRESS 300B Instruction Manual

THÖRESS
300B Mono Power Amplifiers
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INSTRUCTION MANUAL
Thank you for purchasing the THÖRESS 300B Mono Power Amplifiers. These single
ended triode amplifiers, hand-made in Germany, have been tailored around the highly
regarded directly-heated 300B power triode for an output of up to 10 watts into a 4, 8 or
16-ohm load. The zero-feedback circuit incorporates an extraordinary single-stage driver
topology which employs two EL803S power pentodes operating in triode mode. This
driver stage is so powerful that it could (theoretically) drive a loudspeaker load with
ultra-low distortion on its own. We believe that these state-of-the-art amplifiers offer the
highest possible sonic performance in the class of 10-watt amplifiers, when combined
with a pair of highly efficient loudspeakers such as our FR20 or 2CD12 speaker models.
Please read the following explanations and instructions carefully to get the most out your
amplifiers.

A NOTE ON INPUT SENSITIVITY

Please note that our Mono Power Amplifiers are specifically designed to have a
comparatively low input sensitivity. Such the idle noise of the preamplifier can be more
easily kept within inaudible limits when combined with high-efficiency to very-high-
efficiency loudspeakers such as our 2CD12 speaker model or even large horn systems.
This design choice has the further benefit of avoiding the problems of channel imbalance
and sub-optimal frequency response which may result when the preamplifier's volume
controls rests at too low an angle of rotation. Should speakers with somewhat lower
efficiency be used it may, however, be observed that the volume-knob(s) of the
preamplifier need to be turned to a somewhat larger than usual an angle of rotation for
convenient loudness.
Please note that low input sensitivity of the power amplifier is a desirable feature
when driving highly-efficient speakers. It should by no means be suspected, that the
amplifiers do not have enough power to drive the speakers accurately when the
volume-knob(s) of the preamplifier needs to be turned to a somewhat larger than
usual an angle of rotation for convenient loudness.

DIRECTLY HEATED TUBES AND AMPLIFIER PLACEMENT

It is well known that directly heated triodes (DHT's) such as the 300B (or 845) power
tube are rather sensitive devices. By their nature, these tubes are namely heated with thin