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-high­efficiency 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

wires mounted with spring suspensions to mica supports pressed into the glass envelope.
The fragile heater wires can break very easily under shock or over-tension, especially
when they are glowing or when they are still hot.

DHT-s are fragile devices and must be handled with exceptional care!

Never pull out a DHT out of its sockets as long as it is still hot!

Never move or even transport a DHT as long as it is still hot!

Moreover, the heater wires are able to swing very easily. As a result, severe vibrations of
the glass envelope will be converted into audio signal and passed on to the speakers. This
effect will be especially pronounced when very high-efficiency speakers are to be driven
by the amps. In view of this effect called tube microphony, amplifiers using DHT’s need
more considerate placement than amplifiers with the more widely used indirectly-heated
tubes in order to prevent sonic artifacts from entering the audio signal due to power tube
tube microphony.

Is is strongly advisable to place DHT amplifiers on a rigid rack, shelf or platform,
carefully decoupled from the floor or wall, and as far away from the speakers as
possible, in order to prevent sonic artifacts from entering the audio signal due to
power tube microphony.

300B POWER TUBES

In our amplifiers the control-grid bias voltage cannot be trimmed in any way to adjust the
idle current of an individual power tube. This is a very reliable and stable way to achieve
grid bias, but it makes it especially important to use tubes that meet tight specifications.
The average 300B tube draws an anode current Ia of 70mA for an anode-cathode-voltage
of 400V when the grid bias is set to 90 volts. Allowing for a 10% tolerance in anode
current with respect to this quiescent point, only tubes with values Ia between 63 mA and
77 mA should be used to equip the amplifier. Although matching the power tubes is not
very critical, a difference in the Ia values of the two tubes of a matched pair should be
preferably not more than 5 mA. The use of 300B tubes with questionable tube data and/or
poor quality will lead to inferior sound quality or in (extreme cases) even to serious
damage to the amplifier. If there is any doubt that a certain 300B power tube actually
fulfills the stated requirements, the idle current can be easily determined by Ohm’s law
by measuring the DC voltage-drop over the winding resistance of the output transformer
primary while the amplifier is powered on.

In view of the high voltages involved, measurements within the powered-on
amplifier circuitry should always be carried out by an experienced technician only !

When the power tubes are replaced for whatever reason, it is strongly advised to use only
high quality matched pairs tested for the above specs supplied either from the
manufacturer or a “decent” tube supplier.