Thoress F2A11 Instruction Manual

THÖRESS

F2A11 Stereo Integrated Amplifier

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INSTRUCTION MANUAL

Thank you for purchasing the THÖRESS F2A11 Stereo Integrated Amplifier. This truly
unique tube amplifier offers the music lover the possibility to build a purist setup
consisting solely of a CD/DVD player or sound card, the amplifier itself and a pair of
highly efficient loudspeakers such as our FR20 or 2CD12 speaker models. Of course, the
amplifier may also be combined with a preamplifier, such as our Full Function
Preamplifier or a stand-alone phono stage, in which case a high-gain unit such as our
Phono Enhancer is strongly recommended.

Three line level inputs are provided, selectable with the source dial located at the back of
the case. The volume of each channel is controlled separately via high-grade rotary
potentiometers (custom made by ALPS in Japan). These parts are carefully matched to
ensure precise channel balance with respect to the rotation angle.

Although reasonably priced, the Stereo Integrated employs the highly regarded new-old­stock Siemens F2A11 beam power tetrode operating single ended triode mode. The
amplifier is hand-made in Germany. It shares the non-compromise approach of the larger
THÖRESS Mono Power Amplifiers and is entirely hard-wired. Every detail of the
internal construction has been carefully considered for reliability and ease of service.
Therefore our smallest amplifier model is by no means an entry level component, but a
puristic state-of-the art device which is likely to be sonically superior to any competitive
product regardless of price and thus will easily measure up to the expectations of even the
most critical and experienced music enthusiast or professional user.

POWER TUBES

The Stereo Integrated is a single ended amplifier equipped with the highly regarded
Siemens F2A11 beam power tetrode operating in triode mode. These new-old-stock
professional grade tubes are known not only for meeting very tight specs and being
extremely reliable, but also for offering excellent subjective sound quality (when
operating triode mode). They are certainly on par with the finest directly-heated triodes
such as the 2A3 or 300B.

The power tubes are biased via low tolerance 220 ohm cathode resistors. 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. Note that an average

(ideal) F2A11 tube draws a cathode current of around 75mA when the grid bias is set to
16 volts at an anode-cathode voltage of 390 volts. When the power tubes are replaced, it
is strongly advised to use only high-quality matched pairs tested for the above specs and
supplied from the manufacturer. If there is any doubt that a certain F2A11 power tube
actually fulfills these requirements, the idle current can be easily determined by Ohm’s
law by measuring the DC voltage-drop over the respective cathode resistor 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!

DRIVER TUBES

The THÖRESS F3A11 Stereo Integrated Amplifier uses a carefully tested new-old-stock
12SN7GT tube in the driver stage. The use of driver tubes with questionable parameters
or of poor quality will lead to inferior sound quality and a higher sensitivity to
microphony. In extreme cases, damage within the circuit may occur! It is therefore
strongly advisable to use only the carefully tested tubes supplied by the manufacturer.
When the amplifier is to be equipped with “fresh” driver tubes, proceed as follows:

1. Carefully clean the tube pins with a dry brush.

2. Spray a few drops of highly viscous oil (such as Ballistol) on a cotton-tip.

3. Then use the tip to apply a thin oil film onto the contact pins and the middle pin of the
octal base.
Tubes handled this way will move in and out of the sockets more easily and will help to
increase the life expectancy of these parts.

The use of a driver tube of questionable quality may lead to degraded sound quality
and inferior signal-to-noise ratio.

Never switch on your Integrated Amplifier until all tubes have been placed into
their respective sockets.

Never pull a tube out of its socket while the amplifier is switched on.

S ETUP

To set up the Integrated Amplifier, proceed as follows:
Do not connect the amplifier to the mains until the following steps have been taken:

1. Switch off all powered devices which are to be used in the setup.

2. Set the power switch of the F2A11 Integrated Amplifier to the off position (power
switch lever directed downwards to the power inlet).

3. Place all tubes into their respective sockets very carefull. Note, that the F2A11 heaters
are connected in series and thus none of the tubes will glow/operate unless both tubes
have been properly installed into their sockets to form a closed chain with respect to the
heaters.