(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.
