CIRCUIT DESCRIPTION
The input stage of these amplifiers consists of a 12AX7 current sourced, long-tailed balanced pair direct
coupled to the complimentary pair of EL84M tubes. An internal DC restorer ensures that the bias volt-
age remains correct over the entire audio signal cycle.
Each set of output tubes are arranged in a push pull configuration. The EL84M screen grids are oper-
ated at approximately 340 volts, the output tube plate voltage is about 408 volts. The power supply con-
sists of a large power transformer with energy storage that is far greater than necessary. Multiple de-
coupling filter sections are used with load regulation obtained through zener diode circuit providing con-
stant current loading.
Turn-on in-rush current limiting is provided by a thermistor, bias voltages are automatically set by inter-
nal circuits, volume by a stereo volume control on top. A tube cathode fuse protects the output stage in
the event of a catastrophic vacuum tube failure. Finally, a rear mounted power line fuse provides overall
protection for the amplifier.
VACUUM TUBES
The EL84M output tubes do not need to be matched. That's because the DC restorer circuit eliminates
that need. Please note, we have selected the “M” version of the EL84 because it has a higher plate volt-
age rating than the “non-M” tubes made by many manufacturers. This is how we achieve a full 20 watts
per channel output power by using just one pair each of EL84M tubes. We do not guarantee that replac-
ing these tubes with non-M EL84s will work. Red plating may occur (meaning the tubes will glow red hot)
Looking at the amplifier from the front, the two smaller 12AX7 tubes go right behind the “roll bar”, the
EL84M tubes are installed in the four sockets behind them.
VOLUME CONTROL
Under normal circumstances, this control should be left in the maximum (fully clockwise) position. If you
have a noisy preamp, turning it halfway down and centering it at the 12:00 o'clock position will reduce
the preamp noise by half, or 6 dB. It should always be turned all the way down when connecting or dis-
connecting any input or output in your system. Connect RCA patch cords from your preamp to the RCA
audio input jacks on the back of this unit.
Note: You may also directly connect a piece of “line level” source equipment to this amplifier with no
preamp and use the volume pot on the amp to adjust the volume. Phonographs cannot be directly to this
amplifier without the use of a phono-prestage preamplifier.
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