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cabinet that may impede the flow of air around the unit. Do not place this amplifier directly on
carpeted floors.
3.
Do not operate the amplifier plugged into an ordinary extension cord. Heavy duty extension
cords (14 ga. or heavier) have adequate wire size and will not overheat.
4.
If you plan on plugging a pair of these amplifier into a switched outlet on a preamplifier be
sure that preamp outlets can handle 1500 watts total.
5.
Never connect or disconnect inputs or outputs while the amplifier is turned on. Loudspeakers
can be damaged or destroyed by the high power available from the amplifier.
6.
The output tubes do get hot when the amplifier power switch is on. Do not touch the output
tubes when the unit is on. If you need to change an output tube, wait 10 minutes after
powering the unit down before you touch the tubes.
7.
Turn the amplifier off when not in use to save energy and extend the life of the tubes.
8.
Additional safety information is located at the back of this manual.
CIRCUIT DESCRIPTION
The input stage of these amplifiers consists of a 12AX7 current sourced, long-tailed balanced pair
direct coupled to a second long-tailed balanced pair comprised of a 12AT7. The 12AT7 drives the grids
of the output tubes through a pair of coupling capacitors that provide low frequency loop-gain
stability. A 6AL5 DC restorer ensures that the bias voltage remains correct over the entire audio signal
cycle.
The output tubes are arranged in push pull parallel, three up, three down for a total of six in all. The
KT88 screen grids are operated at approximately 360 volts provided by a separate power supply
formed by one-half of a voltage doubler supplying the plates with 725 volts. The power supply
consists of a large power transformer with energy storage that is far greater than necessary. AC
filament voltage is biased to approximately 70 volts. Multiple decoupling filter sections are used with
load regulation obtained through resistive constant current loading.
Turn-on in-rush current limiting is provided by a thermistor, bias voltage adjustment by a rear panel
bias control pot, feedback gain by a top-mounted front panel switch, volume by a volume control pot
and bias current is displayed by a front mounted panel meter. 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 output tubes do not need to be matched, as the sound of the amp does not at all depend on
matched output tubes. That's because the DC restorer circuit eliminates the need to match tubes. The
only caveat is that the output tubes should all be from the same manufacturer and same type.
Looking at the amplifier from the front, from left to right, the first tube is a 12AX7, then a 12AT7, and
finally the 6AL5. The six output tubes are; KT88s.
Note: Replacing the KT88 output tubes with KT120s
will not yield a 305 watt amplifier as the internal circuits, tube voltages and transformer windings are
different.
