Owner`s manual

2 | P a g e

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

KT120 screen grids are operated at approximately 390 volts provided by a separate power supply

formed by one-half of a voltage doubler supplying the plates with 785 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; KT120s.