14
Output Tube Bias Adjustment
As shipped from the factory, the output ‘bias’
adjustments are set for a nominal 65mA per
KT150 tube. Under these idle conditions the
tubes are each dissipating approximately 27
watts of their 75 watt rating. This point of
operation provides ‘enriched’ Class AB1, and
will satisfy the most critical listener.
For best results, operate and adjust the
Reference 150 SE at 120V AC. Adjustment
must be made under zero-signal conditions
after at least 15-20 minutes of uninterrupted
stabilization time.
A digital voltmeter capable of accurate
measurements with 0.1 mVDC resolution
is preferred for accurate adjustment (must
have 3 ½ digit display). Use the plastic
alignment tool provided to make the
adjustment. The measurement points are
yellow and black banana test jacks next to
each output tube. Adjust the ‘bias’ for a
voltage reading of 65mVDC (.065 Volt DC) at
the blue bias pot for one of each output tube
pair as follows, noting the voltage setting
of the larger V number tube in each pair is
slaved to the adjustment setting of its lower
V number companion tube: for example,
adjust V5 for 65mVDC and measure its
companion tube V7 to verify a reading of 57
to 73mVDC. Repeat procedure by adjusting
and measuring V9 to read 65mVDC and verify
V11 reads 57-73mVDC; adjust and measure
V6 to read 65mVDC and verify V8 reads 57-
73mVDC, and finally adjust and measure V10
to read 65mVDC and verify that V12 reads 57-
73 mVDC.
Hour Counter
An LCD hour counter of elapsed tube
operating time can be viewed through the
top cover near the front, mounted on the
right main circuit board (see illustration
page 6). This displays accumulated hours
of vacuum tube service life. If the amplifier
is unplugged from A.C. supply, the total
accumulated hours are retained. Adjacent
to the hour counter is the hour counter reset
button; after replacing vacuum tubes, press
this button to reset the hour counter back
to zero. Note that once the hour counter has
been reset, it is no longer possible to
recall the previous hour count.
Cooling Fan Speed Adjustment
The two D.C. cooling fans located at the
rear of the Reference 150 are adjustable in
speed. Locate the red colored dip switch
under the top cover at the top near the
center of the back panel. For highest fan
speed move both white tabs on the red dip
switch to the upward position; for medium
fan speed move either one of the tabs to
the downward position, and for low fan
speed move both tabs to the downward
position. For maximum cooling and
extended tube life, use the highest fan
speed possible. Be sure to first turn off and
unplug the Reference 150SE from its power
receptacle before unscrewing the top cover
to access the switch. Refasten the top cover
before resuming operation. Do not operate
the Reference 150SE with fans disconnected
or if one or both fans should stop running.
Operation
