INITIAL ADJUSTMENT—BIASET
Plug the line cord into a conventional 120 volts
AC
line
(alternately 100 volts, 220 volts or 240 volts), and turn the
power switch on. It is safe to operate the Mark VI without
connection to the input or the output. The red lamp in the
power switch and the lights in the meter should glow. While
the tubes are becoming warm, on the front panel turn the
meter range switch to the "Adjust for 0" position, and turn
the Biaset potentiometer to approximately its center of rota-
tion. A bladed screwdriver adjusts these recessed controls.
Although the meter will begin showing a bias reading
quickly, it will take at least 30 seconds before the output
tubes draw their correct current. After almost a minute,
adjust the Biaset potentiometer to read "0" on the meter.
Allow the amplifier to operate this way for three or four more
minutes, and readjust the Biaset control to read "0" once
again, if necessary. There may be some drifting of the read-
ing as the output tubes are initially used, but this will sta-
bilize within an hour, after which no further adjustment is
required at this time. It should be understood, however, that
this voltage is in direct proportion to line variations and
therefore it is normal for there to be small changes. These
will have no effect on performance.
After initially setting the bias, turn the meter range switch
to the next (—3 dB) position.
Although the adjustment of Biaset is semi-permanent, it
is good practice to check this voltage three or four times
during the useful life of the output tubes (1,000-1,500 hours).
If the amplifier is used continuously, Biaset should be
checked at least weekly. If the amplifier is in a mobile instal-
lation, we recommend checking the Biaset every time the
amplifier is set up in a new location. Of course, it must be
checked and reset at times of output tube replacement or
other types of repair or maintenance work.
The Biaset adjustment must be made when no signal is
going through the amplifier, for it is normal for this voltage
measurement to vary considerably with signal going through
the amplifier.
If it is not possible to set bias—the control range of the
potentiometer is insufficient to obtain the correct "0" read-
ing—do not operate the amplifier until the cause of the dif-
ficulty has been established. Operation with incorrect Biaset
can lead to damage to the output tubes and/or to other
components.
OUTPUT METER READING
The action of the meter supplied is "fast" enough to re-
spond to substantial musical peaks, but no meter can
accurately indicate the magnitude of transient waveforms
which occur in music. However, with an
accurate
4, 8 or 16
ohm load, the meter is a measure of output power. A square
wave test signal puts out approximately 11% more voltage
than the equivalent sine wave meter indication.
To protect the meter from "pegging" (overdrive), it is
recommended that the —3 dB position of the recessed meter
range switch be used where the anticipated power levels are
not known, and when starting any test. Switching to suc-
cessively higher ranges (—9 dB or —15 dB) until the loudest
passages indicate near "0" will yield the most information.
The accompanying chart lists 4 ohm, 8 ohm and 16 ohm
load power outputs in watts for each meter range.
SCALE
-3dB
METER RANGE
-9dB
-15 dB
+ 3
120
30
7.6
0
60
15
3.8
- 3
30
7.5
1.9
-10
6
1.5
0.38
18
