Break-In Period; Tube Replacement - Cary Audio Design CAD 120S MKII Owner's Manual

OPERATION
On the rear panel of the CAD 120S MKII is a bias adjustment potentiometer for each
channel. This is a screwdriver slotted shaft control. After the amplifier has been
powered up for a few minutes observe the meter readings. The proper reading should be
140 ma. plus or minus 10 ma. To make the bias adjustment insert a flat bladed
screwdriver into the rear mounted bias adjustment potentiometer and turn clockwise to
raise the bias or counter clockwise to lower the bias reading. There is some interaction
between channels. Adjust the right and left channel bias a few times until you have an
equal reading (plus or minus a few ma) on each channel. The bias reading level is not
critical in respect to a range of a low reading of 125 ma. to a high reading of 170 ma.
The preferred reading should be approximately 140 ma. plus or minus 10 ma. The DC
bias current value will fluctuate with varying in house AC wall voltage. As air
conditioning systems cycle and power loads in the neighborhood change, the bias reading
will fluctuate a bit. This is normal and therefore not critical.
Another word about the meter readings. The louder you play the CAD 120S MKII the
more current it will draw. Playing loud, especially in the ultra-linear mode will peg the
meters at 200 ma. This is normal and there is a protection circuit built into the metering
circuit to prevent damage to the analog meters. Pegging the meters is fun!

BREAK-IN PERIOD

The tubes, capacitors and output transformers take approximately 100 hours use to settle
in for peak performance. The CAD 120S MKII will sound good right out of the box but it
will improve within a short time. After the first 50-100 hours you will notice increased
depth and tighter bass. This break-in period is true with quality vacuum tube audio
amplifiers.

TUBE REPLACEMENT

If it becomes necessary to replace the tubes in the amplifier, a matched set of output tubes
of the same name brand should be used. A new tube kit is available from Cary Audio.
You should get years of service from the rugged KT-88 output tubes. The input and
driver 6SN7 tubes can last even longer. Other output tubes may be used in the CAD
120S MKII power amplifier; 6L6, 6CA7, KT-66, 6550C, KT-90, KT-99 or EL-34 tubes
are all acceptable for use in the output stage of the CAD 120S MKII. Use the same
current setting for any of these tubes as described in the bias set up.
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