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6
SETTING THE TUBE BIAS
Tools required: bias tool (provided with amplifier)
The output tubes (the big ones) need to be “biased’ with a grid voltage that
controls the flow of electrical current through the tube. Tube biasing on the Stereo
90 amplifier has been designed to be both simple and effective. On some
amplifiers biasing is done for two or more tubes simultaneously. With this
method, the total current flowing through the group of tubes may be correct, but
the current flow through any individual tube may vary significantly from the
optimal value. With the Stereo 90 amplifier the bias is set individually for each
tube so that the correct operating point is assured. An added benefit to this
approach is that, should a tube fail prematurely, the single tube can be replaced
without having to purchase a matched set. Use the following procedure to set the
tube bias:
WARNING – There are dangerous and potentially lethal voltages inside this
amplifier. Do not touch any part of the amplifier other than as described
below. If you are uncertain about any of these instructions, please contact
your dealer and have them bias the amplifier for you.
1) Allow the amplifier to warm up for at least 30 minutes.
2) Turn the preamplifier volume completely off so that no signal is entering the
amplifier.
3) Remove the top cover. The bias tool is snap attached to the inside top of the
deck lid.
4) Locate the rotary switch and associated small potentiometers (they are blue
and have a small screw in the top). There are four, one for each tube. Note the
biasing meter.
5) Position the rotary switch into the corresponding tube position (ie. V1 for
output tube V1).
Note – There is a small leakage current of 5mA that flows through the
meter’s shunt resistor during the tube bias setting operation. To accurately
account for this, set the bias current at 5 mA lower than the desired setting.
6) Using the bias tool, slowly turn the screw on the potentiometer that is adjacent
to the switch until the bias meter reads ~ 35 miliamps (actually 40mA.) Turn
the screw clockwise to lower the bias and counterclockwise to raise the bias.
Note that there are two ends on the bias tool. Using the end with the recessed
screwdriver will greatly facilitate this operation.
7) Repeat the above steps for each of the four output tubes.