Bias Adjustments - General - DYNACO Stereo 80 Owner's Manual

Bias Adjustments - general

The Stereo 80 incorporates integrated circuits that monitor the actual quiescent (idle) current of each
output tube. For each output tube there is a separate bias adjustment potentiometer accessible from the
front panel of the amplifier. Directly above each adjustment potentiometer is the indicator lamp for the
tube which it adjusts.
The front panel switch labeled Bias Display On/Off must be ON for bias adjustments. The Bias Display
On/Off switch (optionally) can be left on while listening to music. If it is left on there will be a "light-
show" display effect which varies with the music material.
During bias adjustments each indicator lamp shows three different conditions. When it glows green it
means that the bias is set too low for the output tube. When it glows red it means that the bias is set too
high. When the lamp is not lit (neither green nor red) it means that the bias setting is correct and is in the
range of 74 to 87 millivolts, (measured across each 2 ohm output tube current sensing resistor - R20, 21,
120, or 121). The correct bias current setting for the EL34 output tubes used with the Stereo 80 is:
40ma +/- 5ma
Before bias adjustments are made, the amplifier must be fully warmed up. As a minimum requirement,
the amplifier should be turned on for 15 minutes prior to bias adjustments. Letting it idle for a full hour
so that all components come up to full thermal equalization is also a good idea, but not usually required.
Notes: For newly installed output tubes, the first three hours is the "burn-in" period. During this time
bias settings need to be closely monitored because they go higher (by themselves) while the tubes
burn-in. If a tube is operated with its bias setting a lot higher than it should be, the tube could
actually burn out or cause other amplifier components to fail.
The output tubes in your new Stereo 80 have already been burned-in and properly biased at the
factory. Nevertheless, when you first start using your amplifier you should check the bias
settings using the Output Tube Bias Adjustment Procedure on the following page.
As mentioned earlier, AC line voltage levels can vary greatly. Even after proper output tube bias
adjustments are completed on your amplifier, temporary changes in the AC line voltage can make it seem
that the bias settings are not correct. If the AC line voltage is low, one or more of the bias indicator lamps
may continue to glow green after the initial one-minute warm-up period. Likewise, if one or more of the
bias indicator lamps glows red after the initial warm-up period, a high AC line voltage condition may be
indicated. If a low or high AC line voltage condition is only temporary, you do not need to re-adjust the
bias settings..
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