4
Bias
Regulators for tube
V1 V2 V3 V4
The bias measurement system
The bias measurement facility makes it easy for you to check and adjust the idle current of the
output tubes. Getting the bias setting right for all four tubes is critical both for the sound of the
power amplifier section and for the service life of the tubes. It guarantees consistent sound quality
over the entire lifetime of the output tubes.
This is why we have built a bias measurement facility into the V 70 that allows you to carry out the
adjustment yourself without the need for test equipment. Thanks to the use of precision op amps,
this system allows you to set the bias to an accuracy of 0.3%, making it superior to any other ad-
justment method.
The use of selected output tubes only makes sense if the idle current is adjusted accurately, as is
clearly shown in Fig. 1 Technical Data.
Turn the Input/Mode selector switch clockwise until the bias LED lights up showing that the meas-
urement circuit has been activated. The 3 LEDs above each adjustment screw in the display panel
indicate that the bias setting is either too low, correct, or too high. Use the screwdriver supplied to
adjust the bias.
Turn the screws clockwise to increase the current. To ensure accuracy, all inputs are automatically
disconnected when the measurement function is selected.
The adjusting screws are fully insulated. There is no risk of electrical shock and the adjustment
procedure is completely safe.
The LED display:
top row: red LEDs Setting is too high
centre row: green LEDs Setting is OK
bottom row: yellow LEDs Setting is too low
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