Octave V 70 Class A, Owner'S Manual

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7. TUBES
7.3 BIAS Control Display
The quiescent current of the power tubes is controlled with the BIAS measuring device. The correct
BIAS setting of all four power tubes is important for the sound quality of the power amplifier and the
life of the tubes. It guarantees consistent sound quality over the entire life of the power tubes.
Real Class A concepts do not require a separate adjustment of the quiescent current, the quiescent
current automatically adjusts itself to the optimal value with this circuit.
This value remains largely constant during the life of the power tubes and therefore does not
require any correction. Nevertheless, it is of course sensible that the BIAS can be checked to
detect a prematurely aged or damaged power tube.
Therefore, a BIAS measuring device was integrated in the V 70 Class A, which enables the user to
check the BIAS of the four power tubes. The BIAS is shown with three colored LEDs, yellow for
quiescent current too low, green for correct quiescent current and red for quiescent current too
high. The display "too low" or "too high" indicates a faulty or worn out power tube. These tube(s)
should be replaced.
The LED display
Red LED only BIAS is too high, this tube should be replaced
Green + red LED
BIAS is OK the tube is working, but the value is unfavorable because it
is slightly outside the tolerance range
Green LED only BIAS is OK All tubes are within their tolerance range
green LED BIAS is OK the tube is working, but the value is unfavorable because it
is slightly outside the tolerance range
Yellow LED only BIAS is too low, this tube should be replaced
If the quiescent current in a tube rises far into the "red, quiescent current too high" range, the
electronic fuse can be activated as a result, this is indicated by the red protection LED. The BIAS
LEDs then all go back to "yellow".
Important note:
Switching the power selector from high to low and vice versa has no effect on the BIAS display