21
7. TUBES
How to set the BIAS
Make sure that no signal is playing through the amplifier when you are adjusting the BIAS.
It is
sufficient to turn your preamp volume control to 0; there is no need to disconnect the speakers.
The tubes should have reached operating temperature. The unit should run at least 15 minutes
before you adjust the BIAS. If you notice high BIAS at one or more of the output tubes in cold state
you should reduce the BIAS in any case.
Procedure:
1)
Turn the volume control on your preamp to 0.
2)
Set the bias selector switch to V1. The green LED next to the trim pot for tube V1 will light up
to show that you have chosen to adjust the bias for tube V1.
3)
The bias setting for this tube is correct when the green OK LED on the display illuminates.
If the LED illuminates, the bias is set too high.
Turn the trim pot counter-clockwise until the OK LED illuminates.
If the yellow - LED illuminates, the bias is set too low.
Turn the trimmer clockwise until the OK LED illuminates.
Repeat this procedure with the other three tubes
4)
Switch the bias selector back to its "off" position.
If there is no change in the BIAS of a tube while turning the BIAS regulator, the corresponding
output tube is faulty.
Note:
It is not possible to adjust the bias if the amplifier's electronic protection has
cut in (as indicated by the red "off" LED in the display), or if the Ecomode has
powered down the unit. When you activate the measuring facility under these
circumstances, the
-
LEDs will illuminate for all tubes. You must fix the fault or
switch off the Ecomode before using the measuring facility. If a defective tube
is causing the problem, you should locate and replace it.
(See Troubleshooting
Chapter 9)
