Velleman projects K4040 Assembly instructions - Manual (Page 20)

20
Switch off the mains voltage.
Turn all trimmer potentiometers, RV1 to RV8, fully anticlockwise.
Mount the 8 tubes, V1 to V8, type EL34 (or C6A7, CV1741) into their sockets (check the position of the
notch).
Standby current adjustment
No measuring equipment is needed for the following adjustment, the standby or bias current of each tube
can be seen from the LED scale on the front. The individual tube to be checked can be selected using
the small DIP switches. RV1 corresponds to V1, RV2 to V2, etc.
Set all 8 DIP switches to the OFF position (up). Each switch has an associated tube, never set more than
one switch ON (down) otherwise you will get an incorrect reading of the LED scale.
Switch the mains voltage on and wait until the green LED is lit continuously.
Adjustment of the left-hand channel:
Switch the first switch of SW2 (left-hand DIP switch) ON (down).
Carefully adjust RV1 until the second or third LED lights up (LD4 or LD5).
Switch OFF the first switch (up).
Switch ON the second switch (down).
Carefully adjust RV2 until the second or third LED lights up.
Complete the adjustment for all tubes of the left-hand channel.
Adjustment of the right-hand channel:
Ensure that all switches of the left-hand channel are OFF (up).
Switch the first switch of SW3 (right DIP switch) ON (down).
Carefully adjust RV5 until the second or third LED lights up (LD4 or LD5). Switch OFF the first
switch (up).
Switch ON the second switch (down).
Carefully adjust RV6 until the second or third LED lights up.
Complete the adjustment for all tubes of the right-hand channel.
Now wait for around 10 minutes.
Repeat the complete adjustment but adjust until one of the two green LEDs lights up. After adjusting all
tubes, the voltages of around 0.4V can be checked.
30. Setting up
IMPORTANT: Connect an 8.2 Ohm 5W resistor between the output terminals
(0 and 8 Ohm) of both channels. The output of a tube amplifier must always be
loaded. Make sure that there is a good contact with the loudspeaker terminals,
the connections of the resistors may have to be thickened a little with solder.
!
ATTENTION: The following adjustment is a reference for the bias current of
the power tubes. Be very careful in setting them. Also respect the order of
adjustment. No signal may be connected to the input.
!
Setting - up