F
ILTERS
M
AINTENANCE
4 5
Typical filter characteristics for Bass and Treble filters:
T
UBES
P
LACEMENT
:
CHANGING THE TUBES
- Although it
may not be necessary due to tube
malfunction, changing the tubes once in a
while will regain the tone and bring back
high performance. There are however
some points that are important:
1.
When changing the power amp tubes,
both
tubes must be changed, and the
tubes need to be a matched pair.
Recalibration of the bias is always
necessary after installing new
power tubes
.
2.
The power tube driver, 12AT7 should
be changed every second time the
power tubes are changed.
3.
The input tube, 7025, must be selected
for low microphony, low noise and
high gain. Alternately, you may use
an 12AX7 or ECC83 here.
4.
The buffer and balanced out tube must
be same type as original, 12AU7.
SETTING THE BIAS
- Whenever the
power tubes have been changed it is
necessary to set the correct bias to ensure
long life operation. This is the procedure:
1.
Power the amp at let it warm up for 5
minutes.
2.
Connect a voltmeter to the bias
measurement point VA at the rear
panel.
3.
Switch on the amp by toggling the
standby switch.
4.
Adjust with the associated bias
trimmer so that the readout on the
meter is 35mV +/-2mV.
5.
Repeat for other tube using point VB.
6.
After first calibration repeat once for
verification of bias setting.
IMPORTANT:
Incorrectly set
bias may cause permanent damage
to the amplifier. If you are not
100% sure on how to do this,
please get in touch with a service
center.