Hammond XK-3C, Owner'S Manual

Page: 79 / 126

Setting the Parameters
79
B I A S V O LTA G E A N D N O N L I N E A R
DISTORTION
To make the vacuum tube function as a audio amplifier, a minus voltage called bias voltage
is added to the input terminal (called grid) as well as the audio signal.
Although, the bias voltage is generally fixed so that the distortion of the output wave-form
of the tube is at the minimum, the bias voltage can be changed when the tube amplifier is
overdriven on this model.
By this parameter, you can adjust the characteristic of distortion to a setting that you like.
The change the bias voltage gives to the sound distortion may not be very noticeble, de-
pending on the drive amount or the drawbar registration. So the best way to check it pre-
cisely is as follows:
1. Set the drawbar registration at “00 8000 000”.
2. Keep pressing the [TUBE AMP] button and display the OD TUBE page.
3. Set the Pre-Amp - Type at “S-U7”.
4. Set the Drive near 40. (Make sure it is not the maximum value.)
5. Move the cursor to BIAS U.
BIAS VOLTAGE AND NONLINEAR DISTORTION OF THE TUBE
6. Press the chord on the keyboard as shown.
Try to change the value of BIAS U, pressing the keys. The degree of distortion is slight near
“0”, but you can hear the distorted sound “rrr” getting loud, if you change the value to-
wards “+” or “-”. Such a distorting sound is called “nonlinear distortion”. Likewise, try to
change the pre-amp. type to S-X7 and also the value of BIAS X.
NOTE: On the vintage B-3/C-3, such distortion may occur as they get older. On this model, you
can create such distortion intentionally.
GRID
The grid is one of the poles of the tube. In the
tube called triode used on this model, there are
3 poles, the cathode, the plate and the grid. The
electrons are attracted towards the plate if the
heated cathode is loaded with voltage. By chang-
ing the voltage on the grid, the voltage flowing
to the plate greatly changes. It is called amplifi-
cation operation of the tube.
NONLINEAR DISTORTION
An amplifier without distortion amplifies input in
proportion (=linear) with output, as shown in
the figure below.
CLIPPING DISTORTION
If the drive is at the maximum, the end of the
waveform is cut (off), as shown in the figure
below. It is called clipping distortion.
If the bias voltage is changed, input and output
are not in proportion with each other, as shown
in the figure on the left. The distortion caused
by this is called non-linear distortion.