Hammond B-3, Owner'S Manual

Page: 81 / 130

81
Setting the Parameters
GRID
The grid is one of the poles of the tube. In the
triode tube used on this model, there are 3 poles,
the cathode, the plate and the grid. The elec-
trons are attracted towards the plate if the heated
cathode is loaded with voltage. By changing the
voltage on the grid, the voltage flowing to the
plate greatly changes.
NONLINEAR DISTORTION
An amplifier without distortion amplifies input in
proportion (=linear) with output, as shown in
the figure below.
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.
CLIPPING DISTORTION
If the drive is at the maximum, the end of the
waveform is cut (off), as shown in the figure
below. This is called clipping distortion.
Bias voltage and Nonlinear Distortion
To make the vacuum tube function as an 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 your taste.
The change the bias voltage gives to the sound distortion may not be very noticeble, depend-
ing on the drive amount or the drawbar registration. The best way to check it precisely is as
follows:
1. Set the Drawbar registration at “00 8000 000” (either manual).
2. Locate the TUBE AMP - OD page.
3. Set the Drive near 40. (Make sure it is not the maximum value.)
4. Move the cursor to BIAS.
5. Play the chord on the manual as shown.
Change the value of BIAS, pressing the keys. The degree of distortion is slight near “0”, but
the distortion increases, if you change the value towards “+” or “-”. Such a distorting sound is
called “nonlinear distortion”.
NOTE: On the original B-3, distortion occasionally would occur naturally as the instrument would
age. On this organ, you can create natural-sounding distortion intentionally.