THE AMPLIFICATION SYSTEM
The electrical impulses which produce the tones of the Hammond Organ are
given their original amplification by a preamplifier located in the console, and are
then transmitted to the power amplifiers which are located in the tone cabinets. It
will be noted that no power transformer is included in the preamplifiers shown in
figures 1 through 9, the required plate current being supplied by the power
amplifier in the first tone cabinet. Later models of preamplifiers have a complete
power supply incorporated within them.
A tone control is included in all preamplifiers whereby the relative intensity of
the high and low frequencies may be changed to suit acoustical conditions by varying
the amplitude of the higher frequencies. On tremulant equipped consoles this control
will be found under a screw cap located toward the right end of the chassis, while
on consoles equipped with the Hammond Vibrato this tone control will be found under
the cap marked HI IMP INPUT. Selective vibrato consoles have the tone control
located midway on the preamplifier chassis.
A microphone or phonograph pickup may be used with the organ if special
circumstances make it desirable. On tremulant type consoles the input terminal,
marked P on the preamplifier, goes through a screen bypass condenser to the screen
of the input tube. This terminal is normally grounded, and the input device should
have an impedance of 500 ohms or less in order not to reduce the volume of the
organ. A signal level of a volt or more is required to drive this point, and
therefore it is suggested that the microphone or phonograph be connected through a
suitable preamplifier having an output impedance of about 200 ohms.
On vibrato consoles the input terminal, located under the cap marked HI IMP INPUT
on the preamplifier, goes to the grid of one input tube. This circuit has an input
of 1 megohm impedance and requires an input signal of about 60 millivolts maximum.
Most preamplifiers used on selective vibrato type consoles are equipped with a
standard phonograph input jack. The input impedance is approximately 1 megohm and
the circuit requires a maximum input signal of about ½ volt.
The push-pull signal line from the preamplifier output transformer to the tone
cabinets has a total impedance of approximately 200 ohms. As it is connected
directly to the grids of the power amplifier input tubes, practically any number of
power amplifiers may be connected in parallel.
The section on cables and plugs shows methods of connecting amplifiers to
the console.
When making tube replacement, output tubes in the amplifier should be checked for
similar plate current readings. If tubes have been in service for a considerable
length of time it is usually advisable to change all tubes at one time rather than
to try to match new tubes to the old ones.
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