Bell & Howell - Projector - Filmosound 641 - 642 ... - Cine Information

AMPLIFIER PART NO.027097
The amplifier used in the Model 652 projector
has a rated output of 10 watts and a rated load
impedance of 16 ohms. It is designed to accept
signals from either a Photo-diode or a microphone.
The insertion of the microphone jack
automatically disconnects the photocell
circuit. The sensitivity has been arranged to
give an adequate gain margin over that
required to obtain the full power output from
a fully modulated film. A single tone control
is provided which gives a lift variable from 0
to about + 8 db. at 10 k/c
The first stage uses the pentode action of
a triode pentode valve type 6BR8. The anode
load resister of this stage is very high (1.5
megohms) and the anode current is very small.
This stage is worked under what are known as
'itarvation condition^ 1 and although this does
restrict the signal handling capacity it also
allows a very large voltage gain to be obtained.
The screen supply for this first stage is derived
from the cathode of the following stage which
is the triode section of the 6BR8. The anode
of fie first stage is directly coupled to the grid
of the second stage which is used as a phase
splitter. This phase splitter is of the conventional
split load type, one output valve
deriving its drive from the anode and the other
/n the cathode.
The output stage is a conventional pushpull
stage employing two EL 84 valves.
Negative feed-back is taken from the
secondary of the output transformer and fed
back to the cathode of the first stage. The
tone control is in the feed-back circuit and
is effective by operating on the frequency
characteristic of the feed-back signal.
The power for the exciter lamp is
supplied by an R.F. oscillator which employs
a cathode coupled Hartley circuit.
The exciter lamp is rated at 6 volts 1 amp
76
and is operated with approximately 5? volts
to it in order to increase its life. The
lamp voltage may be varied by means of
a resistor (VR3) in the screen circuit of
V4 the oscillator valve. The power for
the amplifier and the oscillator is provided
by means of a double wound transformer/
the primary winding of which also
acts as an auto transformer to supply the
correct voltage for the projector motor.
The anode supply is rectified by means of
an EZ 81 rectifier valve.
PHYSICAL FEATURES OF THE 652
AMPLIFIER
The three amplifier valves, the oscillator
and rectifier valves are all mounted
on one printed circuit board. This board
is of thick material and ruggedly constructed
to prevent damage due to handling
and transit. All heavy components have
been kept off the printed board in order
to ensure reliability with rough handling.
The mains transformer is mounted away
from the amplifier and is situated in the
projector just beneath the motor. The
servicing of this amplifier can, in most
instances/ be performed without the
removal of the amplifier from the projector.
The chassis has been kept shallo'
for this purpose. The photo cell employefor
reading the signal from the optical
sound track of the film is a Germanium Diode
Because the Germanium Photo-diode used
in the soundhead works into a lower load
impedance than the Caesium-Silver photo
cells previously used, the input impedance
of the amplifier is relatively lov* The
lower impedance and the somewhat greater
signal level obtained form the Photodiode
(about 10 D.B.) leads to comparative
freedom from noise pick-up troubles and
consequently an improved signal to noise
ratio.