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

The amplifier used in the Model 641
projector has a rated output of 6 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 photo cell 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 + 8D.B. at 10 K. C.
The first stage uses the pentode section
of a triode pentode valve type 6BR8. The
anode load resistor 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 "starvation conditions 11
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 the 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
from the cathode.
The output stage is a conventional
push-pull 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 feedback
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
and is operated with approximately 5 volts
AMPLIFIER PART NO. 026641
43
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 641 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 shallow for this
purpose. The photo cell employed for
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 low. This
lower impedance and the somewhat greater
signal level obtained from the Photo-diode
(about 10 D.B.) leads to comparative freedom
from noise pick-up troubles and
consequently an improved signal to noise
ratio.
The 641 projector is fitted with a
6 lf x 4" elliptical speaker connected across
the output. A 16 ohm resistor is connected
in parallel to limit the power fed into it.
This internal speaker is - automatically