Page 17 - 2015-03

Primary winding + 300 volts
4 volt
Secondary
winding

To
Exciter
lamp 6V6
Feedback winding

Above: The Bell & Howell oscillator circuit. The capacitor C28 adjusts the oscillator
frequency and therefore the output voltage.

to passing 3/4 of an amp in current, so using a
valve as a rectifier or to smooth the resultant By using a low voltage - high current exciter
output was not very practical. lamp, the hum that would normally be caused
by the AC on the filament can be minimised.
As we are talking about the days before semi-
conductors (diodes, transistors, etc), there were The Kellart-Victor is probably the best know
very few choices when low voltage, high cur- 16 mm projector to use this method. Others
rent was involved, except for what were called included the Pathescope Son (front cover) and
'metal rectifiers'. These were mostly selenium most of the 35 mm cinema projectors before
rectifiers and were expensive, bulky and often the 1960s.
not greatly reliable.
Bell & Howell came up with a method that
The most common method in early equipment was used in their early 16 mm sound projec-
was to feed the exciter lamp from AC - yes AC, tors (621, 620, etc.). They used a 6V6 valve
but with a difference. There is a phenomenon (the 6V6 is a medium level power valve with
technically known as “thermal inertia”. a rating of about 5 watts), and configured it as
oscillator.
This can be best demonstrated when you com-
pare the operation of your car headlights (low The arrangement was very smart - the 6V6
voltage - high current) with a domestic 60 watt was connected to the primary winding of a
globe (high voltage - low current). transformer, which had a feedback winding
and low voltage secondary. The valve was
The car headlights don't extinguish immediate- wired with positive feedback and operated as
ly, but take a short time to go dark - about a an oscillator.
second in most cases. This is the time it takes
for the white-hot filament to cool down. The 6V6 oscillating at a frequency in the
Whereas the mains operated 60 watt globe goes lower RF (radio frequency) spectrum at about
off as soon as the switch is flicked. 50 KHz.

March 2015 REEL DEALS 17