Western Electric 407A-4, Instruction Manual

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unidirectional current which after being filtered by choke
coil L51P and condenser C51P is connected to the anode cir­
cuits of the 298A water-cooled amplifier tubes. The seventh
tube, V7H, with its anode and filament both connected to
busses, is provided for use in case of failure of any of the
other tubes. This tube
may
be quickly connected in place of
a faulty tube by transferring its pair of disconnecting
links
(
D8H to Dl9H inclusive
)
to these busses. The filament
of V7H is run at reduced voltage when not in service and is
raised to normal voltage by shorting R9H by means of the
"Spare Tube" switch D4H. The blower MBlH is thermostatically
controlled and provides a means of cooling the tubes when
the air temperature exceeds 30° C
(
86°
F . ) .
Its motor may
be disconnected by means of switch D5H. Relay S8H prevents
the application of high voltage to the rectifier until the
filaments are energized and the motor switch D5H is closed.
Protection from accidental contact with the high-volt­
age circuits of the rectifier is afforded by the mechanical­
ly interlocked switches DlH, D2H and D3H. DlH grounds the
three-phase high�voltage supply and short-circuits the high­
vol tage condenser. D2H energizes the door-lock circuit and
D3H opens the high-voltage circuit breaker.
Relays SlH to S6H, inclusive, light lamps on the con­
trol panel to indicate arc-back failures of the rectifier
tubes. Under normal operating conditions, the anode cur­
rent of each tube is a pulsating direct current in the form
of rectangular blocks which last for approximately 120
electrical degrees of each cycle. This unidirectional cur­
rent produces a voltage drop across the respective relay
shunts RlH to R6H. Copper-oxide rectifiers XlH to X6H are
poled in such a way that this unidirectional voltage across
the shunts produces no current in the relays. If, however,
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