Section 5
OPERATING AND MAINTENANCE NOTES
TUBES - The 4CX800A / GU74b tubes used in the ETO 91/3 are very rugged and normally
operate with a large margin of safety. They should provide outstanding service for many
years if not damaged by abuse- especially overdrive or blockage of cooling airflow. Allow at
least twelve inches (31 cm) unobstructed clearance around the air intake and exhaust areas. Stacking
equipment is not recommended.
Never allow key-down plate current to exceed 1.5 amperes for more than one or two sec
onds. If you do and a plate current trip occurs, it will automatically reset in about 4 seconds if
the amplifier is returned to receive (key-up). Never allow the red GRID LED to stay brightly
illuminated for more than a second.
Frequent on-off AC power cycling may shorten tube life. It's better to leave equipment in
standby for several hours than to cycle power repeatedly on-off-on-off over the same period. *
INTERLOCKS — The ETO 91/3 is equipped with a cover interlock switch intended to remove
primary power from the amplifier, and a crowbar to short-circuit the high voltage to chassis
whenever the cover is lifted. These interlocks are designed to protect against dangerous
electric shock resulting from accidental contact with potentially lethal voltages inside the
amplifier.
WARNING! WARNING! WARNING! WARNING! WARNING! WARNING!
ALWAYS DISCONNECT THE AC LINE CORD FROM THE POWER SOURCE BEFORE
REMOVING THE TOP COVER FROM THE 91/1 FOR ANY REASON!
Cover interlocks are intended only as back-up protection against accidents. Never depend on
them! Always disconnect the power cord from the AC mains before removing the cover!
Interlock switches should never be disabled for any reason except by a skilled and experi
enced technician.
FUSES — Never replace any fuse with one of a different type or greater current rating.
Blowing of one or both primary line fuses indicates that the maximum safe average power
capability of the amplifier has been substantially exceeded or that an equipment failure has
occurred. USE ONLY 20 AMP, 250 VOLT RATED FUSES.
Slow-blow fuses F3 and F4, located above the primary line fuses, may prevent damage to the
step-start resistors and HV rectifiers in the event of abnormal tum-on conditions or HV
faults. If the AC interlock is defeated and primary power is applied while the HV crowbar is
closed, the step-start fuses normally will blow.
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