Instruction manual

AL-800 Instruction Manual
11
The 3CX800A7 is a reasonably rugged tube. The anode can handle very large anode dissipation
overloads for short periods of time due to the thermal mass of the external anode. Care must be
taken to avoid exceeding the temperature ratings of the tube's ceramic-to-metal seals. The life of the
tube in this amplifier may be prolonged if adjustment periods are kept short and a brief "cool-down"
periods are provided between lengthy adjustment periods. Avoid shutting the main power off
immediately after lengthy full power CW or RTTY transmissions. After lengthy full power RTTY
or CW transmissions, allow a one or two minute "cool-down" period (without transmitting) before
shutting off the ac power switch.
The most sensitive element in any indirectly heated oxide cathode tube will always be the control
grid. A fraction of a second of incorrect tuning can cause control grid damage. This is especially
true in amplifiers that use fuses or resistors to protect the grids. Fuses and resistors are much too
slow and too unpredictable to adequately protect control grids. In this amplifier, the 3CX800A7 grid
is protected by a fast-acting electronic circuit. This circuit rapidly disables the amplifier when
excessive grid dissipation occurs. Never remove or defeat this circuit. Doing so will void the
manufacturer's warranty.
In the AL-800, a grid current of 75 mA (at 100 watts of drive) produces the maximum rated grid
dissipation of 4 watts. This amount of grid current will also safely disable the amplifier within a few
milliseconds.
WARNING:
NEVER drive this amplifier with more than 100 watts of short term average envelope power.
NEVER remove, defeat, or modify the internal electronic grid protection circuits.
Maximum power output normally occurs with a maximum of 15 to 20 mA of grid current on CW,
or 12 mA indicated (25 mA peak) during a SSB two-tone test. The grid dissipation is well under one
watt with 25 mA of grid current and 100 watts of drive.
The cathode of the 3CX800A7 must reach full operating temperature before the tube is operated.
The cathode becomes fully operational 180 seconds after the application of the full heater voltage.
The warm-up timer in this amplifier assures that the cathode reaches the proper operating
temperature before the amplifier can be operated. Never defeat the warm up timer or alter the heater
voltage.
The continuous commercial plate current rating of the 3CX800A7 is 600 mA. Brief periods of plate
current exceeding 1.2 amperes (during tuning) will not cause loss of emission or shorten the amateur
life of the tube in this amplifier. For maximum tube life, plate current should be maintained below
700 mA on normal amateur CW operation, 600 mA on FM, RTTY, and other "steady" carrier
modes. The life a metal oxide cathode is directly related to the accumulated heater operating
time, not the transmitting time.
The application of heater or filament voltage causes thermal stresses from rapid and uneven
temperature changes in the tube's heater. Avoid unnecessary main power switch cycling to prevent
needless heater life reduction.