AMERITRON AL-800 - General Information; Safety Interlock; Alc

AL-800 Instruction Manual
6
General Information
Safety Interlock
An interlock switch is closed while the amplifier's top cover is in place. This switch allows ac line
voltage to reach the power transformer. The interlock will open and remove line voltage from the
transformer primary if the top cover is removed.
WARNING:
High voltage can kill!! Accidental contact with the voltages in this amplifier can be lethal.
For your personal safety please observe the following precautions:
1.) NEVER defeat the interlock.
2.) NEVER remove the cover with the amplifier connected to the power line.
3.) ALWAYS allow the capacitors to discharge for several minutes after unplugging the amplifier
and before removing the cover.
4.) ALWAYS select the high-voltage (HV) function of the Multimeter to check the high voltage
potential. Do not remove the cover if voltage is indicated.
5.) ALWAYS ground the tube anode (outer metal surface) to the chassis through a low value, high
wattage resistor before touching anything inside the amplifier.
6.) ALWAYS be cautious of heat. Many components inside the amplifier operate at high enough
temperatures to cause burns.
7.) NEVER make any unauthorized component or circuit modifications to this product. The only
acceptable source for modifications is Ameritron or a source approved by Ameritron.
Unauthorized modifications almost certainly will increase the risk of equipment failure or
personal injury.
ALC
The AL-800 is one of the few amplifiers using a "true" automatic level control ALC circuit. In this
amplifier, the ALC actually samples grid current and power supply loading to determine the ALC
voltage. The front panel "ALC SET" knob adjusts the level of grid current where ALC action
begins. The ALC action begins softly over a range of a few milliamperes to minimize distortion.
This soft-touch ALC is an Ameritron exclusive.
The ALC circuit is located on the METER BOARD P/N 50-0800H-3 (behind the
"MULTIMETER" switch). IC301 (pin 9) samples the voltage drop (caused by grid current)
across R106 in the main power supply board. ALC voltage is derived by comparing the grid shunt
voltage to the voltage from the power supply (pin 10).