AL-800H Instruction Manual
6
Power Supply
Heater Supply
The heater circuit of this amplifier satisfies
all
requirements of the tube manufacturer related to tube
performance and life. The heater voltage and inrush current are controlled by the power transformer's
internal resistance and impedance, heater choke resistance, heater wiring resistance, and the step-start
circuit. For maximum tube life, NEVER replace any circuit components or wiring with substitute parts.
Always consult the factory before modifying any section of this amplifier
Plate (HV) Supply
The plate supply uses a full wave doubler circuit with 200 amp surge rated diodes and a heavy duty 32
pound grain oriented transformer with internal thermal protection. Filtering is accomplished by a bank of
high quality computer grade capacitors totaling 63 mFd. Large 50k-ohm 7-watt bleeders are used for safety
and superior voltage equalization. The transformer allows user selection of fourteen different line voltages
(See the TRANSFORMER CONNECTIONS
section on page 13 for details).
Power is supplied through
"OFF-ON"
switch S2. A 10 ohm resistor (R27) limits the line current during
the filter capacitor charge time to lower component stress. When the filter capacitor charging current
decreases sufficiently, RLY102 shorts the 10 ohm resistor. This applies full power line voltage to the
transformer. The 10 ohm resistor is protected from high-voltage supply shorts by a two-ampere fast-blow
fuse (F101) during start-up. If F101 or the 10 ohm resistor (R27) fails the amplifier will not start. If F101
fails from a momentary HV to ground fault, meter protection diode D117 may also fail. See the METERS
section that follows for more information.
Meters
Current Meter (plate and grid):
The plate and grid current meter is located on the far left side of the front panel. This meter indicates the
plate current (Ip) on the right-hand meter scale. This scale has a small picket every 0.05 ampere, a large
picket every 0.25 ampere, and indicates 1.5 amperes at full deflection.
The left-hand meter scale indicates the grid current (Ig). The small pickets on this "Ig" scale appear every 5
mA and the larger pickets appear every 25 mA. The full scale "Ig" reading is 125 mA.
The plate and grid meters in this amplifier will normally indicate maximum grid current and maximum RF
output near the same "PLATE" tuning setting during adjustments. Maximum grid current and
minimum
plate current also generally occur at or near the same "PLATE" tuning setting.
NOTE:
If the grid and plate meters always track (move together in exact step) as the tuning
controls are adjusted and if they show the same approximate amount of pointer movement,
diode D117 on the power supply board could be shorted.
D117 protects the overload circuit and the meters. This diode is located near the electrolytic capacitors on
outside edge of the main circuit board. D117 will usually short if there is a large high-voltage-to-chassis
current fault. The grid and plate current meters will not read correctly if this diode fails. The overload
circuit may repeatedly trip before full power is reached if D117 is shorted.
