Ameritron AL-800 Service manual
AL-800 Instruction Manual
TABLE OF CONTENTS
Unpacking Instructions....................................................................................................1
Features ........................................................................................................................... 3
AL-800 Technical Specifications....................................................................................4
General Information ........................................................................................................6
Safety Interlock....................................................................................................6
ALC.....................................................................................................................6
Dynamic Bias ......................................................................................................7
Timer - Overload Circuits ...............................................................................................7
Grid Overload Protection ....................................................................................7
Thermal Overload................................................................................................8
Cathode Warm-up Timer.....................................................................................8
Power Supply ..................................................................................................................8
Heater Supply......................................................................................................8
Plate (HV) Supply ...............................................................................................8
Meters..............................................................................................................................9
Current Meter (plate and grid):............................................................................9
Multimeter...........................................................................................................9
Multimeter Functions.......................................................................................... 9
Wattmeter Circuit................................................................................................10
Peak Envelope Power vs. Average Power........................................................... 10
Tube Life......................................................................................................................... 10
Export Modifications.......................................................................................................12
Technical Assistance.......................................................................................................12
Installation Guidelines and Suggestions..........................................................................13
Location Of The Amplifier.................................................................................13
Ventilation...........................................................................................................13
Power Connections.............................................................................................. 13
Grounding............................................................................................................14
Transformer Connections................................................................................................15
Jumper Connections........................................................................................................15
Interconnections .............................................................................................................. 15
Operating Instructions And Guidelines...........................................................................16
Front Panel Controls............................................................................................16
Driving Power .....................................................................................................17
Tube and Component Life................................................................................... 18
Additional SSB Notes .........................................................................................18
AM (Amplitude Modulation) Operation.............................................................19
Audio Distortion.................................................................................................. 19
FM (Frequency Modulation), RTTY, and DIGITAL OPERATION ..................19
ALC Metering, Controls, and Adjustments.....................................................................19
ALC Metering Functions.....................................................................................19
ALC Controls...................................................................................................... 20
ALC Adjustments................................................................................................20
Rear-panel "ALC LIMIT" control....................................................................... 21
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AL-800 Instruction Manual
ALC Limit Adjustment (rear panel)....................................................................21
"ALC SET" control .............................................................................................22
Tuning ............................................................................................................................. 22
QSK Operation................................................................................................................ 25
Periodic Maintenance......................................................................................................26
Parts List..........................................................................................................................27
Timer / Overload Board (50-0117-2).................................................................. 27
Power Supply / SWR Board (50-0800-1)............................................................28
Meter Board (50-0800-3) .................................................................................... 29
Tuned Input Board (50-0800-2) ..........................................................................29
AL-800 Tuned Input Chart.................................................................................. 29
AL-800 Main Chassis Parts List..........................................................................30
Frequency Coverage........................................................................................................31
Amplifier Tuning Log .....................................................................................................31
Notes: ..................................................................................................................31
Schematics....................................................................................................................... 32
Power Supply / SWR Board................................................................................ 32
Tuned Input Board...............................................................................................33
Meter Board......................................................................................................... 34
Timer / Overload Board ......................................................................................35
Main Chassis.......................................................................................................36
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AL-800 Instruction Manual
Ameritron AL-800
1200 Watt Linear Amplifier Instruction Manual
The Ameritron AL-800 is a 1200 watt PEP output linear amplifier that operates from 160 through
15 meters. The AL-800X and AL-800Y are export models that operate from 160 through 10 meters.
Export modification instructions are only available with proof of a valid amateur radio license.
The AL-800 uses a single 3CX800A7 tube in a class AB2 grounded grid circuit. High-quality
power supply and RF components combine with an accurate peak-detecting directional coupler,
front panel adjustable true ALC circuit, electronic grid current and thermal overload protection, and
automatic bias switching to make this the best featured and most reliable single 3CX800A7
amplifier available. The AL-800 and AL-800Y are factory-wired for 120 volt, 50/60 Hz ac line
voltage. The AL-800X is factory wired for 240 volt, 50/60 Hz ac line voltage. All models are
easily converted to other supply voltages between 90 and 250 volts ac.
WARNING:
NEVER APPLY POWER TO THIS AMPLIFIER WITH THE COVER REMOVED!
CONTACT WITH THE VOLTAGES INSIDE THIS AMPLIFIER CAN BE FATAL!
PLEASE READ THIS MANUAL BEFORE ATTEMPTING TO OPERATE EQUIPMENT!
Improper or abusive operation of this amplifier can damage the tube or other components in
this amplifier. Damage caused by improper or abusive operation is not covered under the
warranty policy.
Unpacking Instructions
Remove the 3CX800A7 tube from its shipping carton. Carefully unwrap the tube and inspect it for
visible damage. Look for a dented anode cooler and broken or bent pins. Roll the tube over slowly
and listen for loose internal materials. Contact either your Ameritron dealer or Ameritron
immediately if any damage exists.
Carefully lift the amplifier from the shipping carton, and place it on a firm, level surface. Inspect
the amplifier for shipping damage. Contact either your Ameritron dealer or Ameritron immediately
if any damage exists. Remove the cover screws with a #2 Phillips screwdriver. Save the screws to
re-secure the cover. Carefully lift the rear of the cover while sliding the cover back. The AL-800 is
shipped with the rubber chimney, fuses and fuse caps packed inside the amplifier. Additional
screws will be included in the fuse pack if they are required.
WARNING:
Never remove the cover while this amplifier is connected to the power mains!
Contact with the voltages in this amplifier can be fatal.
The tube must be installed in its socket prior to operation. Observe the pin configuration on the tube
and the socket. Two of the pins are separated by a large space. Carefully observe the pin alignment
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AL-800 Instruction Manual
to be sure the tube pins are centered on the socket's metal contacts. Firmly press the tube into its
socket. Do NOT twist or force the tube into the socket.
Install the rubber chimneys over the tube. The seam of the chimney must face the fiber panel. The
bottom of the chimney has notches that must fit over the grid connectors near the base of the tube.
The heavy copper strap anode lead connects to the spacer on the center panel with the 6-32 screw
and nut provided. Install the anode spring clip connector on the tube's anode cap.
The top cover should now be installed with the vent holes on side of the cover positioned near the
electrolytics. The vent holes on the top of the cover should be on the right side of the amplifier
(front view). Secure the cover by installing the corner screws first. Install all the screws loosely
until every screw is in place. Tighten the screws after they have all been installed.
Install the fuses and fuse caps on the back of the amplifier. This amplifier uses two 20A fuses for
standard 120 Vac operation. If you wish to operate the amplifier on a line voltage other than 120 V,
the jumpers on the power transformer primary windings must be changed. If the line voltage wiring
is changed, the correct fuses must be installed. See the TRANSFORMER CONNECTIONS
section on page 15 for more information.
CAUTION: Always use fast-blow fuses rated at 250 volts or higher. NEVER use standard
automotive fuses or slow-blow fuses.
Note: Ameritron will NOT be responsible for shipping damage due to improper packing. The
packing materials used to ship this amplifier are specially designed to prevent damage. All
packing materials should be retained for future shipping. Replacement packing materials
may be purchased from Ameritron if original packing materials are unavailable.
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AL-800 Instruction Manual
Features
1. The AL-800 uses a rugged high-gain 3CX800A7 tube.
2. A time-delay circuit provides a 180 second warm-up to eliminate potential damage to the tube
cathode.
3. A grid overload circuit quickly disables the amplifier if the grid current becomes excessive.
This feature prevents excessive grid current from causing distortion or damaging the tube or
other components.
4. A thermal overload automatically disables the amplifier if excessive heating of the power
transformer occurs.
5. A dynamic bias circuit eliminates hundreds of watts of unnecessary heat generation in the
power amplifier tube. The result is cooler operation and longer component life.
6. A multi-voltage heavy-duty transformer with a unique "buck-boost" winding allows
adjustment of the primary voltage to 14 different voltages centered on 115 and 230 volts. This
versatile Ameritron feature allows the user to select the optimum primary voltage for maximum
performance and life.
7. The tuning and loading controls have vernier 6:1 reduction drives for smooth tuning.
Logging scales allow quick and repeatable control adjustments for rapid band changes.
8. The AL-800 has two illuminated cross-needle panel meters. The left meter provides a
continuous reading of grid and plate currents. The right meter reads peak RF power output on
one scale and Plate Voltage (HV), Reflected power and SWR (REF), ALC detector voltage
(ALC), and ALC adjustment level (ALC SET) on the other scale.
9. Heater and plate voltages are maintained using the "STBY/OPR" switch. This allows the
amplifier to be conveniently bypassed for "barefoot" operation.
10. A front panel "ALC SET" control allows convenient adjustment of the ALC threshold. The
unique ALC circuit samples the grid current and power supply voltage.
11. "XMT" and "OL" LED's on the front panel indicate proper operation of the amplifier.
12. A rear panel 12 Volt auxiliary output jack provides up to 200 mA at 12 Vdc for accessories
such as the ATR-15 Antenna Tuner.
13. A step-start circuit limits the inrush current to the power supply and tube heater. This circuit
extends the life of the amplifier components.
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AL-800 Instruction Manual
AL-800 Technical Specifications
*
Input:
Circuit type: Pi-network, slug tuned coils
Maximum VSWR at resonance: 1.3:1 or less
Minimum 2:1 VSWR bandwidth: 20% of center frequency
Maximum drive power permissible: 100 watts PEP
Typical drive for full power output: 85 watts
Output:
Circuit type: Pi-L, Pi-network
Typical SSB PEP voice operation: 1200 + watts
CW continuous operation: 850 watts
1/2 hour PEP two-tone test: 1200 watts
1/2 hour continuous carrier (RTTY): 700 watts
Frequency Coverage: 1.8 to 21 MHz amateur Bands. (Export models
include 24.5 and 28 MHz)
Third Order IMD: -35 dB or better (at rated output)
Efficiency:
CW: approximately 65%
SSB: approximately 65% (envelope crest)
Power Supply:
Circuit type: Full wave voltage doubler
No load voltage: 2600 V
Full load voltage: 2250 V
Full load current: 1.2 A
Regulation: 12%
Transformer: 32 lb. E-I lamination grain oriented
Capacitors: 63 mFd total, computer grade
Normal line current at rated CW output: 16A at 120 Vac
Normal line current at 1200 watt PEP output: 17 A at 120 Vac
Power line current in standby: 0.9 A at 120 Vac
Tube:
Type: 3CX800A7
Continuous dissipation: 800 watts total
Warm-up time: 180 seconds
Estimated life: 8,000 hours ICAS
* Exact performance measurements may vary due to the accuracy of test equipment and the measurement methods
used. (Prices and specifications subject to change without notices.)
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AL-800 Instruction Manual
Metering:
Multimeter: Peak forward power (continuously). The second scale switches between peak
reflected power (and SWR), ALC threshold, ALC output voltage, and high voltage.
Current meter: Plate and grid current (simultaneously) on separate scales.
ALC:
Negative going, 0 to 10 volts, adjustable.
Relay:
Requires contact closure or sink of +12 Vdc at 100 mA. Back pulse protected.
Connectors:
RF: SO-239
Line: NEMA 5-15P, standard 120 V three wire
Others: RCA Phono
Physical:
Dimensions: 14-1/2"D x 14-1/4"W x 8-1/2"H
Weight: 68 lbs.
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AL-800 Instruction Manual
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).
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AL-800 Instruction Manual
Pin 8 of IC1 will go negative if the grid shunt voltage exceeds the voltage from divider R311 and
R2 (chassis mounted control), or if the supply voltage sags from excessive line voltage drop. Pin 8
is followed by current buffer Q303 that sources the negative ALC voltage to the ALC output jack.
Dynamic Bias
Conventional bias circuits allow high power linear amplifiers to dissipate hundreds of watts during
low or no signal periods. This creates needless heat, since virtually no dissipation is required unless
the amplifier is being driven with large signal levels.
The AL-800 contains an exclusive bias circuit that reduces the idling (quiescent) current very close
to the tube's cut off region. The power amplifier tube in the AL-800 has a full resting period of very
low dissipation between dots and dashes on CW and between words on SSB. The lower idling
current dramatically reduces component temperatures on both CW and SSB. If only a few
milliwatts of RF power are applied to the amplifier, the quiescent current will increase. Linearity
remains excellent with this circuit because the tube's conduction angle is never reduced below class
AB.
The dynamic bias circuit is located on the Power Supply Board P/N 50-0800-H1. Diodes D101 and
D102 rectify a small sample of the RF drive voltage. This voltage is applied to the base of dc switch
Q101. If Q101's base is driven with a few microamperes of current from the RF sampling circuit,
Q101's collector will pull the base of PNP transistor Q102 low. This turns dc switch Q102 on.
When Q102 is on, zener diode D103 is connected between the collector and base of Q103.
D103 sets the operating bias. This zener applies forward bias to Q103's base whenever the collector
voltage of Q103 exceeds the voltage of D103. This forward bias will turn Q103 on harder and will
reduce the collector voltage. If the collector voltage is less than the breakdown voltage of D103,
Q103 will move towards cut-off and the collector voltage will increase. Q103 is, in effect, a current
buffer for zener diode D103.
Timer - Overload Circuits
CAUTION: Never modify or disable protection circuits without factory approval. Doing so
with void all manufacturer's warranties.
Grid Overload Protection
This amplifier contains a fast acting electronic grid protection circuit to enhance tube life. The
amplifier will quit operating and the "OL" and "XMT" LEDs will both illuminate if the safe limit
of grid dissipation is approached or exceeded. The grid overload protection circuit is reset whenever
the "STBY-OPR" switch is placed in the "STBY" position.
Note: This circuit has a much faster response time than the grid meter. This circuit also
measures the peak grid current. The operator may not observe the grid current
meter climbing before the overload circuit responds and disables the amplifier.
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AL-800 Instruction Manual
This circuit is located on the Timer / Overload board P/N 50-0800-5. IC501 (pin 5) senses the
voltage across the grid shunt (power supply board, R106) through terminal 1. If the voltage at pin 5
exceeds the regulated voltage at the junction of R505 and R511, the output of IC501 (pin 7) will go
high and turn on Q504. Q504 will energize the coil of RL501. RL501 will latch "on" and remove
the voltage from terminal 3 and relay RLY101 (power supply board).
Thermal Overload
The transformer in this amplifier contains an internal temperature sensor. This sensor disables the
transmit function if the transformer's temperature ever happens to approach the limit of safe
operation. Thermal overload is indicated if the "OL" LED illuminates without the "XMT" LED
being illuminated.
The amplifier will not operate during a thermal overload indication. The thermal protection system
automatically resets after several minutes of a non-transmitting cool down period.
Cathode Warm-up Timer
Indirectly heated transmitting tubes normally require a few minutes of "warm-up" time before plate
current can be drawn. This protection is provided by IC501 (see Tube Life on page 10). Pin 2 of
IC501 is connected to a regulated reference voltage. C509 begins to charge when the main power
switch is placed in the ON position. The voltage at pin 3 of IC501 will exceed the voltage on pin 2
after approximately three minutes, causing the output of IC501 (pin 1) to go high.
Q503's collector pulls low whenever the output of IC501 (pin 1) is high. When the collector of
Q503 is low, Q502 and Q501 are forward-biased. This allows 12 volts to appear at terminal 5.
Terminal 5 supplies 12 Vdc to the keying circuits and the plate and grid current meter lamp.
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 making modifications that may affect tube
life.
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 15 for details).
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AL-800 Instruction Manual
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 highÂvoltage-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.
Multimeter
The multimeter is the meter on the right. It continuously reads the forward peak envelope power on
its left-hand scale (FWD). This scale is calibrated in 100 watt steps up to 2 kW.
The right-hand scale of this meter serves multiple functions, including measurement of the high
voltage (HV), reflected power (REF), SWR, ALC voltage output (ALC), and relative ALC threshold
(ALC SET). These four metering functions are selected by the "MULTIMETER" switch. See
Multimeter Functions for more information.
Multimeter Functions
HV: The multimeter will indicate the dc plate voltage of the PA tube when in the HV position. The
correct scale to use is the ACL / HV scale, which has a picket every 100 volts. Two zeros must be
added behind the numbers indicated on the meter scale (i.e. multiply by 100), so that "25"=2500
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AL-800 Instruction Manual
volts and "20"=2000 volts. Do not operate the amplifier if the high voltage is over 2700 volts with
the amplifier on standby. See the Transformer Connections section on page 15 for
information on correcting excessive high voltage.
REF: The multimeter measures the antenna (or load) peak envelope reflected power in the REF
position. The full scale reflected power reading is 500 watts. This scale is marked every 10 watts
below 100 watts, and every 100 watts from 100 to 500 watts.
Note: The SWR of the load can be measured when the "MULTIMETER" switch is in the REF
position by observing the different red SWR curves. The forward and reflected meter
pointers will cross each other on, or near, the correct SWR curve.
ALC: The multimeter measures the output voltage of the ALC detector when in the ALC position.
The full scale ALC reading is 35 volts and is read directly from the ACL / HV scale. The meter
should flick upwards occasionally during normal ALC action. See ALC Metering,
Controls, and Adjustments operation on page 19 for more information.
ALC SET: The multimeter measures the approximate grid current that will produce ALC activity
when the meter switch is in the "ALC SET" position. One zero must be added to the reading on
the ACL / HV scale and the result divided by 2 for this function. For example, ALC action will
begin at approximately 75 mA of grid current (typically between 60 mA and 90 mA) when the
"ALC SET" control is adjusted until the meter reads "15."
Wattmeter Circuit
The AL-800 wattmeter circuit uses an accurate directional coupler followed by a true peak detector
circuit. This circuit will accurately determine the true peak envelope power (PEP) of normal voice
waveforms. If the load SWR is high, the true power reading can be obtained by subtracting the
reflected power from the forward power reading.
Peak Envelope Power vs. Average Power
Peak envelope power (PEP) is often misunderstood. PEP is not associated with the sine wave or
root-mean-square (RMS) power, and it has no fixed ratio to average power. There are no formulas
that allow accurate conversions between average and peak voice waveform power.
PEP is the power at the crest (highest point) of the RF waveform. On SSB, the average power can
vary from a few percent of the PEP to over fifty percent of the PEP. Generally, the PEP on SSB is
two to five times greater than the average power. On CW or other constant amplitude modes (such
as FM or RTTY), the PEP is always equal to the average power.
Tube Life
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