OM POWER OM2500 HF User manual

Instruction Manual
OM2500 HF
SHORTWAVE POWER AMPLIFIER
OM POWER, s. r. o. 930 30 Báč 126
SLOVAKIA
E-mail: om[email protected]

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TABLE OF CONTENTS
1. GENERAL INFORMATION
1.1. Introduction ……………………………………………………………….. 4
1.2. Specification …………………………………………………………….... 4
1.2.1. Parameters …………………………………………………………………. 4
1.2.2. Protection Circuits ………………………………………………………… 4
1.2.3. Indicators ………………………………………………………………….. 5
2. SAFETY INSTRUCTIONS ……………………………………………… 5
3. GENERAL DESCRIPTION ………………………………………………. 6
3.1. HF part ……………………………………………………………………. 6
3.2. Power Supply …………………………………………………………….. 7
3.3. Safety Devices ……………………………………………………………. 8
4. INSTALLATION ………………………………………………………… 8
4.1. Grounding ………………………………………………………………… 8
4.2. Coaxial Cable …………………………………………………………....... 8
4.3. Control Cable ………………………………………………….…………... 9
4.4. Main Supply ………………………………………………………………. 10
4.5. Cooling ……………………………………………………………………. 10
5. OPERATION ……………………………………………………………… 10
5.1. Operation Elements ……………………………………………………….. 11
5.2. Tuning of Power Amplifier …………………………………………….. 12
6. MAINTENANCE ………………………………………………………. 14
6.1. Indication of Fault Conditions …………………………………………. 14
6.2. Fuse Replacement . .………………………………………………….... 15
6.3. Tube Replacement …………………………………………………....... 16
6.4. Cleaning ……………………………………………………………...... 16

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7. APPENDIX ……………………………………………………………... 16
7.1. Band Blocking …………………………………………………………… 16
7.2. Block Diagram of OM2500 HF Power Amplifier (USA version) ………... 17

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1. GENERAL INFORMATION
1.1. Introduction
The OM Power model OM2500HF is a manual tuning, full legal limit amplifier, designed for
heavy duty use on all short wave amateur bands from 1.8 to 29.7 MHz (including WARC
bands) and all modes with no time limit. It is equipped with one GU84b ceramic tetrode.
1.2. Specification
1.2.1. Parameters
Frequency Coverage Amateur Bands 1.8 –29.7 MHz including WARC
Power Output 1500w PEP continuous output all modes - *no time
limit*
Input Power 40 to 60W for full output power
Input Impedance 50 Ohm, VSWR < 1.5 : 1
Power Gain typically 15 dB
Output impedance 50 Ohm unbalanced
Maximum output SWR 2:1
SWR protection: automatic switching to STBY , when reflected power is
350W or higher
Intermodulation distortion 36 dB below nominal output
Suppression of harmonics < -52 dBc second, < -65 dBc third
Tube GU84b Ceramic tetrode
Cooler Centrifugal blower + axial blower
Power supply 240 V - 60 Hz
Transformers 2 pcs of toroidal transformer 2,0 kVA
Parameters 485 mm x 200 mm x 455 mm [19.1” x 7.9” x 17.9”]
(width x height x depth)
Weight 38 kg ( 84 lb )
1.2.2. Protection Circuits
There are 8 special protection circuits used in the amplifier. They are activated when one or
more of next parameters exceed defined values or some unwanted occasion occurs.
VSWR too high
Anode current too high
Screen current too high
Grid current too high
Mistuning of PA
Hot switching protection
Soft start for protecting your fuses
“switch-on blocking “ at opened amplifier

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1.2.3. Indicators
There are couples of LED and bar graph indicators visible on the front panel to inform you
about value of some parameters or operation status:
Bar graph indicators Power output - 50 LED
Reflected power –20 LED
Current at screen Ig2 –10 LED
Anode voltage, anode current, tuning –30 LED
LED Indicators Current at control grid (Ig1 –2 LED)
WAIT –preheating of tube (180 sec)
STBY –standby
OPR –operation condition
FAULT –failure, switching off for abt. 4 sec
2. SAFETY INSTRUCTIONS
WARNING!
DANGEROUS HIGH VOLTAGE!
The power amplifier is using high voltage up to close 3000V DC, which is very
dangerous for human life! Read next safety instructions carefully first, before you will
start to install and operate power amplifier! NEVER VIOLATE NEXT RULES!
WARNING! NEVER ALLOW CHILDREN to play around PA or to touch power
amplifier or connected cables in working condition, or to push anything into the case
holes!
WARNING! The amplifier contains high voltage circuits. Never turn the amplifier on
without the upper lid in place. DO NOT ATTEMPT TO SHORT OR BYPASS safety
switch under upper lid!
WARNING! The OM2500 HF amplifier is neither to be used in a WET or HUMID
environment nor to be exposed to RAINFALL!
WARNING! Do not turn the amplifier on without having connected the ANTENNA or
properly rated DUMMY LOAD! A hazardous HF voltage may build up on the antenna
connector after turning the amplifier on with no antenna or dummy load connected!

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WARNING! Before opening the upper lid of the amplifier make sure that power supply
has been disconnected AT LEAST 10 minutes allowing the electrolytic capacitors to
discharge fully. Disconnect power cord from the outlet!
CAUTION! The amplifier must be installed in such a way that free flow of hot air from
the tube is allowed. The amplifier must not be installed in a constrained surrounding
(i.e. tight shelves etc.)
CAUTION! The amplifier must be properly grounded during operation.
CAUTION! During operation the amplifier must be installed in such a way that the rear
power supplies are accessible.
CAUTION! The amplifier is an A category product. In a household it can influence
other electric appliances. In such cases the user is to take proper actions to mitigate this
disturbance.
CAUTION! Make sure that all screws holding the case together are properly in place
and tightened before carrying the amplifier with the handles.
CAUTION! Read this manual carefully. Fallow all of instructions during installation
and operation to avoid damage to the amplifier not covered by manufacturer´s
warranty! Do not attempt to perform any change of hardware or software!
3. GENERAL DESCRIPTION
3.1. HF part
In this amplifier a tetrode GU 84b is used in a grounded-cathode circuit (input into control
grid). This amplifier achieves excellent linearity by the voltage stabilization of the control
grid bias and the screen voltage. The power input is given to the control grid, using a
broadband input circuit with an input impedance of 50 Ohms. This adaptable input circuitry
ensures a good input SWR (better than 1.5:1) on all amateur bands.
The output of the amplifier is a Pi-L circuit. The ceramic capacitor for TUNE and LOAD are
divided. This enables the amplifier to be tuned exactly and makes it possible to easily return
to the previously set positions after band changes.

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Top view on the opened OM2500 HF
Tetrode GU84B PWR meter Blower Switch-on board
Output Pi-L Circuit Tuning capacitor Power supply board
Subpanel
3.2. Power Supply
Power supply of the amplifier is comprised of two of 2,0 kVA toroidal transformers. A soft
start is provided using relays and resistors.
The high anode voltage is made by combining 8 x 350 V (total 2800V) @ 2A. Each has its
own rectifier and filter. In the high voltage circuit, safety resistors are employed to protect the
amplifier against overload.
The source for screen grid is regulated by parallel stabilization with BU508 transistors and
delivers a voltage of 360V at 100mA.
The -120V for the control grid is regulated with zener diodes.

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3.3. Safety Devices
Control and monitoring circuits ensure control and safety during malfunctions of the PA.
These are on the Control board, which is located on the chassis subpanel.
4. INSTALLATION
NOTE
Read this chapter carefully prior you will start installation. Before unpacking inspect shipping
woody container first, if it is not damaged. Keep all of packing parts for possible future
shipment. Check unpacked power amplifier. If you find some damaging, contact your dealer
immediately to keep full warranty.
During installation go step by step according to next parts.
4.1. Grounding
CAUTION
The amplifier has to be grounded properly! Connect the screw on the rear panel of the
amplifier to your local grounding system with a copper cable, use a cross-section of 4
mm2 at least.
Connect your transceiver to the same grounding system of your shack carefully!
Use minimum length cables and make certain that the connections are both physically and
electrically sound. With poor grounding, you may risk damaging your equipment, having
problems with TVI/BCI or your transmitted signal may be distorted.
4.2. Coaxial Cable
The output of the transceiver is to be connected to the input of the amplifier via RG58 or
similar cable. For the connection between the power amplifier and the antenna, RG213 or
similar coaxial cable suited for high power is recommended. Both the INPUT and OUTPUT
SO-239 sockets with Teflon insulation is used.

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Rear view of the amplifier OM2500 HF
MAINS CABLE FUSES OUTPUT
CONTROL INPUT
4.3. Control Cable
Control cable maintains TX / RX switching of the PA (TX GND). The cable is shielded. On
the side of the power amplifier a CINCH-socket is used. On the side of your transceiver you
have to use a socket suitable for this transceiver. During transmitting the middle pin is
connected to the ground.
The relays of the OM2500 HF have to be switched earlier than HF is applied (cold switching).
Modern transceivers they have a time delay between PTT switching and power output.
CAUTION
If you are using and older transceiver or transmitters without time delay, we
recommend to connect the PA in such a way that the transmit/receive switch (foot switch
for example) is connected with the KEY IN socket of the amplifier. The KEY OUT
socket is to be connected with the PTT socket at the transceiver.
The amplifier is equipped with two safety devices, which ensure that the output relay is not
switched under power mistakenly (hot switching).
KEY IN RCA Phono - Input signal PTT switching voltage / current 5V /2 mA)
KEY OUT RCA Phono - Output signal PTT (maximum switching of 30V / 50mA)

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4.4. Main Supply
WARNING!
Power Amplifiers OM2500 HF are manufactured for several different power systems.
For US territory only one mains cable is used for two phases (2x120V) system. Cord is
not removable from PA. Amplifier is equipped with two mains fuses (see rear panel).
CAUTION
Be sure you got PA with properly terminated line cable, corresponding with your power
system’s outlet. If not, contact your dealer. In such a case you should make the necessary
changes using a licensed electrician.
WARNING!
Be sure that your power system is correctly wired and properly rated! To use
adequately sized and connected grounding system is also very important.
4.5. Cooling
CAUTION
The amplifier must be installed in such a way that free flow of hot air from the tube is
allowed. Do not obstruct air intake and exhaust areas of the PA.
The centrifugal blowers provide the necessary cooling of the amplifier, even during long
contests. The main blower is activated by switching the PA on and it is turned off when
cooling is finished (approx. 1-5 min after switching off the PA depending on the temperature
of the tube). The supplemental fan is turned on depending on the temperature of the air exiting
from the amplifier. It is switched on at 70°C and switched off at 60°C.
5. OPERATION
WARNING!
Before switching PA on, make sure that amplifier is grounded, antenna or dummy load
is connected, and line cord is putted to the outlet. Do not allow children or unauthorized
person to go close to PA, to touch cables or connectors, to push anything into the case
holes!
CAUTION
Before switching PA on, check all connections between PA and TCVR.
CAUTION
Do not turn PA on for at least 2 hours after unpacking it and locating in its operating
location. Especially when amplifier is moved from a cold place to a warm one because

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not visible condensation may develop, and this could result in damage to the high
voltage circuits of the PA.
CAUTION
Never try to change antenna output during a transmission to avoid warranty loss.
NOTE
When you decide to have a short operating break, place the amplifier in the standby
mode rather than switch it off.
5.1. Operation Elements
There is couple of operational elements accessible or visible on the front panel.
BAND- Band selector switch
TUNE - Anode capacitor for tuning, tuning of higher frequencies to "0",
lower frequencies to „100“.
LOAD - Output capacitor tunes antenna load resistance to amplifier.
Capacity is low at „100“ and high at "0" on the scale.
OFF - You switch off the amplifier by pressing this button.
ON - You switch on the amplifier by pressing this button.
After 3min of warm-up delay, the amplifier will be ready for operation.
OPR/STBY „OPERATE“ sets the amplifier ready for transmit operation.

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In STBY, if WAIT-LED is on or the amplifier is OFF, the amplifier
is in bypass-mode and your transceiver is directly connected to the
antenna. Maximum allowed power in bypass mode is 400 Watts!
RF OUTPUT Bar graph –shows output power .
REFLECTED POWER Bar graph –shows reflected power from the antenna.
Maximum level is 350W otherwise amplifier switches to
STANDBY mode.
Ig2 Bar graph –measures the current of the second grid from -20mA to
+80mA.
HV/IP/TUNE Bar graph –measure the anode voltage, anode currency or tuning of PA.
5.2. Tuning of Power Amplifier
The OM2500 HF amplifier is operated in class AB. Thus it’s possible to obtain a maximum
output power at excellent linearity. For this purpose the amplifier has to be tuned carefully.
WARNING!
Before starting tuning you have to check if the right antenna or a 50 Ohms load
resistance is connected at the antenna output!
CAUTION
The operation of a mistuned PA will cause malfunctions, the increase of grid current
(the GRID-MAX-LED will light up) and problems with TVI/BCI.
CAUTION
The grid-current is shown with 2 LED diodes. It’s normal if the green LED is flashing or
may be shining a little bit during peak operation. If you overload the amplifier the
output power increases the grid current at very small rates and the red GRID-MAX-
LED is shining and the safety devices switch the PA to STBY. You must decrease the
input power.
CAUTION
In SSB mode you will have good output power if the green LED lights up a bit. The
current of screen grid is measured and shown in a Bar graph Indicator. The amplifier
has to be tuned in such a way that the current is between - 20 mA to +50 mA. At
currents beyond these values the operating point will be shifted and IM-products will be
rapidly increased. If a value of + 70mA is exceeded, the safety devices will switch the
amplifier to STBY mode.
1. Set the multimeter switch to the HV position
2. Set the OPR/STBY switch to the STBY position
3. Press the ON button
The amplifier is prepared for operation with the following automatic steps:

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The toroidal transformers are switched on step by step.
The cooling blower for the final tube is switched on.
The multi-meter bar graph measures the high voltage; the normal value is 2.8 kV
The WAIT LED lights up
CAUTION
After switching on, please confirm that the blower is operating properly. Air must be
flowing from the ventilating aperture above the tube. If there is any concern, or no air
flow, press the “OFF”-button immediately !
Heating the tube takes about 150 seconds. After this time the WAIT LED goes out and the
amplifier is ready for operation.
4. Reduce the power output of your transceiver to the minimum!
5. Switch OPR/STBY to OPR position (OPR LED lights up)
6. Choose the TUNE position of multimeter
7. Transmit (mode CW) and increase driver power to 10W (OUTPUT power will be abt.
500W)
CAUTION
If the input power is higher than 15W and the power amplifier is not correctly tuned,
the safety devices will switch to STBY.
After switching the amplifier to STBY, the amplifier will automatically reset and switch back
to OPR mode after approximately 2 seconds.
A tuning table is delivered with the power amplifier. Select a band with band switch and
choose the setting of “TUNE” and “LOAD” according to the table.
NOTE
Delivered tuning table was made for 50 Ohm loading of PA (dummy load). Each amplifier
should have different values depending on used frequency and used type of antenna. Make
your own table valid for your real conditions.
Band
Tune
Load
1.8
70
40
3.5
75
30
7
30
85
10
25
45
14
70
70
18
32
50
21
25
60
24
60
66
28
40
70

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8. Set TUNE knob in such a way, that the TUNE-LED lights up maximum left.
9. Set LOAD in such a way, that the TUNE LED on the TUNE scale lights up under
the “V” sign. If it is possible to obtain the LOAD in 2 positions, set the position
that is father to the right
10. Repeat tuning several times according to 8 and 9
11. Increase the input power until an output power of approximately 1500 W is reached.
12. Repeat steps 8 and 9
13. Set TUNE to maximum output power
After this procedure the amplifier is tuned correctly and ready to give 1500W output power in
all operation modes.
At optimal tuning and full output power a positive 50mA current goes through the second
grid. On 24 and 28 MHz bands optimal tuning can be achieved when one or two LEDs are lit
up to the left from the position “V”. If less output is desired you can simply decrease the load
of the transceiver.
CAUTION
Should the amplifier demonstrate any malfunctions during tuning or should it not
behave in accordance witch the description, interrupt the tuning procedure immediately
and check the amplifier! Be sure to have not done any mistakes in choosing bands or
TUNE/LOAD values! Be sure that VSWR is not higher than 2:1 and input power is
LOW!
After excluding possible human mistakes you will be able to work for long time with this
amplifier!
6. MAINTENANCE
6.1. Indication of fault conditions
OM2500 HF has the following indication LED on the front panel:
GRID MIN - indication of first gird current
GRID MAX - max. First grid current exceeded
HV - measuring of anode voltage by bar graph
IP - measuring of anode currency by bar graph
FAULT - fault
OPR - amplifier in operation mode
STBY - amplifier in standby mode
WAIT - heating of tube after switching on the amplifier
NOTE
Should a fault condition appear during the tuning or operation of the amplifier, the safety
circuits of OM2500 HF will react. The amplifier will be turned to STBY mode. After approx.
2 sec the control circuits will switch the amplifier back to OPR.

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CAUTION
If the fault will repeat 3 times after each other the control circuits will turn the amplifier
to STBY. Bringing the amplifier to OPR is enabled by using the OPR/STBY switch only.
After reaction of safety circuits, the FAULT LED will be lit up for approx. 5 sec, depending
on the nature of the fault.
Flashing LED signalizes:
IP - anode currency exceeded
HV - low anode voltage
FAULT - reflected output exceeded
GRID MAX - first grid currency exceeded
- screen grid currency exceeded
GRID MAX + HV - maximum load power exceeded
GRID MAX + IP - zero output power during tuning
HV + IP - tuning fault, incorrect tuning of the Pi-L output circuit
In case your OM2500 HF amplifier is not working correctly, please contact the
manufacturer or your dealer.
Manufacturer’s contacts: OM POWER, s.r.o.
930 30 Báč 126
SLOVAKIA
Email: om[email protected]
Dealer in USA:
Array Solutions
2611 North Belt Line Road
Suite # 109
Sunnyvale, TX 75182
Tel: [phone]
Email: [e-mail]
6.2. Fuse Replacemeent
The user is allowed to change mains fuses (6,3 x 32mm), accesible from the rear panel, only.
In the case of interrupted fuse (fuses) inside the power amplifier, contact your dealer, please.

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6.3. Tube Replacement
In the case of vacuum tube damaging, contact the manufacturer or your dealer for ordering
new one. You will get instructuions how to change tube and how to preset proper parameters,
too.
If you are not comfortable to replace vacuum tube itself, contact your Service Center, please.
In the USA call or write to:
Array Solutions
2611 North Belt Line Road
Suite # 109
Sunnyvale, TX 75182
Tel: [phone]
Email: [e-mail]
6.4. Cleaning
To prevent damage of amplifier surfaces and plastic components do not use aggressive
chemicals for cleaning. Do not open the amplifier for cleaning. Outer surface may be safely
accomplished by using piece of soft cotton cloth moistured with clean water or window
cleaner.
7. APPENDIX
7.1. Band blocking
For US market, manufacturer supplies power amplifiers with 24Mc and 28Mc band
mechanically blocked. For licensed HAMs dealer will release the lock upon request and on
presentation of a valid license.

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7.2. Block Diagram of OM2500 HF Power Amplifier (USA version)