OM3501A Power Amplifier.

I have been using the OM3501A PA since summer 2019. The amplifier is very solidly made. Large housing and high-quality components, as well as protection for almost all possible accidents. The amplifier's protective system protects the GU78B tube. If the tube is damaged, the amplifier's protective system protects the amplifier from damage :). The amplifier is fully automatic and never forgets to switch and tune the antenna. Control is done via a computer, and the PA does not have to be on the desk, it can be placed anywhere where there is a connection to a computer network or the Internet. I like this. This is best described on the manufacturer's website.

Modification for the GU-78B cathode heating power supply.

When using the amplifier, a voltage drop in the power grid is noticeable. It can even be 10%! The amplifier displays a yellow message "Heating voltage error". This is bad news, the destruction of the cathode and the significant reduction of tube life begins. In addition, when the cathode emission is reduced, the power of the amplifier decreases. The modification involves the use of a stabilized power supply to heat the tube cathode. I used the ready Mean Well RSP-150-27 power supply. And now we have realized two important factors extending the life of an expensive tube. The tube cathode heating has a soft start, since the power supply limits the maximum current of the cold heater. The tube cathode heating is stabilized and regulated, with -3% or 26.2 V DC setting. The optimal cathode heating supply voltage is: -3 + 1% depending on the operating time and the condition of the tube. It is not worth following the tube manufacturer's instructions for a heating voltage of 27V ± 5% - this guarantees only 1000 hours of operation. We mount the power supply under the front panel. Here is the wires for connecting the cathode heater, just connect the 230V AC from amplifier power transformer TR1. We disconnect 27 V AC from the TR1 transformer and connect the existing 10A fuse. You must adjust the resistor in the voltage measurement system through the amplifier controller. This is the R14 resistor on the INPUT BOARD. I changed from 47 kOhm to 37.4 kOhm. This is because I changed the cathode heater power supply from 27 V AC to 27 V DC. The cost of modification is only 10% of the price of the tube.

Installation of Mean Well RSP-150-27 power supply

Mean Well RSP-150-27

Calibration of the heating voltage meter


Connecting the heating fuse

Heating fuse

Hot air stream from the GU-78B cooling system output

The temperature on the display is not the actual tube temperature! This is the relative temperature for the controller, based on which the additional fan is turned on. The DS18B20 digital sensor used in PA measures temperatures from -55°C to + 125°C. I used a digital thermometer with a PT100 sensor that measures up to +400°C. The extract air temperature from the tube cooling system is 150 ++ ° C! It's the air temperature, the tube itself is even warmer. The maximum surface temperature of the tube is limited by the manufacturer to 200°C. The additional fan turns on at 60°C (actually about 120°C) turns off at 55°C (actually about 100°C). I noticed that an additional SUNON DPA200A fan lowers the actual air temperature at the tube outlet by 15-17°C. No problem in SSB mode, the air temperature rarely reaches 90+ ° C. In digital modes it is worse, with FT8 I have 120+° C  after 3-4 minutes and an additional fan works all the time. On RTTY 170+ ° C after a few minutes.

We check the cooling of the tube

The amplifier for cooling the GU-78B tube uses a blower from ebm-papst model D2E097-CH85-48 and SUNON fan model DPA200A 2123XBL.GN. The blower air flow is 180 m3/h in the free air. The GU-78B heatsink causes pressure loss. The amount of loss depends on the air flow. So the real air flow of the blower will be 140 m3/h, at pressure losses around 100 Pa. The SUNON fan has an air flow of 165 m3/h at 85 Pa. This will partly compensate for the loss of pressure. The total airflow in the cooling system will be around 170 m3/h. See the charts below. The GU78B tube with a full power dissipation needs an air flow of 320 m3/h and with pressure losses of 55 mmH2O (550 Pa). The maximum tube temperature is 200° C, the working temperature recommended by the manufacturer is 150-160° C. If we heat to 180-200° C, the tube will not work for a long time. This is not an electrical parameter to be exceeded :). The graph below shows the power dissipated on the tube. The amplifier cannot work continuously at full power.

And that's why the user manual says:

Notice 1: If the tuning process takes more than 1 minute, allow for a short break to prevent temperature overloading of the PA.


PA cooling

Bias current of the GU-78B tube

The OM3501A amplifier automatically sets the bias current to 500 mA. We can also set the current in the service menu manually - “SET EBS2 MANUAL”. The tube manufacturer recommends a bias current of  850 mA. Are we checking what effect the current has on the output signal quality? The output signal is analyzed with the RF Vector signal meter SMΩRF from MicroHam. I use two sensors, an input and an output amplifier. With it we can measure IMD3 in two tone signal. Transceiver used for measurement - Yaesu FTdx101D generates a signal with an IMD3 of -41.7 dBc. The result may be different for each tube. Measurement results in the table.

Bias current mA






OM3501A  IMD3 dBc






Gain dB






Tube plate power dispation kW






Conclusions: We have to choose better signal quality or lower amplifier power consumption. CW, FT8 - 500 mA tube bias currents will be fine. SSB - 750-800 mA required. Can be adapted to the preferred mode.