LF Amplifier - low power output

[seymour12]

Hi guys,
recently I have made what is supposed to be a 250W power amplifier for the 137kHz LF band (for learning purposes only, link to schematic is below). I am using a 30V 5A 150W SMPS as the main power source, and a 15V 2A linear PSU for the bias/pre-amp. The amplifier seems to be working fine, except its putting out significantly less power than expected. With the 150W SMPS I can't get over 50W on my 50 Ohm dummy load (rated for 250W), while my SMPS shows full power consumption (for example, 5A at 30V). I have a LPF connected between the amp output and the load. At the load I have a clean sine wave. Does anyone have any ideas? The efficiency this way is basically 30% which is very low, I am losing the power somewhere. I'd appreciate any input, thanks.


S

 

[Externet]

What components get hot ?
The supposed 250W are peak or RMS ?
Supplying 30V x 5A = 150 W power in, times efficiency will never go beyond 150 W.

 

[seymour12.]

The common-mode choke (T2) is quite hot, as is the heatsink on the transistors. The dummy load also gets warm after a while. The 250W are supposed to be RMS, if I understand correctly, but this requires a stronger PSU than I have (30V at 12A or so). I understand that with my PSU I cannot exceed 150W, that is not my aim. With an expected efficiency of 60-70 % I am aiming for a 100W RMS with my current setup, of which I'm only getting 50.

 

[Externet]

That common mode choke is.... in the power supply ?
Are you feeding a high enough level signal to the amplifier input ? If you can raise that input level, what happens ?
What is the amplifier consumption current in Amperes while delivering 50W ? Is the 30V stable ? Please measure.

It has been a while I want to build a transmitter at 137 KHz, but only at 1 Watt. Perhaps some day...

 

[seymour12.]

The common mode choke is part of the amplifier - it splits the supply between the two stages. In the schematic i attached in my first post it is designated as T2.
I have tried adjusting the bias and also the signal input level, but at one point the output just doesn't increase beyond 50W, while my PSU is putting out 5A at 30V, so 150W. The 30V are stable in that case.

 

[73's de Edd]

Are you using proper frequency relevant design protocol for the wire, windings, core and construction of T1-T2 and T3 ?

 

[Sir Kit Board]

How are you measuring the output power?

Can you show the circuit diagram for the LPF?

 

[seymour12.]

@73's de Edd - I am following the original construction techniques as specified for this amplifier, which is a well tested design. The ferrite cores for T1/T2 are a bit larger than what was originally used but work fine at these freq's, and the output toroid T3 is also a bit larger, it barely increases in temperature when im putting out 50W. Although as I said before, T2 runs quite hot.

@Sir Kit Board I'm measuring the RMS voltage across the 50 Ohm dummy load with an oscilloscope and computing power as Vrms^2/Rload. The signal across the load is a clean sine wave. The power used by my amplifier in total is shown on the display of my Basetech 150W variable SMPS. The schematic of the filter is attached.