This particular circuit is very useful ,especially in cases where the measurement of the amplifier output is to be done. The maximum power of the amplifier, which is to be measured must be only half the resistance R2  that acts as the  load for the amplifier.A very good result can be obtained if the meter scale is calibrated,and with a little more hard work.

Circuit Diagram & Parts List 

Tips

1.  If you cannot get a 1A no problem , make one with four 1N 4007  Diodes.
2. The resistance R2 must be rated double the power of amplifier you are going to      measure.
3. To calibrate  the watt meter, Connect the output of amplifier to to port 1.Also connect a AC voltmeter to port1.
4. Set R1 fully off.That means full counter clockwise rotation if wired correctly.
5. Adjust the output of amplifier to read 20V rms on the voltmeter.This is the equivalent of 50Watts on 8 Ohms load.
6. Set R1 for a full scale deflection on meter M1. Now fix the wiper of preset R1 using some adhesive.
7. Now decrease the voltage across the port 1 by decreasing the output power of amplifier under test  as per the the table shown below and mark the readings on the meter scale.The watt meter is now ready to measure.

 Table for Calibration