1000W Tube Amplifier Interference Pattern of the MV-Rectifiers

The line frequency is 50Hz - when the signal frequency is turned to 25Hz, the pulsing load causes this interference pattern through the mv-rectifier tubes. In some way this visualises the 120 degrees phase angle. The frequency counter is a HP 521 full tube device, the noise you hear, is caused by its fan. But you can also hear the load choke during low frequency interfernce.