WARNING

Valve equipment uses high voltages. Most early mains radios and televisions have a live chassis, so be bloody careful. I use an isolating transformer which not only helps to protect me but is necessary if you're going to connect earthed test gear to the chassis of the equipment. If you don't have an isolating transformer, make sure that the chassis is connected to the mains neutral. Don't go killing yourself, you might regret it later.

An AF signal generator produces an audio signal. This can be switched between a sine or square wave at frequencies ranging from a few Hertz to many thousand Hertz. OK, so you have an audio amplifier with a distortion problem. Stuff the AF signal generator into the input of the amp and connect an oscilloscope to the output. If you pump a nice sine wave into the amp, you should see a nice sine wave at the output, which will be displayed on the scope. Distortion will show up on the scope as a bent or chopped off sine wave.

Let's assume that the audio amp has three stages - pre-amp, driver and PA valve. Put the scope on the output of the pre-amp and check the display. Looks OK? In that case, move the scope to the output of the driver. Still looks good on the scope? In that case, the distortion is being produced in the PA stage. Having narrowed the problem down to one stage, you can then check voltages etc and, hopefully, find the fault.

Obviously, the output from the amp will display a larger sine wave on the scope than the input - this being the idea of an amplifier. Keep the signal generator output as low as you can otherwise you may overdrive the audio stages and cause your own distortion which will totally confuse you. And don't forget to set the signal generator to a frequency which is audible to the ear, such as 1kHz.