I look for fuzz on the waveform during all the power and distortion tests, but ultimately I find you need to attach a real speaker, run all different waveforms at an uncomfortable level for both me and the speaker, and look for that same fuzz. I like a nice old analog scope with a tack sharp trace. Usually you don't need all that much bandwidth, but you do need to see very subtle anomalies. I don't have a standard load to simulate a speaker and IMO the values necessary to do this are impractical. Oddly, I don't find musical signals to be of much use in ferreting out problems. I do find it useful to test the system in it's installed configuration, with real cables and loads, rather than just on the bench.
Not a lot.
Overall loop oscillations are likely to be at inaudible frequencies, but they may induce distortion which you may hear.
RF oscillations within a stage may also cause distortion, and may be heard on a nearby radio receiver. Also look for changes in DC supply current when parts of the circuit are touched by your finger or screwdriver - beware of RF burns if it is an output stage!
Overall loop oscillations are likely to be at inaudible frequencies, but they may induce distortion which you may hear.
RF oscillations within a stage may also cause distortion, and may be heard on a nearby radio receiver. Also look for changes in DC supply current when parts of the circuit are touched by your finger or screwdriver - beware of RF burns if it is an output stage!
Oscilloscope and a function generator is simply must have if you tend to do some serious business with electronics. Ok, digital storage oscilloscopes especially the quality ones are pretty expensive but analog oscilloscopes will still do the job very well. There are bunch of used scopes on ebay in good conditions for a fair price. Btw, I got mine Instek GFG-8219A function generator for 150 bucks on ebay. Almost new, slightly used 
If a function generator should not be present, you can take a simple triangle / square wave generator.
But an oscilloscope is mandatory! 20MHz is sufficient.
Test the amplifier with small signals, high signals and in overdrive.
Repeat the test with a 100nF capacitor in parallel with the load resistance (8ohm)
The amplifier must not start to oscillate.
But an oscilloscope is mandatory! 20MHz is sufficient.
Test the amplifier with small signals, high signals and in overdrive.
Repeat the test with a 100nF capacitor in parallel with the load resistance (8ohm)
The amplifier must not start to oscillate.
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OK Lineup, I reveal old trick for person who has nothing but amplifier. Get yourself an old transistor radio with AM. You know, the small ones that take a 9V battery and have a 2' or so pull out antenna. Probably a collectible today! Tune it to a place with no stations at the lower end of the dial and put it near the amp. If the amp oscillates or generates RF in the power supply section, there's a good chance you'll hear it as a growl, tone or whistle in the AM radio. Those little radios make surprisingly good RF detectors and can be used as a "sniffer" probe.
Oscilloscope and a function generator is simply must have if you tend to do some serious business with electronics. Ok, digital storage oscilloscopes especially the quality ones are pretty expensive but analog oscilloscopes will still do the job very well. There are bunch of used scopes on ebay in good conditions for a fair price. Btw, I got mine Instek GFG-8219A function generator for 150 bucks on ebay. Almost new, slightly used
OK, but is it any good ? .........
I'm not sure what do you mean. The quality of that function generator? Well, it works perfectly. No instability, clean output signal. I'm satisfied. Besides, this generator costs over 250 bucks in stores so I'll guess 150 bucks over ebay is a good bargain.
I have several such cheaper radios.
It could give a hint of oscillation.
To buy an old oscilloscope at eBay is a good advice.
People upgrade and the cost of the old oscillosope is not high.
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