Simple Tools: Detecting Oscillation Above The Audible Range

There are a lot of folks around who think that a scope and a signal generator are the answer to every problem large or small, and you should have one before you go to brush your teeth or eat breakfast.....where was I? Oh, I remember.

On a website which shall remain nameless, a person had a problem that was related to tube rattle and a poster suggested it was "hyper sonic oscillation" and that the guy had to have a scope and a signal generator to suss it out.

Problem is, most amp owners, unless they're going to get into the trade, don't have a scope or a signal generator and wouldn't know what to do if they did have one.

Here's what I answered.

You don't need a scope and a signal generator to detect any high frequency oscillation above the audible range. All you need is a little common sense and some simple tools.
Here is what you do.
Hook up your bias checker and start at a low level with no signal input. While you're doing this go and get a transistor AM radio with an antenna (remember them?). Tune it between a couple stations and turn it up about half way. Lay it on top of the amp with the antenna extended.
Then advance the volume in your amp until you're reached full tilt.
If there is any HF oscillation going on two things will happen.
First, your bias current will go crazy and all the frequencies radiated by the oscillation will make your transistor radio hiss like a cobra.
I figured that out all on my own while building a tube matcher and have since used that trick on several amplifiers. I'm sure the old radio men knew all about it too. 

The moral of the story is this. All of the problems we have with guitar amps today have existed ever since there have been amplifiers with tubes. Most of the old radio men like Everitt Lincoln have passed on to the great ham shack in the sky, but they knew that there were plenty of ways to skin any cat you might come across. This is one I figured out on my own but I am sure they were using it too.