You can turn the voltage up to 15 Volt without any problems. The circuit diagram advises a power supply voltage of 6 Volt but I use it with 10 Volt and I feel that works better. I set it all up on a bread-board and it worked like a charm. For the NPN's I used the 2N3904's (I also tried the BC547 and it works just as well) and for the TUP I used a BC559. I tried it with a 1N5819 and that works just as well. I used a AA119 for the diode, but you can also use a Schottky Diode if you don't have a Germanium one. TUT is the Transistor Under Test, TUN stands for Transistor Universal NPN, TUP is Transistor Universal PNP and DUG stands for Diode Universal Germanium. (The graph for the PNP version requires you to invert the Y-channel (channel-2) on your scope, so that will look normal on almost any scope.) That's what you get with such a simple design. However most analog scopes won't allow you to invert the X-channel, usually only the Y-channel, so you'll just have to get used to that. The traces are displayed from right to left which is the wrong way around for a normal graph display but you can change that by inverting the X channel on your scope.
Mind you, I only tested it on a bread-board so if you build a print for it, it might yield a better result. This gives you a bit more control over how the traces look. (However you won't have this problem if you use an analog scope.) But the plus point is that you can easily make the frequency adjustable with a 555. I've tried it with a 555 but on a digital scope the lines are a little bit thicker than with a transistor based square-wave generator. You could replace the square-wave generator formed by T1 and T2 for a NE555 square-wave generator if you wish (beware of the input voltage if you do, so you don't blow up the 555 and make sure the duty cycle of the squarewave is 50%). Instructions for that are at the bottom of this article. You can build a PNP version of this circuit. However it does not work with FET transistors, like Jfet's, P or N channel mosfets etc. This circuit can handle all sorts of components, NPN transistors (bi-polar junction transistors), diodes etc.
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