Wow, it's obvious that I don't have nearly as good of a grasp of this as I thought I did! I'm going to spend the morning reading all the responses and sources and come back when I can post an informed, coherent reply!
Thank you guys for your great, informative responses.
Avalanche noise is generally an order of magnitude lower for 5V and below sources (typically zeners') when compared to 12V+ sources, even when current is adjusted for. Further the reverse bias transistor used in the OP example doesn't enter the breakdown zone until about 8-9V, though I have found a few examples of the 2N3904 (the transistor typically specified) that entered breakdown at lower voltages, the mean tends to be around 9V, with noise peaking near 12V.
The 'best' current for maximum noise varies with the device; however, the ones I have tested have generally been 5-50 uA.
Further, there have been several people who have tested a variety of such transistors, and the 2N3904 tend to produce the most noise.
I am limited to a voltage source of 9V for the final project, unfortunately, so I chose the 2N2222 as opposed to the 2N3904 because, even though it might not produced as much noise, it looks like it's easier to push into breakdown at lower voltages according to the example I cited in the OP. If I'm off base here, let me know.
I disagree. my original circuit was stolen from a 12v design. I dropped it to 5v, changed the NPN bias to get 2.5 ish midpoint and enough gain to exceed the upper and lower hystersis points of the 393. it didnt work...until I realised that the key point is the current through the noise source is quite critical. i guess most mfrs try to reruce noise and though zeners are inherently noisy there is a point on their curve where they are most noisy. we are talking nanocurrents here, any help you can get to boost the starting point is a bonus...I rebiased the zener to get the exact datasheet current point for peak noise and it burst into life....
albeit with the nyquist sample rate problem, so im not cel3brating yet, just my 2c that 5v circuits can be made noisy with a bit of care,a datasheet and a calculator
So you're using a Zener diode to generate avalanche noise as opposed to transistor breakdown? Does your circuit look something like this, getting the noise from the zener and then using the two transistors to amplify that?