allanhurst:
But the noise output will quite be small...
... I'm not so sure. I'm hoping that the noise will be huge (say 100mV peak). I've toyed with a 20V Zener diode. If you just add a simple voltage follower and shove it up a small tranny speaker, you can actually hear the hiss! It's quiet, but quite audible nevertheless. Pretty amazing to hear pure quantum mechanics going on atop your bench! The mechanism seems similar.
I can't just yet power up the tube as I need to build a buffer /safety circuit for my PC which I'm using as a rudimentary oscilloscope. I just know that I'm going to blow it up anyway.
ERNIE1 amplified the regulator noise via two pentode stages but they were probably running the rest of the gating hardware on higher voltages than 5. Assuming 50 x 50 gain, that is a lot though. I have the same pentodes to experiment with.
That was the point of the question. I was going to output a PWM signal to an analogue volt meter and use that to indicate if the noise signal was clipping the input range. You would then be able to adjust the level to a nominal setting. It's starting to look as if I was worried unnecessarily.