I just lit up my QS92 voltage stabiliser! My first valve. At the moment the noise seems to be approximately 15mVpp which is less than I expected. 50dBV gain should get me to full Arduino levels which is achievable.
I recorded the noise signal to a WAV file using the PC (44.1KHz @ 16bits). A quick entropy calculation gives me a randomness rate of 192Kb/s. Factoring down for an Arduino (10KHz @ 8bits) should be 24Kb/s. I can live with that. Beats the pants off /dev/random.