MarkT:
What is the output impedance - as I said its all about impedance.
I don't know, but the tube will run on about 2mA via a throttling resistor. The noise will be taken directly off the tube's anode and then amplified by other tubes. A pentode's grid impedance is pretty high. The biggest extraneous noise will be mains hum @ 50Hz but that will not effect the randomness extraction. RFI is irrelevant as the circuit can only sample frequencies at a maximum of 5kHz if I run the Arduino at full pelt. There's also a 100kHz SMPS for the 260V tube voltage but again that's above the sample limit.
I'm concerned about the 500Hz PWM output from the Arduino. That's right in the sample range and it might affect the analogue readings. I'd be creating my own auto correlation. Hence this question since I have scant experience of Arduinos (Arduinoes?)