allanhurst:
any idea what pentodes they used?
CV4002s are military wired versions of the pin based CV4014 that I'm using. I've consulted some valve aficionados but no one seems able to guess the operating frequency of this circuit. I'm concerned with the small 1nF inter stage coupling capacitors. They suggest frequencies above normal audio. The RC constant on the cathode resistor /bypass suggests perhaps a minimum of 16KHz. I'm hoping to re spec the bandwidth of the amplification stages but valve design is really hard 
I think that I'll have to resort to the Monte Carlo methodology and just try some stuff. It's perhaps apt to use randomly chosen components for a random number machine...
I'd really really like to avoid MHz frequencies as I only have a PC and hammers. The Arduino can only sample 5kHz analogue anyway.