Work on the circuit continues! An unexciting but important problem to be overcome in any electronic designs is that different chips and components require different voltage supplies if they are to function properly and not explode. We want to be able to power the finished synth from 9v (eg battery or a standard guitar FX pedal power supply) and also 12v for modular synth compatibility. However the AM radio part of the circuit can't handle more than 6v and some other parts we are working with need 5v. So we need to somehow divide the voltage. There are a few methods of doing so. One which appealed to me because it's a counterintuitive hack using very few components is to misuse an LM386 amplifier IC as a voltage divider (an idea I found on this forum thread). You can just connect the +9v and ground pins to your power source, don't put any input signal to the amplifier and it sends out half the voltage! But though it's small and easy, it's not all that accurate and can only chop the input voltage in half rather than give you exactly 5v, so in the end we decided it's better to do it properly with a real voltage regulator. These only require the addition of 2 capacitors to get them working, simpler to use than I expected. For testing we tried the NCP7805 as pictured above, but that is overkill for our power requirements and would take too much space on the board, so we switched to the similar but smaller 78L05. Below you can see it putting out 5.04v, definitely close enough for our purposes. We have been using this to power our test circuits.
In other news, we have the radio part of the circuit working very well now, amplified/distorted using a CD4049 Hex Inverter (another classic creative misuse of an IC, as popularised in Nicolas Collins book Handmade Electronic Music). It picks up both noise and discernible radio signals in an interestingly chaotic way and is highly playable by touching the circuit or even waving hands over it.
For our general research into radio-electronics-as-noise-instruments, and low-voltage tube amplifier + distortion circuits, radio expert Mike Kana very kindly sent us a wealth of rare and interesting vintage components which will be very useful for future Isn'tses circuits! Thanks Mike!