Good to hear 
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I laugh because I have forgotten to put in IC chip and wondered why the module wasn’t working .
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This looks incredible. You’re like the nicer version of Eric Barbour.
I wonder if you or anyone else has tried using car radio tubes in a synth module? They called them space charge tubes… they were specifically designed to operate with 12V on the plate, to be used in automobile radios. Really interesting page here with more info: Space Charge and Other Low-Voltage Tubes
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Hey that’s a nice page. Too bad there are no european codes included, so i can’t say if i used any. I’d have to google them. I have a lot of Philips/Mullard and telefunken tubes and i’m not sure if i ever found something about special low voltage tubes by these manufacturers, so i’m wondering if it might be an american thing?
I do know that a lot of the “standard” european radio tubes were also used in car and battery operated radios. It’s why the heaters are 6.3 or 12.6 volts.
If you’re wondering about performance though, most small signal tubes work just fine on 12 volts. Especially the miniature 7 and 9 pin tubes. You do sometimes have to play around with resistor values.
High voltage is really only needed for power tubes, because it is converted in the output transformer to a higher current.
Tubes with similar characteristics will probably act similar in the same circuit. If the pinout is the same, it will often work as well. Some are way more stable than others, which can be a good or a bad thing 
Tubes optimised for low voltage application will probably work great but for the distortion modules it might not be what you want. People want clean sound in their car radios. I’m guessing the oscillator in this thread will be very stable with a space charge tube, but it works just fine with several pentodes i tried it with.
If you have any space charge tubes just try them out
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Regarding distortion, I didn’t even think about that. I want nonlinearity and brokenness from my tubes, so it makes sense that running 12V tubes on 12V is just… running the tubes as they were meant to be, which is the opposite of interesting (in my mind).
I’ve got a few heptodes/pentagrid converters on hand and I’m going to try making some kind of module with them. I’m thinking some kind of chaotic LFO type of thing, with audio in to the first grid and CV in to the second, and maybe have the audio input normalled to an internal oscillator of some kind. Essentially using the tube as it was designed to be used, just with completely different types of signals?
I am a totally self-taught amateur though, I have no degree or schooling to speak of so I might just be inviting severe frustration?
Edit: also I feel like an idiot, I just read over your VCO post again and realized you are indeed using a car radio tube already in that circuit.
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That lfo idea sounds very cool, i’ve never heard of it being used at such low frequencies though. On the other hand, I don’t see any reason why it would only work at radio frequencies.
Heptodes are also cool for logic gates. I’m still looking for a way to make vca’s with them, which also shouldn’t be hard. The youtube channel Usagi Electric has some cool videos about tubes at low voltage if you want to check something out.
Also, there is a schematic on google for a ech83 guitar preamp. I put sam’s fuzz for a fiver in front of it on a dual pot. It’s not quite optimal for synth levels yet, but leaves room for experimentation
Usagi Electric is actually where I learned about heptodes and got the idea from!
I have a 1R5 heptode on hand, it was designed for use in battery-operate radios and has a heater voltage of only 1.2 volts. No idea if it’ll work but we’ll see?
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Does the EF80 oscillator of the schematic in the first post really pull 0.45A? Was it measured by you or someone else?
Today I acquired an EF80, and came across this thread. But if it really pulls 0.45A, then it is not very suitable for my current case, and I won’t bother with this schematic.
Came to think of Cat Girl’s tube VCA. It’s updated by now and is named CGS765.
It utilizes a Soviet 6Ж24Б (1J24B) tube.
I got the old model but haven’t tried it yet …
The EF80 is specified to draw a filament current of 0.3A at 6.3V according to my ‘Electronic Tube Handbook’ published in 1979 by “De Muiderkring”.
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Thanks Jos! That number is at least a bit better. 
Maybe I should just give 8lit a go. Woupd be a waste not to use that beautiful tube.
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Note, the current is the current when switched on (not when switching on). Make sure to use a resistor in series to the filament to reduce the inrush current. When cold the resistance of the filament is much lower than when warm. In my versions of the valve caster I use a large (being able to withstand the heat the filament current will produce) 15 Ohm resistor.
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