I seriously thought I couldn’t get a tube sawtooth oscillator to work on +/- 12V… this seems to work just fine, with all the usual caveats of tubes. I’ve only thrown the circuit together after having simulated it. There may still be problems I’ve not yet found.
The design is a classic Phantastron oscillator with the anode output buffered with a transistor to help with the retrace time, and to prevent interference working its way back into the circuit through the output jack. The diagram shows an EF80, though I used a 6AU6 because I had one handy. Any sharp-cutoff pentode with the suppressor grid separate on a pin will do.
If you’re thinking of building this and are going to buy a tube to play with, try and get a 12AU6 - note that’s a 12AU6 and not a 12AU7. This is a 12V filament version of the 6AU6, and was intended to run off the 12V battery in cars for car radios. Though, they still used a high voltage for the radio, which was converted from 12V with a mechanically switched power supply. The 6AU6 and 12AU6 are 7 pin tubes, so get the right base for them while you’re at it.
Here’s the pinout for the 6AU6 (and 12AU6):
The major problem I’ve found with tube oscillators is offset nulling - that is, setting it so 0V = 0Hz. No more, no less. This is a huge problem when you’re playing bass notes.
The tricky circuit on the cathode handles the nulling. It’s basically a +/- 0.6V (diode drop) regulator with a trimmer connected across it so the cathode voltage can be adjusted by that much. Depending on the characteristics of the tube, you may need to increase the nulling voltage by adding more diodes in series to the existing ones, with ground in the middle. That, or use Zener diodes.
To tune, make sure your 1V/Oct converter is in tune by tuning it on treble notes. The 0Hz nulling is negligible at high frequencies. Now tune the converter so the oscillator plays a high C (or what have you). Go down the octaves and adjust the Null setting so the oscillator plays the same note.
I’ve found the null can go out of tune. It’s usually beneficial to have the null control accessible from the front panel through a hole to access it with a screwdriver.
It’ll distort differently to a silicon VCA if driven too hard. Also it’s more difficult to wreck if you give it too much voltage (assuming you don’t do that to the filaments…)
Watch out for those part numbers! The 12AU6 is for the oscillator. The VCA uses 12AU7’s and a 6BL8. You’ll need to run the 6BL8 off 6V or step 12V down to 6V somehow too.
I assume 256b meant the heater, which according to a quick googling needs 6.3V 0.45A and you’re not going to pull that current through a 10k resistor
(If you have 12 V and want 6.3V 0.45A, I guess you could put a ~12 ohm 3 W resistor in series, but not a valve guy so not sure if there’s something to watch out for if you do that.)
That’s 100% correct, a 12 ohm resistor in series with the filament will let it run at 12V at 0.45A. It’d be as efficient as a 7806 regulator, and a lot simpler.
Not wise, unless they’re the controlled heater type. They have an A after the name (12AU7A, 6BL8A, etc). You can only get them as new-old-stock. They’re intended for series strung television sets where all of the filaments run off 110V AC mains.
“Controlled” might be a bit misleading in this case, for lack of a better word. The controlled warm up type of tubes have a different filament characteristic, though I’m not sure exactly how it works (probably because it’s tube design secret sauce?). The idea is if you string this type of tube in series with others, the controlled characteristic helps prevent voltage surges across individual tubes, which would reduce life span.
I made 2 valve casters and both tubes have an individual power line for the filament (so they are not in series). To limit the initial current of the filament I put a large 27 Ohm resistor in series. When monitoring the current drawn you can clearly see the PTC character of the filaments.
How do i get it to oscillate? I just built it but it doesn’t do anything
I’m not sure if i’m feeding it right, i just inserted a voltage on the point where the expo converter connects.
Am i doing something wrong?
Only thing i changed is the 1uf capacitor to 680nf because i didn’t have one
EDIT:
I forgot the transistor, obviously. It works great now!
