Dropping resistors are inherently noisy. The more you drop across them, the noisier they are. Make sure you use a large wattage resistor, it can help.
Rectifying from AC to DC can halve your power factor as it loads down your power supply more. This means it can draw up to 900ma to power just that 450ma heater. Running a power supply near it’s limits will increase noise and ripple. Any power supply noise and fluctuations will be directly projected onto the tubes filament emission.
The Valve Wizard, in his article about filament supplies, states a minimum capacitance of 4700uF for a 300mA DC heater to get the ripple low enough. Additionaly, it is good practice to bypass electrolytics with smaller unpolarized caps in the 100nF region.
!!!Watch the inrush current if you are going to try the big capacitor!!!
If you want to try AC heaters: try using antiparallel diodes to drop the voltage. Mind their current rating though.
Edit: You can see in another topic why i don’t recommend ATX power supplies but i’m still using one. If you have several tube modules, the inrush current can be very high, it actually fried my linear power supply. It helps to have your power supply over spec, even if calculated current draw is ok
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Thank you so much for the tips. It’s a huge help. I am planning on having more tube modules so the PSU issue is something I’m going to need to address sooner rather than later. I was thinking I would wire up a second transformer just for heaters, like the tube amp guys do, instead of loading down my main linear PSU.
The hum is weird. It sounds almost like the heater is acting like an oscillator.
Yes you can use a transformer just for the filaments. Reference the center tap to the ground of your main psu. You can make an artificial center tap with two resistors or a pot so you can balance the hum. Look up ‘artificial center tap’ or ‘humdinger’.
I’ve been thinking about doing the same thing.
I’m more of a tube amp guy than a synth guy, and i’m not familiar with your circuit, but i can certainly tell you that unstable filament power supplies can cause weird problems. Also noisy switching power supplies can cause weird problems, i once built a vacuum tube opamp that would modulate the heater noise on the output.
Hope you can fix your problem 
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Okay, I’ve built this on stripboard, it works pretty well. I’ve resorted to using a small DC buck converter I got on Amazon to drop the 12V to 10V for the tube heater (edit: actually I am using a 9VDC wall wart I have, and plugging it into the buck converter through a panel I made so it is a totally separate power source for the heater)
I tried to make it as clear as possible, but that just means it’s as clear as possible to me, and not to you, so if you have any questions I’ll do my best to help. There’s still a little bit of hum, but it’s not enough to bother me personally.
Honestly I should probably change the labels for the knobs. After playing with it some more, it acts more like a mixer.
Feel free to modify or add whatever to it. There’s probably a lot more that can be done with this.
Here’s a crappy video of me running a pocket operator through it: 10LE8 vacuum tube distortion/mangle/whatever module - YouTube
Here’s the panel art I made, I printed it on waterslide decal paper and use that:
I’m going to try to make a blog/whatever to post this stuff on, I’ve been messing with other tubes and stuff. That’s where the ‘asphodel meadows’ stuff comes from.
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Brutal sounds, sexy panel, Kosmo format, verified stripboard. 10/10 post.
BTW, are you in Japan? Is this the first Kosmo sighting in Japan?
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LOL! No. I wish. I live in Tennessee. I’m just a weeb.
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Ahh, arigatōgozaimashita, distortion-san!
Man that thing sounds amazing!
I don’t have the tube you used but i am going to try to get two pentodes to act like a dual control pentode.
Great post man 
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Did anybody who also built this make any alterations ?
Can you give me some extra informations on what specifications the caps and resistors should have ? 
That would be great !
Such an awesome sounding bad little thing ! ! !
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