Safety valve module

right so finally down to the safety valve, I think it will be one of the next modules ill make for the module a month (there will be 3 this coming month though haha, midi-cv,bufferedmult,mini mixer)

but after playing with the safety valve quite a bit now. regarding what its primary use is, which is a distortion sort of modular “effects pedal” its just got a lot simpler.

CV inputs? the thing is I don’t think there is anything to control. The amount of gain you can sort that out with what is going into it. so a VCA before it does the same job.

the tone knob is rather naff and im thinking of ditching completely.

in my mind right now its a 5cm wide module. with a gain knob. a volume knob, an in/out and a bypass switch.

the only thing I can think of is Adding cv to the bypass switch, via a small IC relay so its like a hard true bypass.

The VCA iteration not good enough to justify a module in my opinion.



i am happy with every module that’s just 5cm. my case is getting full.
2 inputs would be nice i guess.


ok interesting! 2 inputs ill see what I can do


In one of your YT vids you run double valve modules in like a feedback, probably not too unlike the splashback? Anyway I thought it sounded badass


thats a fair point, adding a feedback to the module!


i was thinking a little bit more about this. i don’t know if it’s technically even possible, but how about turning this module in something like the ms20’s external signal processor?
since you are also working on filter designs anyway? like add a hpf, lpf and then somehow generate control voltage and trigger signals out of the audio input?

i like running drum machines through the ms20 esp and then taking the generated cv from the esp to pitch to create a melody or something and mix it with those distorted drums.

@lookmumnocomputer I’ve build 2 and added vactrols to all 3 controls of them and … so far never used them. The frequency control is not very impressive as you say, so that doesn’t add much. The other 2 settings have a big effect on the total volume of the module, so changing those dynamically would also involve adjustments to the circuit the output is sent to. So, I agree with you, CV doesn’t add much. The bypass is something different. That can come in handy and a relais or hand operated switch can do that. I combined bypassing the circuit with switching on off the heating filament of the tube. This makes the bypass introduce a delay when deactivated, but I’ve been able to live with that without a problem, so far.

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Ordered all the bits for this, now I’m looking at the stripboard layout - can anybody tell me what the hell the danging red wire in the upper right hand is about? It looks unnecessary…

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I think it’s supposed to go to the level pot


Oh, ok. Makes sense.

There were some issues with this Stripboard layout, if I remember correctly. Just search for safety valve in this forum and take a look…

Ah, right you are. Looks like this is the winner: Valve Distorting Vca Problems

I dont know if I call this a video about problems - but there is some unexpected behavior to cover.

1 - A bit of a highpitched hiss that gets louder when near the tube.
2 - Triangle wave fading in.

I’m not sure about the feasibility of this - but it would be neat to have a thermocouple on the tube that could control the brightness of the power led. I would also like to move that led behind the tube socket to sort of simulate the old tube glow. And then perhaps have a indicator LED where the power one is currently that lights with the output signal for some visual feedback.


Were your speakers on while you recorded that? The valve can be microphonic (picking up vibrations from the speaker), so blocking it with your hand would have a weird effect. If you tap it with your finger, it’ll go pinggggg

I’m guessing that the fade-in behavior was because the tube wasn’t all the way warm yet, if you give it more time to warm up then it won’t do that.


I turned my speakers way down at 2:00 - so the audio at that point was going directly into the line in on my compy.

I unplugged my speakers and listened with my headphones and it is still very much still there. Its almost a ‘bacon cooking’ kind of static and it has a distinct threshold when I turn the CV input pot to about 10oclock. Tapping the tube has no effect at all.

And yes, it does seem like after things warm up that the triangle wave works as it should. I thought I had checked that before!

In general it is rather advisable to avoid simply touching a tube with your fingers, so maybe make gently with the Pliiinng :wink:


Thankfully I am not THAT cavalier with my electronics.

Close, but not quite.

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Hi, it is difficult to say anything about the signal level dropping and slowly rising when you switch from the saw to the triangle. I take it that is what you find ‘kinda weird’. Otherwise, please explain what it is you find strange as we the readers do not known your equipment.

If that is what you mean, this could be a problem of the signal source responding differently to the load the valve caster constitutes, or the valve caster responding to a change in signal from another output. In both cases it could be a problem related to output or input impedance changes. A well functioning valve caster would not have that effect, or at least the 2 that I build don’t.

If you have an oscilloscope you could check what is happening to the signal you feed into the valve caster. You could try to check this by ear by sending the signal into the valve caster and into a second channel of your mixer. If you mute the channel with the output of the valve caster you will hear if the input signal changes when you choose a different source.

B.t.w. it is normal for the sound of a triangle wave to sound less aggressive / less loud than a saw tooth.

I just found it odd that the vca would work as expected with both a sawtooth and a square wave, and then have different results with a triangle with the slow signal gain boost. But it seems to just be an issue of warming up.

This hiss on the other hand is an unknown. Most likely something is not grounded properly and that is what is generating the sound as well as its intensity when moving my hand near the tube.

So the effect passes / is not repeated once the tube has been switched on for a few minutes?

This sounds like a grounding problem to me. It should not be that sensitive at a normal gain factor.

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