Showing my version of the safety valve (introduced by Sam The Safety Valve - Valvecaster For SYNTHS DIY How To ( actually I build 2 ) for euro rack format here. I added a bypass switch to the circuit which connects the input to the output immediately and switches off the current to the heating filament. Furthermore I added vactrols to the gain, tone and volume controls.
The panels were 3D-printed. If anyone is interested in the STL-file, just let me know.
Note that in the original board layout safety valve published by Sam the text accompanying the Tone and Gain potentiometers are swapped. So ‘Tone’ should be ‘Gain’ and vice versa.
nice! good 3d panels. and the offset for the valve. how do you find if you turn off the heater the delay to turn on the effect? also is there any pops or change in oscillator tunings around this on the same power supply??
It will indeed take a minute or so to heat up the tube and to get the effect going, but I’m not using it in a way I want the effect to be on or off within a track. The switch is meant to preserve some energy / put less load on the power supply when I’m not using it (but using the rest or the rack) and because it switches the input signal to the output as well when in bypass mode, I do not have to rewire a patch when I’m not using the safety valve. When switching to bypass, I hear no pops, and since I do not have any oscillators near it I can not say anything about any change in their tuning.
Vactrol = an LDR controlled by a LED, both enclosed in a light proof tube. Connect the LDR and a serial resistor to the mid point of a potentiometer and put a control voltage on the LED to control the LDR’s resistance. This is an easy way to control electronics often used with analog synthesizers that lack control inputs. Have a look at Sam’s video vactrol-lemon-squeezy about the subject.
I have looked around but I can not find anything on what kind of LDR and LED you need to use for your vactrol! Any? I assume that the ‘resistance in darkness’ determines the coupling between the signals, but does it matter? LEDs any color? or better white? I want CV on the gain as well
You may want to read this and that, especially the last post first. I tried several LDR’s and ended up using a LDR that is 2 mega Ohm when not lit and using a standard LED. Make sure they are shielded from any outside light! It all boils down to the linearity of the LED’s light emission and the linearity and sensitivity of the LDR responding to that, and of course what it is you want. To make it more complicated, it also depends on where in a circuit you are going to use them. There are no easy answers here. I chose the 2 mega ohm, because the control was quite linear. Of course you can buy prefabricated ones. Do a search for Vactrols – VTL5C and you’ll find plenty (e.g. here).
Best Practice: you want the LDR’s value not to influence the circuit it is connected to when the LED is off. So its impedance should be higher than e.g. the potentiometer you are connecting it to. You also may not want the LDR to create a short circuit when the LED is lit, in that case you could add a serial resistor to prevent that from happening.
The vactrols I made, were made of 2Mohm LDRs and a bright LED isolated from sun light by black shrinkwrap material. Codebeat, a friend of mine, told me that he made vactrols himself using liquorice. He told me he would use liquorice with a low sugar content (probably because it prevents it from burning when heated up). He would glue the LDR and white LED together and then dip them in melted liquorice to get a compact vactrol ( the ones you can buy are quite bulky ). They will then harden when cooling down. It may appear strange at first, but liquorice is a very black substance and to keep light away from the LDR and the fact that you can mould it makes liquorice quite a good choice. You can find some more of his thoughts on vactrols here. You may also want to consider using something like a H11F1M (or similar). This is a 6-Pin DIP Bilateral Analog FET Output Optocoupler (datasheet). These are dirt cheap and given the LDR is replaced by a FET they should be really fast ( a typical LDR is not ).
