Valve Distorting Vca Problems

Yesterday I made the Safety Valve 2.0, but unfortunately it is not working :frowning:
While checking the stripboard and the schematic, I noticed that on the stripboard the pins 4 and 5 at the valve are mixed up (can someone doublecheck?)
I fixed my stripboard but still no sound. When I measure the output, I only get a constant 12V. When try to ‘follow’ the sound (I just use a cable which goes into my mixer) I can hear the sound very lout after the first stage of the valve and going into the second one (at pin 7) but at the output of the valve and into the opamp it is very quiet. Is this what I need to expect? Then it looks like the opamp just ignores the input and sends out 12V at its output… Any ideas what to check?

I changed the TL071 for a TL072, which I had lying around and changed the stripboard accordingly, but this should make no difference, right?

I’m gonna build this next week so i’ll give you an update on mine when it’s done.


There should be a decoupling capacitor between pin 6 of U1A and the opamp’s input pin 3.
You only want the AC signal to amplified / buffered, so you have to get rid of the DC voltage that is present on pin 6 of U1A. That pin will always carry a relatively high DC value near 12 volts. If you connect a capacitor to pin 6 and put this directly into an amplifier ( set volume low ) you will hear what I mean. So basically the schematic contains an error.

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I’m not sure what SW1 is for. It basically causes a problem by connecting the high voltage on pin 1 of U1B to the grid of U1A. That grid’s DC-value should be lower than pin 6 of U1A otherwise that tube will stop amplifying. The capacitor C3 will only allow the AC signal from U1B to control U1A and decouples the DC settings of U1B from those in U1A. This is similar to what I meant with my remark that there should be a capacitor between U2 and U1A.

Thank you for looking into it! Hm… that is strange, but there is definitely no additional capacitor in the video

Maybe @lookmumnocomputer can clarify a little bit? At least on the mixup of pins 4 and 5 (stripboard vs schematic) ah, and the connection from +12V to the lvl pot is also missing in the stripboard picture.

Well I build 2 of those myself, I used the schematics for safety valve version 1. You can see that there is a capacitor from pin 6 of the tube to the ‘top side’ of the volume potentiometer. The middle pin of the potentiometer is connected to the + input of the TL071. So this will only amplify the AC signal it gets.

Note that the GAIN and TONE labels in this diagram are swapped.
For reference I include the schematic published by matsumin which I think was the inspiration for the Safety Valve. This does not have an opamp but here also you can see that from pin 6 there is a capacitor C3 that leads the signal via the tone and volume potentiometers to the output. So also in this schematic the capacitor makes sure that there will not be a DC voltage on the output jack.
You will find decoupling capacitors as a general principle in lots of amplifier designs.


Maybe Sam drew the diagrams after building the valve casters? Because if he made them following the diagram you included he would have had the same problems you are experiencing.

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It works!! Thank you so much!! :smiling_face_with_three_hearts:
I put a cap between the output of the valve and the next step (I made a picture)
Thank you so much!! I used only 100nF, because I could not find a 1uF, can you explain, what this changes?
And also the bypass switch does not work, but I have to check that again! It only bypasses if I remove the output from the opamp… any ideas?

valve schematic


A smaller capacitor implies a higher resistance to low frequencies. So you will hear less low frequencies in your output. You can always either exchange the capacitor once you find the desired value or use a few in parallel to get to the value you want.


Cool! Is this how the tone pot works in the safety valve? Not changing the cap, but the resistor and therefore a filter?

Indeed. The combination of the resistor and the capacitor are a filter. The lower the value of the resistor the more the capacitor will short high frequencies to ground ( and you will not hear them at the output ). Which frequencies this affects depends on the resistor and capacitor values.


Any tips on the bypass?

The bypass switch is apparently supposed to work by connecting the input jack to either the output jack or pin 2 of the valve. Use a continuity tester to make sure that’s the way it switches, and of course make sure you’ve correctly identified the valve pins.

So was adding that capacitor the only thing you changed from sams original strip board diagram?

The bypass should work maybe add a diode at the output of the opamp or something like that.

Changed the cap and also the pins, as mentioned earlier. But now when I tried to solder it down, it stopped working again… I mean I have sound, the bypass makes the signal more quiet, but now I have no distortion effect anymore… this is strange, but my board is a mess right now. I just decided to stop here and make the simple safety valve…

Maybe it was the capacitor? I changed it from 100nF to 2.2uF…

I’ll give it a build and if mine works fine then you can just build it again luckily it’s a small circuit.

Is the 2.2uF capacitor you used bipolar or is it an elco? Oh, and is it oke?

Looks good to me
BTW: one of the best tools I got! They cost like 10€!


That looks like an elco to me so you need to put it the right way around. The plus side should be on the side of the tube.

Yup, checked that! I have now removed all my cables and am rewiring (and resoldering) everything as the safety valve you posted :wink: hope that goes better!