Valve Distorting Vca Problems

I red it wrong and had pin 6 and 9 from the tube swapped, now it’s working.

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If you leave R5 at 150K you get a big drop in volume if you switch to bypass. At least this happens in my build.

I also found that the CV input behaves like a gate. Turning that level pot gives me either nothing or the full signal plus some very nasty sounds in between. I guess this is due to the BC107 put in backwards.

Hey guys! First time posting! Greatly appreciate all the hard work and input you have all given.

I am nearing completion of the strip board layout but was wondering if someone could explain the multitude of +12v, ground, and -12v points at the top right corner. Still learning to read schematics. Noob here.

Do I need all of those connections going out or just one of each?


I think I just answered my own question but it brings another. Ha!
So it’s a 10 pin eurorack connections.
NEW QUESTION: can this simply be powered by a 12v wall wart or something? More like a pedal.

The safety valve no, it uses opamps which are configured to use ±12V here.
But the circuit the safety valve is probably based upon runs on a 9V battery:



Thank you for the info!
Guess I’ll just commit to building more eurorack stuff :wink:

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Did you see that there is a new version of the safety valve done by Sam?

It’s pretty much the old version, but now as pcb project and a (hopefully) error free schematic :wink:

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Saw that and it looks rad!

Thank you and dav for your work!


No smoke but a lot of tears on my side…

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And after months in the “Box of Shame” my Valve VCA distortion is alive.
Had all sort of problems, “hot R27”, crosstalk, a leg of a potentiometer shorting all to GND and many more…
Here some pics.

I was wondering if its possible to have more low end, maybe changing the CAP on the valve?
I want to make a double one and add some features, if anyone of you have a KICAD file for it and maybe with TL072 it would be really appreciated.
Thank you Sam and all the others for the fun project and help.

P.S. Anyone with a tutorial on how to use the metal wire to protect and secure the valve? Mine is just useless as it is.


you can get projection cages for them at most guitar amp part stores (eg tonefactory)

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Very nice that you got it working!

Looks like the wire clip is for a taller type of tube like a pentode, triodes are usually a bit shorter, you’ll need a shorter one or a shield like above .

To increase the bass response, you would usually change the coupling capacitors or the cathode bypass capacitor.

This circuit doesn’t have a fixed cathode resistor to bypass, because it is replaced with the bc107. I can’t tell you what adding a bypass capacitor between the collector and emitter will do, but this is where it would be in a normal vacuum tube circuit.
I don’t think anything will release the magic smoke if you try this, but i don’t know if it will help either. 22uF should give full gain over the entire audio range.

I calculated the cutoff frequencies from the input and interstage high pass filters and they are both below 20hz. So these shouldn’t attenuate anything within the audio range.

I have experimented a lot with playing bass guitar through the safety valve distortion, which is very similar, and found that low notes can still have trouble with getting through at all.
This circuit needs a hot signal. Too low signal will kind of fart out. I use a boost circuit in front of it. Like a electro harmonix lpb-1. I made it in a panel. It really helps but also will make it distort more



i built this following the new layout here posted that later was confirmed to be working:

yet is not working…

the switch of bypass works, in the sense that on one side let the unchanged sound pass, but on the other side it just turn off everything.
pots and other switches do not affect at all. only working thing is the led on the power on/off
i checked the stripboard and connection and everythihg seems fine.

maybe i burn something without realizing it? i previously had some short somewhere, which i solved…
how to understand if my lamp is still ok? or if the chip is ok?


edited: my pots are very near to each other… but not the leg, i mean the round part… could it be a problem?

To test the TL071 you can build a voltage follower on a solderless breadboard (or if you don’t have one, use a small piece of stripboard and a DIP socket):


Any voltage you put at INPUT (-8 to +10 V) should give the same voltage at OUTPUT.

If the tube doesn’t show signs of physical damage it’s likely to be OK.

For further troubleshooting refer to:


is there any difference about the tl071CP or tl071ip? all of them named tl071should work right?

about valve im using this: ECC83/ECC83S-JJ TESLA GOLD PIN (JJ Tesla)

The only difference between them is the better operating temperature of the CP version. Unless you are using the TL071 in freezing temperatures, there should be no difference.

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should i post in that topic for general advice/troubleshooting for continuing talking about my issue or i can continue here?

i bought more tl071 chips, mounted and nothing changed, …
pin 1 of tl071 is not shorted to ±12,
it’s mounted correctly (there is no U on the chip only a dot, which i consider as the U side)
+vcc pin connected to +12v and -vcc pin to -12v

still it’s not working… the module only bypass and lets the untouch sound pass.
nothing seems burning, nothing seems to get hot…
im almost sure i connect everything as it should …
polarity of capacitor is correct.
as i said previously there were some rails which were shorted between each other: i solved and checked with multimeter that they are not anymore… yet maybe those shorts burned a capacitor or something else?

sorry for noobing… what kind of voltages could I check and where?
maybe i burned a capacitor… i don’t have multimeter to check capacitance… to test one without that kind of multimeter online they say to take it out/desolder from circuit…

list of component:
R1- 220R
R2 - 220K
R3 - 470K
R4 - 1M5
R5 - 150K
R6 - 470R
R7 - 10K
R8 - 10K
R9 - 20K
R10 - 20K

C1 - 47nF
C2 - 10uF
C3 - 10uF
C4 - 47uF
C5 - 47uF

Q - bc107

thanks in advance

Use an audio probe as described in the post I linked (if you don’t have a scope). Plug in an audio signal and trace it to see how far it gets, from the input jack to the switch to the gain pot to the tube input to the tube output and so on. See where it disappears.

Pin 1 is not the output.


Pin 6 is.

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yes, i think i was refering to your troubleshooting thread where you wrote:

" […] Non power pins are shorted to power rails. With power turned off, check continuity between other pins and +12 V, -12 V, and ground. In some cases continuity is okay and in fact part of the circuit design, but in particular op amp output pins (pin 1 on a TL071, pins 1 and 7 on a TL072, pins 1, 7, 8, 14 on a TL074) should not be shorted to ±12 V or ground… […]"

maybe i misuderstood…

anyway the only pin of tl071 that goes to +12 is the +vcc (7) and the only one that goes to -12 is the -VCC (4). other ones do not touch ±12v

mmh ill see what is this audio probe… on youtube i saw how to build one… thanks