Simple tube vca issue with pin 6 no output.

[Norke]

Hi everyone,
i built this on breadboard, it runs from +12 and -12 volts.
Circuit works, besides the error of pin4 and 5 inverted in the schematic layout compared to the veroboard layout(which is good) the rest looks right but i can't make it work by myself on my breadboard.
I built it 3 times already, same issue each time.
I can visualise on the oscilloscope the waveform entering pin 2 and going out of pin 1 and then into pin 7 but i have no signal at all just a 1.2v steady dc fixed voltage at pin 6, no waveform, same thing at the opamp input at the right on the schematic.
Any ideas what can happen?
I verified everything so many times already, the tube filament heats up, i have an oscilloscope and multimeter i can test what ever you suggest.
Forget about the switches for the moment that are on the schematic i dont wire them up just to keep it simple.
But one time i tried to add the bypass switch shown at the left and while the breadboard not powered i could see the waveform that i put at the input socket going into the power suply, i mean by that its a eurorack power suply and there is leds for +12 -12 and +5v that show when its active but they were lightning on even if i disconnected the psu from the power source, the sine wave form at the input was going back into the psu bussboard, if that can even give a hint...
The vca part in the blue dotted square at the bottom was working fine i could see the waveform being modulated from the control voltage input that i feed it.
It's really just that the signal doesnt come out of the pin6 of the tube.
Thanks a lot here's the info about the circuit:

https://www.lookmumnocomputer.com/pr...distorting-vca

[Enzo]

Remove the op amp IC. Does signal now appear at pin 6 of the tube?

[J M Fahey]

Hi everyone,
i built this on breadboard, it runs from +12 and -12 volts.
Circuit works, besides the error of pin4 and 5 inverted in the schematic layout compared to the veroboard layout(which is good) the rest looks right but i can't make it work by myself on my breadboard.

You have 2 multi pin elements shown in your schematic: 9 pin 12AX7 and 8 pin TL071 , BOTH have pin 4 and 5 , so what are you talking abut?

* on 12AX7 pins 4 and 5 are 12V filament, inverting them does nothing, it works both ways.

* on TL071 pins 4 (-V) and 5 (offset adjustment) are NOT interchangeable but are properly drawn on schematic, I didn´t check the Vero layout.

I built it 3 times already, same issue each time.
I can visualise on the oscilloscope the waveform entering pin 2 and going out of pin 1

let me warn you that this is a TERRIBLE design, designer LMNC has very little clue and it has gross errors.
I suggest you build a real tested VCA; LMNC certainly breadboarded it (he shows a picture) and got same results as you: VCA "works" (sort of) but the full circuit does not (best case does it very crudely).

and then into pin 7 but i have no signal at all just a 1.2v steady dc fixed voltage at pin 6, no waveform, same thing at the opamp input at the right on the schematic.

What you see is exactly what that circuit does; that´s why I say LMNC never fully built it and even less used it in a functional Synthesizer.
First tube half "works" (sort of) late gets meager 12V through 220k resistor, grid is at ground voltage, cathode gets some current through reverse biased Q1, and amplifies.
Second tube half gets unbelievable low 1.2V DC, exactly what you measured, because it´s connected at the mid point of a voltage divider: 1M5 to +V , 150k to ground.
I do not expect a 12AX7 to work at all with just 1.2V plate voltage, period.

I suspect LMNC who is clearly a sloppy designer built the tube part only, up to the 1M5 resistor, in which case the second tube half got 12V through it and it worked (sort of) , he got some signal there.
Then he addeed an Op Amp 20X gain stage (10k/470r) he cut and pasted from somewhere else and forgot to add a coupling cap, so 150k loads the 1M5 resistor to get 1.2V DC
So you made no mistake, just built a bad design.

Another GROSS error: Op amp has 20X DC gain, so +1.2V at its pin 3 input will *try* to make pin 6 reach +24V ... which are nowhere available, so it will just slam against the +12V rail.

Another gross error: in bypass mode input signal is sent straight to Op Amp output pin 6 ... which is virtual Audio Ground, so this module: "won´t work when ON, won´t work when OFF"

I firmly suggest you build any other time tested VCA, this one is a mess and probably chosen "because it has a Tube and tubes are better, aren´t they?" which is a silly argument ... "tubes are better" ... where and when they can do their thing, generally in analog amps, specially Guitar ones, but not randomly thrown anywhere without clue.

Just to let you enjoy this a little, add a .1uF capacitor between tube pin 6 (so it can get 12V through R6) and R7 so they are not DC joined.
Circuit should work, Op Amp pin 6 must be near Ground (a few mV offset) instead of the +12V it must be showing now, and signal should appear there.

Bypass path still won´t work as shown, unless you make it a true bypass which puts VCA fully out of circuit.

And yes, Enzo was right: removing the Op amp section, including R7, will make the Tube section work on its own.

OK; please do the tests suggested and post results.

And then build a proper VCA

Your symptoms make sense considering what I wrote above:

Any ideas what can happen?
I verified everything so many times already, the tube filament heats up, i have an oscilloscope and multimeter i can test what ever you suggest.
Forget about the switches for the moment that are on the schematic i dont wire them up just to keep it simple.
But one time i tried to add the bypass switch shown at the left and while the breadboard not powered i could see the waveform that i put at the input socket going into the power suply, i mean by that its a eurorack power suply and there is leds for +12 -12 and +5v that show when its active but they were lightning on even if i disconnected the psu from the power source, the sine wave form at the input was going back into the psu bussboard, if that can even give a hint...
The vca part in the blue dotted square at the bottom was working fine i could see the waveform being modulated from the control voltage input that i feed it.
It's really just that the signal doesnt come out of the pin6 of the tube.

[g1]

You can't just make stuff up (whoever designed it).
Like 'Q1 put in backwards because it sounds gnarlier', or bypassing the coupling cap with SW1.

[Norke]

You have 2 multi pin elements shown in your schematic: 9 pin 12AX7 and 8 pin TL071 , BOTH have pin 4 and 5 , so what are you talking abut?

* on 12AX7 pins 4 and 5 are 12V filament, inverting them does nothing, it works both ways.
a
* on TL071 pins 4 (-V) and 5 (offset adjustment) are NOT interchangeable but are properly drawn on schematic, I didn´t check the Vero layout.


let me warn you that this is a TERRIBLE design, designer LMNC has very little clue and it has gross errors.
I suggest you build a real tested VCA; LMNC certainly breadboarded it (he shows a picture) and got same results as you: VCA "works" (sort of) but the full circuit does not (best case does it very crudely).

What you see is exactly what that circuit does; that´s why I say LMNC never fully built it and even less used it in a functional Synthesizer.
First tube half "works" (sort of) late gets meager 12V through 220k resistor, grid is at ground voltage, cathode gets some current through reverse biased Q1, and amplifies.
Second tube half gets unbelievable low 1.2V DC, exactly what you measured, because it´s connected at the mid point of a voltage divider: 1M5 to +V , 150k to ground.
I do not expect a 12AX7 to work at all with just 1.2V plate voltage, period.

I suspect LMNC who is clearly a sloppy designer built the tube part only, up to the 1M5 resistor, in which case the second tube half got 12V through it and it worked (sort of) , he got some signal there.
Then he addeed an Op Amp 20X gain stage (10k/470r) he cut and pasted from somewhere else and forgot to add a coupling cap, so 150k loads the 1M5 resistor to get 1.2V DC
So you made no mistake, just built a bad design.

Another GROSS error: Op amp has 20X DC gain, so +1.2V at its pin 3 input will *try* to make pin 6 reach +24V ... which are nowhere available, so it will just slam against the +12V rail.

Another gross error: in bypass mode input signal is sent straight to Op Amp output pin 6 ... which is virtual Audio Ground, so this module: "won´t work when ON, won´t work when OFF"

I firmly suggest you build any other time tested VCA, this one is a mess and probably chosen "because it has a Tube and tubes are better, aren´t they?" which is a silly argument ... "tubes are better" ... where and when they can do their thing, generally in analog amps, specially Guitar ones, but not randomly thrown anywhere without clue.

Just to let you enjoy this a little, add a .1uF capacitor between tube pin 6 (so it can get 12V through R6) and R7 so they are not DC joined.
Circuit should work, Op Amp pin 6 must be near Ground (a few mV offset) instead of the +12V it must be showing now, and signal should appear there.

Bypass path still won´t work as shown, unless you make it a true bypass which puts VCA fully out of circuit.

And yes, Enzo was right: removing the Op amp section, including R7, will make the Tube section work on its own.

OK; please do the tests suggested and post results.

And then build a proper VCA

Your symptoms make sense considering what I wrote above:

wow thanks so much for the help!!

I thought it was sounding nice and simple build so i gave it a try, maybe lost too many time on it already but anyway... A friend of mine built it but not on breadboard, on stripboard, apparently his works, i understand it's not a very good circuit, lmnc says it himself but i was at least hoping to make it work like in his video.

Which tube vca schematic would you suggest me to try out? Using +12 and -12 volts with 12ax7.

For pin 4 and 5 i was refering to the tube i'm sorry about that, yes i know it would work but that made me suspicious about the rest of the schematic.

Why do you say it does not work or "crudely" ? It sounds fairly nice on his video... Also why you say LMNC never built it? He show the build on his video, you can see it using with an envelope generator to activate the vca in it.

Yeah i thought that 1m5 voltage dividider was weird, should i reduce to something closer to 220k like the first stage of the tube and reducing also the feedback resistor of the opamp?

Yes indeed i just see a 12v at the output of the opamp pin 6

I wonder how he managed to make it work, lmnc and my friend that built it on veroboard....

I forgot to mention the 27r at pin 5 of the tube gets really really hot, i'm using a 1/4w but i will use a 2w minimum, that is enough?

Yeah i know all the hype around tubes but it does sound good, maybe not better but they have their places and sounds like nothing else.

When i had the bypass switch wired on the breadboard at my first try of it, having the bypass to on (bypassed the circuit) i was hearing the incoming not distorted sine wave at the output for 1 second gradually going to silence after 1 second. That happened each time i switch from bypass to non-bypass, while having absolutely no waveform at the output of the tube circuit just a steady 12v.

Adding the 0.1uf like this?




I removed the opamp.. literally only the opamp... everything around it was still there... and there is no signal at pin 6 of the tube, only a 1.2v steady....actualy 1.46v

I can try adding the 0.1uf but that would not make it work like his does.. isnt it? I'll try for sure but for now i would really like to get it work like it does for him even if its not an ideal circuit.

by the way, the vca part works i can see the wave form modulating from the lfo i feed it on my scope.

Thanks a lot, really appreciated!

[Norke]

So i forgot to remove r7 while removing the opamp and now i can finaly see the waveform at the pin6 of the tube !!!
Now what...? My ic tl071 was new... i have some 072 to try, i'll just change the pins to accomodate but i already tried that before... What is hapening? lol....

[J M Fahey]

Did-you-use-the-.1uF-coupling-cap-as-suggested?

As of what LMNC built, maybe it works, sort of but then it is NOT what he showed in the schematic. Period

+/-12V tube VCA?

Build an SS one instead.
Lots of excellent ones out there.

I only built one synthesizer in my life, ... but it was a full MiniMoog one
And of course it worked, after all it was designed by a bunch of Engineers plus The Man himself

There are many excellent designs out there, don´t attach yourself to the worst one available.

[Norke]

Did-you-use-the-.1uF-coupling-cap-as-suggested?

As of what LMNC built, maybe it works, sort of but then it is NOT what he showed in the schematic. Period

+/-12V tube VCA?

Build an SS one instead.
Lots of excellent ones out there.

I only built one synthesizer in my life, ... but it was a full MiniMoog one
And of course it worked, after all it was designed by a bunch of Engineers plus The Man himself

There are many excellent designs out there, don´t attach yourself to the worst one available.

"SS" ... i dont find anything on google with ss tube vca... could you link pls?

I just found out that keeping the 071 (or a 072) but removing the 150K r7 makes the tube pin 6 signal visible on the oscilloscope, i can see the waveform but it disapear as soon as i put back r7 into the breadboard. I can also see the waveform when r7 is removed at the input pin 3 of the opamp, but it's output is then just weird noise waveform...

[Norke]

The 0.1uf cap you suggested like this?

[Jazz P Bass]

Quote:" "SS" ... i dont find anything on google with ss tube vca... could you link pls?"

The statement that Mr Fayey made used the initials 'SS'.
That implied the term "Solid State".
As opposed to Tubes.

[Norke]

Quote:" "SS" ... i dont find anything on google with ss tube vca... could you link pls?"

The statement that Mr Fayey made used the initials 'SS'.
That implied the term "Solid State".
As opposed to Tubes.

Ah ok but... i want the tube sound lol... not because its "better" but because it's different.

I just tried to add the 0.1uf cap (still waiting to know if that's where you wanted me to place it) and here is the signal on the scope at pin 1 of 072 output (changed 071 for 072 in case my 071 was bad) :




and heres the signal on the scope at pin 1 of 072 output without the 0.1uf cap:




It's still very very low signal and doesnt work like intended

Anyone can suggest a tube vca schematic that's working good?

[Norke]

Did-you-use-the-.1uF-coupling-cap-as-suggested?

As of what LMNC built, maybe it works, sort of but then it is NOT what he showed in the schematic. Period

But one of my friend on Facebook built it from that verroboard layout and it works fine...

[Norke]

So basicly i can see the signal at the opamp input if i remove r7 150k but as soon as i put it then the signal disapear. I checked at the output of the opamp without the r7 and it still outputs nothing...
How would i just modify the opamp circuit for something else to amplify the output of pin 6 of the tube that i see at the moment?

[eschertron]

Let's talk about the 'tube sound' for a moment. Tubes - as in this circuit, single-ended triodes - color the sound by ever so slightly distorting the pure wave form of the signal in an asymmetrical fashion. This gives the 'hollow' or 'reedy' sound from exaggerated (mostly 2nd) harmonics. They do this simply by passing signal. No special control circuitry needed. More signal, more color. The relationship between adjustment and effect is direct and completely intuitive. This, to me, is the primary function of triodes in a tube circuit. After all, if I wanted clean gain I'd use transistors

I suggest you build a tube pre-amplifier circuit - potentially even simpler than the VCA you're attempting to build - which has the sole function of coloring the sound by adding some 2nd-harmonic distortion. This way you can focus on the desired result of getting the tube sound as a function of the circuit and not simply as a byproduct. Divide and conquer, as it were.

The VCA circuit function is to envelope-modulate the signal, not incompatible with coloring the sound, but not really related in any functional way. Hope this helps.

[Jazz P Bass]

You MUST install a 'coupling capacitor' between R6 & pin #3 of the opamp.

Without it, pin #3 sits at whatever dc voltage is coming through R6.

The capacitor will block the dc voltage but allow the varying signal voltage to reach pin #3.

Take a look at this L-1 schematic and you will see what I mean.
L-1_TUBE_VCA_Schematic.pdf

By the way, the L-1 kit is available for purchase.
http://l-1.su/TubeVCA.html