5 tube section vibrato

[jeff]

Hi, I've been looking at the vibrato circuit that uses 5 12AX7 1/2's ala concert 6G12-A.
I've never actually heard one but heard that this type of circuit is a true vibrato, in that it changes the picth of the note. Looking at the circuit i'm trying to figure it out and this is what I've come up with.

The first two sections look to me to be an oscillator which if I understand correctally produces some sort of wave form with a variable frequency. This is then sent to what looks like to me to be a phase inverter so the oscillation is now split into two equal and opposite wave forms. These are now sent to two gain stages's grids. (Here's where I get confused.) Buy putting this signal on the grid it seemes to me that you are changing the gain of the stage, increasing one while decreasing the other and vise versa. When mixed back together the "note" caused by the oscillation is canclled because it is equal and opposite through the two gain stages and is phase cancelled out.

Does this sound right so far?

Now the signal from the preamp is diverted through the 250pF to one gain stage(treble) and through the 220K .005 with .1 to ground(full signal with treble grounded, bass). So it seems to me that the gain on the tube fed by the treble portion of the signal is increased as the gain on the tube fed by the bass portion of the signal is decreased and vise versa.
Is this correct?

If so how is this affecting the pitch(frequency) of the original note? It seems to me to be affecting the tone of the note.

[sluckey]

If so how is this affecting the pitch(frequency) of the original note?

You have a good understanding of that circuit. But it's still amplitude modulating the signal, allbeit in a fancy way. There's no pitch shifting happening.

Sounds like you saw this attachment somewhere?

[tubeswell]

FWIW I had this same question a couple of years ago at the ampage forum. (Got some answers too)

http://music-electronics-forum.com/t10295/

[phsyconoodler]

Actually it only uses three tube for the vibrato section.The other three are for each channel and the phase inverter.
  Fender used to have an amp that had a 'pulse' control on the back of the amp that was used to control the vibrato warble,but I've never actually seen one of those in person.I bet is was an early prototype before they had the three tube vibrato.
  I tried a couple of different ways to do either a magnatone style and a fender style vibrato but ended up stumbling on something quite different.I use a pulse control on the rear of my V-Verb amps to control the warble,but I have no idea if it was like Fender's or not.Mine sounds a lot like a uni-vibe but not quite.
  It's a cool sound,whatever you use.

[PRR]

That's an unusual plan. Leo patented it. Very few amps used it. Why? It may be nice, but it is a LOT of parts, and maybe on balance not worth the effort.

Pitch is not changed (not enough to hear; a phase-geek could sight it in with a microscope).

You have a bass-heavy channel, a treble-heavy channel, and a crossfader wiggling a few times a second. You could set it up with a disco mixer and a crank on a slow motor.

The oscillator runs maybe 2Hz to 10Hz. Often called "LFO". In guitar practice we favor Sine wave.

The two channels are not balanced at 2Hz-10Hz so the LFO "cancellation" is poor. But the LFO rate is well below guitar pitch. A bass-cut after the mix takes the strong thump out so it does not bang or stun the power stage. Two stages of 100Hz bass-cut will put 10Hz down to about 1% of what it is on the vari-gain stages' plates.

It would be simpler to do such a thing in analog chips; still simpler in modern digital audio processors. But that's not common.

Leo believed in it enough to pay a lawyer and the patent office. Leo was no fool. But even a genius can be over-impressed with his own work.

If you dig it, build it. The "LOT of parts" is not a major hurdle for an experienced builder. (You do have to keep your head on straight to get that tangle of interconnections right.)

[jeff]

Having never heard this circuit, could you please describe to me what it sounds like?

With the intensity low does it sound more like an auto wah where the treb goes up while the bass goes down and vice versa?

Does it ever reach a point where the volume is totally off with intensity all the way up?

Is it like a opto trem circuit where the volume goes on and off?

[tubeswell]

Having never heard this circuit, could you please describe to me what it sounds like?

I think its close to this clip by our own Mr Hoffman (albeit that this is the revibe, which has a slightly different trem circuit to the 6G12A, but it is nevertheless the trem sound from that brown trem genre)

http://www.el34world.com/Misc/Music/files/LoveHurts.mp3

[67polara]

I just built the ReVibe from Weber kits.  The Vibrato is just like that clip.  It is not Vibrato in the purest sense of the word but it sure as heck isn't Tremelo.  It is something in between full sounding, filling, musical.  I modified mine a bit to gain control over the speed using a modified volume pedal that when plugged in removes the speed control and hooks it to the pedal.  Talk about having control!

Tony   

[HotBluePlates]

Having never heard this circuit, could you please describe to me what it sounds like?

Doug's demo sounds more like typical trem to me than what I've heard the "5 tube section trem" sound like before. When I've heard it in the past (but this was as much as 10 years ago), it sounded more swirly than pulsating. For that matter, the vibrato in a Vox AC-30 has the same sound, whereas the AC-30's trem is a pulsating volume change.

The sound "swirls" from bassy to trebly and back, but I wouldn't call it the same as an auto-wah effect, which can be pretty quacky.

But Doug's demo definitely seems to sound more like a typical bias-vary trem.

[jeff]

So basically when the oscilating wave is at its peak it's sending a positive voltage to the grid of the tube with the treble portion of the signal increasing its gain and a negative voltage to the grid of the tube with the bass portion decreasing its gain.

This gives me a crazy idea and I don't know if this is possible or would even sound good.

 What if instead of an ocsillating voltage you used a voltage that increased or decreased with the volume of the signal? I don't know if you can do this but is it possible to rectify your guitar signal? After the coupling cap in one of the gain stages have a diode in series with a cap to ground(a half wave rectifier). So as your signal gets louder the voltage would increase. Now set up another one only negative. Now bias the bass tube for maximum gain and the treble for lower gain. Apply the negative biased rect through a resistance to the bass tube's grid and the positive to the treble's. Now the louder you play the more treble/less bass you get. Kinda like an envolope filter.

Not sure if it would work, what do you think?

[HotBluePlates]

This gives me a crazy idea and I don't know if this is possible or would even sound good.

 What if instead of an ocsillating voltage you used a voltage that increased or decreased with the volume of the signal? I don't know if you can do this but is it possible to rectify your guitar signal?

The basic mechanism you're describing is exactly what is used by a compressor and expander. The control voltage generated is used to vary the volume.

To get an envelope filter, you'd need the control voltage to control a tone circuit. There are devices that are voltage-controlled capacitors (called varactors), but I don't know the values available or the range of adjustment as a result of the control voltage. They are more commonly used in radios for using an electric signal to tune the station (which implies a very small capacitance).

I have never looked at the schematic of an envelope follower, and it seems probable that it would be cheaper to buy one than build one.

An idea that occurs to me is that a transistor or JFET could be used as a voltage-controlled variable resistance in series with a cap. That could be a way to have a voltage-controlled tone circuit. Again, it seems a better method to use solid-state for these functions rather than a tube, which then also implies that buying a pedal is better than building one.

[jeff]

To get an envelope filter, you'd need the control voltage to control a tone circuit.

Isn't this kinda like a tone circuit?  You're sending the highs to one tube the lows to another then mixing them back together. by adjusting the gain of each tube you're adjusting the mix.

I have never looked at the schematic of an envelope follower, and it seems probable that it would be cheaper to buy one than build one.

You're probally right. I guess I was just thinking out loud if something like this was possible in theroy.

For that matter, the vibrato in a Vox AC-30 has the same sound, whereas the AC-30's trem is a pulsating volume change.

 So the Vox has both trem and vibrato. Neat. The schematic is a little confusing the way it's drawn I'll have to study it more.  

[HotBluePlates]

Isn't this kinda like a tone circuit?  You're sending the highs to one tube the lows to another then mixing them back together. by adjusting the gain of each tube you're adjusting the mix.

The difference is that it would be a complex way of doing it, and I'm thinking of the simple way: a cap plus a controllable variable resistance.

I'm thinking of something that uses no extra tubes, but uses passive components and a JFET to take the place of a variable resistor (pot using only 2 legs).