Fender Pitch Shift Vibrato

[Willabe]

This is a thread started by kagliostro on trem/vibratos.

Look at reply #7 from Roly, very interesting. (PRR is in the thread too.)

http://www.guitargear.net.au/discussion/index.php?topic=30731.msg325979#msg325979

Also look at the Dorf links that K posted. It seems that Dorf came up with the pitch shift circuit, then Wurlitzer used it, then Gibson (GA83S) then Vox. The guys at guitar gear left out Gibson. All of which was before the Maggi?


           Brad                  

[HotBluePlates]

... It seems that Dorf came up with the pitch shift circuit ...

The guy who came up with the Magnatone circuit has his own series of patents, and they run over a number of years.

I don't know who came up with the idea fist, but they all seemed to cite bit & pieces of each others' work in their patents, while coming up with a circuit just different-enough to side-step the other guy's patent.

I suspect that's also why you see all the confusion over pitch-shifting circuits, because people are confused by the small differences that make a circuit work differently but do the same function as another circuit.

[Willabe]

Did you read what Roly (reply #7) said about the Fender and Vox circuits?

Do you think he's right about the time delay in the circuit?


            Brad     

[HotBluePlates]

Do you think he's right about the time delay in the circuit?

He's unquestionably right. But just be careful how much you read into his answer.

He doesn't mean delay like an audible slapback of a delay pedal. However, if you shift the phase of a signal, you advance or retard it by a fraction of a cycle. That's a delay.

I found a reference online that says the shortest delay audible as a distinct echo is ~30milliseconds; shorter time delays result in the sounds of "doubling", "flanging" and "chorusing".

Roly talks about the 2 filter networks being 90-degrees apart in phase, which is a difference of 1/4 of a cycle.
1 / Frequency = Time for 1 cycle of a given pitch
1 / 400Hz = 2.5milliseconds
90 degrees of a 400Hz cycle = 2.5mS / 4 = 0.625mS

So the delay (difference) when considering a 400Hz pure tone applied to the Vibrato circuit is 5/8 of a millisecond, which is 48 times smaller than what you can perceive as a distinct delay. And the delay time is smaller for a higher frequency.

Obviously Roly isn't saying the Fender circuit is a replacement for your delay pedal.

[Willabe]

Do you think he's right about the time delay in the circuit?
He doesn't mean delay like an audible slapback of a delay pedal. Obviously Roly isn't saying the Fender circuit is a replacement for your delay pedal.

Yes I get that. I was wondering about it being a delay with respect to causing audible phase shift.

So it is true then that the Brown face Fender is/has true pitch shift going on.

PRR has said you can definitely have both trem and vibrato happing at the same time in a circuit.

Here's a clip of Mark Baier owner of Victoria amps playing through 1 of his amps that has the brown face vibrato in it. Vibrato starts ~1:18 with a Strat, there's more latter at ~4:15 with a Les Paul. Sure sounds like it has a lot of trem mixed in with the vibrato to me. Maybe more trem then vibrato but it sure has something more then just trem going on.

He's a pretty good player too.    

victoria golden melody pt. 2


              Brad      

[HotBluePlates]

PRR has said you can definitely have both trem and vibrato happing at the same time in a circuit.

Roly did too, I think, in the thread that was linked.

Basically, look hard at the Wurlitzer circuit discussion linked in the first post on that thread, and compare it to the Vibrotron and Vox's circuit. Roly basically said that the more elaborate form of the circuit as employed by Vox and Wurlitzer was good at creating phase shift without amplitude shift (so you get vibe with little-to-no trem).

Roly also implied that when Fender simplified the Wurlitzer circuit to have a few less R's and C's, it was less-good at preventing amplitude shift along with the phase shift. So you get both.

I don't know that Fender actually copied "Wurlitzer" or how he arrived at the circuit. It does look to me like the bridge formed in the Wurlitzer circuit is not completely copied in the Fender circuit.

[Willabe]

Ok, thanks.

So here's the Million $ question;

Do you think there's a way to make the amount of vibe verses trem adjustable? Like a mix pot? Might be nice to be able to fine tune that but it might be a real pain to do it, if it's even possible.

I forgot to say that I DO like the sound of the brown face vibrato a lot. Doug has made more then a few recordings that he's posted using that circuit (ReVibe) that sound really good. (Of course it helps that he's a darn good player.  ) 

Although I still think I like Sluckeys Warblers, that I built, sound better. It's just a personal preference as they both sound great (if you like that kind of sound Fx) and I think there's a place for both of them in different music/songs.


             Brad      

[HotBluePlates]

So here's the Million $ question;

Do you think there's a way to make the amount of vibe verses trem adjustable? Like a mix pot?

Wire the circuit the complicated way, like the Wurlitzer version (or Merlin's Vibrotron).

You still have the usual oscillator, right? How about taking a 2nd output from that to a buffer stage (say, a cathode follower, if used in your trem circuit of choice). Have an Intensity pot, as a typical trem circuit would.

Make the trem path a 2nd path and its own circuit.


Maybe there's a way to get adjustable trem over-layed on vibe with the Fender circuit. But the methods eludes me. Rather than figure out how to make Fender's "improper vibe" bend to your will, it is probably faster to have 2 parallel effects paths.

[Willabe]

Make the trem path a 2nd path and its own circuit.

it is probably faster to have 2 parallel effects paths.

Ahhh, thanks.


           Brad     

[sluckey]

I don't think the Fender circuit has a pitch shift component. It's just tremolo. Although it has a much more complex and pleasing sound that the standard simple amplitude modulated tremolo. I've studied the 6Gxx Fender circuits. They're all functionally the same. But they're all just amplitude modulators. The unique sound comes from splitting the guitar signal and running one path thru a high pass filter to a simple amplitude modulator and running the other path thru a low pass filter to another simple amplitude modulator. The LFO has two output signals that are 180 degrees out of phase. One of these is sent to each modulator. Then the output of each modulator is recombined with simple mixing resistors. The results is a complex tremolo sound that's hard to describe. I don't hear any pitch shifting in my revibe.

There's a schematic with a simple explanation in my scrapbook.

[Willabe]

Look at reply #7 from Roly, very interesting. (PRR is in the thread too.)

http://www.guitargear.net.au/discussion/index.php?topic=30731.msg325979#msg325979

                Brad     
   

[sluckey]

I did. His explanation sounds good when talking about the Vox circuit. But it's a mistake to lump the Fender into his discussion about the Vox circuit. He begins by saying "I'm not going to pretend for one second that this, and Leo Fender's very similar "Harmonic Vibrato", are anything other than the most difficult to understand circuit you are going to encounter in a guitar amp (and possibly the most widely misunderstood and misrepresented).

The input signal is conventionally amplified, then phase-split to produce an inverted mirror-image.".

THERE AIN'T NO PHASE SPLITTER IN THE SIGNAL PATH OF THE FENDER CIRCUIT! The input signal is simply split. One path is thru a series 250pF cap (high pass) to the grid of a modulator tube. The other path has a .01µF shunt cap and goes thru a .005µF (low pass) to the grid of another modulator tube. It's as simple as that. I'm looking at the 6G12A circuit in case you want to take a look. The Fender circuit does produce a very rich harmonically sounding tremolo that's like no other I've heard, but as far as pitch bending or phase shifting, I don't see it and I don't hear it.

Now the Vox circuit does have a phase splitter in the signal path. Look at the AC-15. The circuit is more complicated and looks nothing like the Fender circuit. It does resemble the Magnatone circuit though.

I've been studying the AC-15 circuit for a while now and am intrigued enough that I will probably build it.

[Willabe]

Isn't this a boot strapped split load?

I don't get it?


           Brad    

Edit; Oh I think I get it, your saying the guitar signal is injected after that tube. So it splits the trem but is not splitting the guitar signal.     

[sluckey]

Isn't this a boot strapped split load?

I don't get it?


           Brad     

Yes it is. But it ain't in the signal path.

[Willabe]

Look at my edit, I was writing as you posted.   


          Brad     

[Willabe]

I wonder what would happen if the circuit was changed to this?

(Couldn't be that easy.)


              Brad    

Edit; The 2 x 1M voltage dividers after the splitter might need to be adjusted.

[HotBluePlates]

The split-load for the trem signal and the split-load for the guitar signal must be different triodes.

But the 2-1/2 tube 6G_A version should be able to be converted to the true vibrato.

I think I'll try re-drawing the Fender circuit to see what's different and where compared to the Vox/Wurlitzer version.

[Willabe]

The split-load for the trem signal and the split-load for the guitar signal must be different triodes.

Ok, why?

But the 2-1/2 tube 6G_A version should be able to be converted to the true vibrato.

I think I'll try re-drawing the Fender circuit to see what's different and where compared to the Vox/Wurlitzer version.

Great!


           Brad     

[HotBluePlates]

The split-load for the trem signal and the split-load for the guitar signal must be different triodes.

Ok, why?

Did you download the Wurlitzer vibrato pdf that Kagliostro linked in the original post?

Fig 11-14 which shows the circuit shows a split-load for the guitar signal prior to a bunch of resistors and caps; I look at the tube as input to 2 points of a bridge, and the output points of the bridge go on the the push-pull stage.

The output of the bridge are 2 signals 90-degrees out of phase (180 degrees due to the split-load, narrowed to 90 degrees by the RC networks). The oscillator signal is applied at the grids of the push-pull stage, along with the 90-degree-apart guitar signals (though each is isolated from the other by R14, R17).

Fig 11-14 assumes a push-pull oscillator signal has already been developed. So you see that each is made into a push-pull signal separately. And the guitar signal gets applied to the R's and C's, while the oscillator is applied to the push-pull mixer stage.