Has Anyone Tried To Do A SS Version Of The Magnatone *True* Vibrato?

[Paul Marossy]

I heard one of these old Magnatone amps the other day and I like the true vibrato sooooooooo much more than tremolo. So I thought I'd try to do a solid state version of it but I'm an idiot - got nowhere with it and just wasted a lot of time. It sure would be cool to be able to get that sound in a little box though. I suppose the varistors could be approximated by an LED/LDR combo (if I could ever get the LFO to actually work) but I don't know if the circuit would even work the same with soon to be completely extinct FETs. Seems like this is one case where you actually have to use tubes because of their unique characteristics (and the secret spec varistors). Or am I once again wrong?

This seems to be in the same category as a Tube Driver starved plate sound using solid state devices, kinda not really do-able. I'd be happy to be wrong about all this... and please inform me if I am so I can feel normal again.

A sidebar: The way that the reversed transistor on the input of the Vox Repeater acts like a variable resistor, can something like that be done to approximate a varistor? That MPSA18 thread got me thinking about this whole thing again, which is why I started this new topic. I might be nuts (well I am) but I thought I'd ask these questions anyway.

[R.G.]

The Magnatone vibrato is done with two (IIRC) stages of all-pass phase shift implemented with tubes. Like all phasers, it generates the wobbling phase shift with all-pass filter stages, using the varistors as the variable resistors. Phasers first generated limited phase delay, then generate a notched semi-comb filter by adding the original signal so there is cancellation at frequencies where the shift is ultiples of 180 degrees.

That is, vibrato can be generated by not adding the dry signal to the phase delayed signal in what would otherwise be a phaser.

There's no magic in the varistors, as you figured out. They act like variable resistors. You can do this with LDRs easily enough.

The variable resistance can be FETs. Or even bipolar transistors or diodes, if you design within the limitations of whatever element is being used as a variable resistor.

The MPSA18 as a variable resistor in the Vox repeat percussion is a special case, or we would use reverwe-mode bipolars for all of our variable resistors. Reverse-mode bipolars have a very limited range of signal sizes for which their distortion is low enough. Diodes can act as variable resistors too - if you are willing to restrict the signal size across them to under 25mV or so to keep the distortion down.

A whole lot of pedal technology (other than distortions, of course) amounts to a search for and adaptations to some method of coming up with an electronically variable resistor.

[Mark Hammer]

I recently made myself a stereo unit, following the Magnavibe schematic, and I can not recommend it highly enough.

In general, the more stages of phase shift one uses, the wider the pitch shift in the vibrato.  The Magnavibe uses one stage, which simply lends a bit of "nervousness" to the sound, rather than turning your guitar into Buffy Saint-Marie or Stevie Nicks.  The Magnatone Panoramic amp also provides stereo vibrato, but uses a single inverted LFO to countersweep the vibrato in each channel of the amp back and forth.  I decided to use a separate LFO for each vibrato circuit, with a dual-ganged pot for adjusting the speed and intensity of the two circuits at once.  The nice thing about having independent LFOs is that they are rarely in sync, removing most of the periodicity of the modulation.

In stereo, the sound is lush and immersive, and most importantly from where I stand, not distracting.  Much the same way that you may only want normal tremolo here and there but don't mind leaving harmonic tremolo on for extended periods, this is also something you don't mind leaving on for a while.

Time Escobedo's "Wobbtron" can do a decent job, though some may prefer using an LDR rather than JFET as the variable resistance.  The second schematic provides a gain-recovery stage, and the pot on the collector allows for adjustment from vibrato to something more in the tremolo range.  You may note some similarities with the EA Tremolo in the design of the LFO circuit.  It's a classic.

Can one mix the two vibratos down to a mono signal?  Yeah, I suppose.  You can also listen to a JC-120 or a Leslie speaker or Axis: Bold as Love in mono as well.  But you know damn well it's not the same experience.

[Paul Marossy]

I recently made myself a stereo unit, following the Magnavibe schematic, and I can not recommend it highly enough.

In general, the more stages of phase shift one uses, the wider the pitch shift in the vibrato.  The Magnavibe uses one stage, which simply lends a bit of "nervousness" to the sound, rather than turning your guitar into Buffy Saint-Marie or Stevie Nicks.  The Magnatone Panoramic amp also provides stereo vibrato, but uses a single inverted LFO to countersweep the vibrato in each channel of the amp back and forth.  I decided to use a separate LFO for each vibrato circuit, with a dual-ganged pot for adjusting the speed and intensity of the two circuits at once.  The nice thing about having independent LFOs is that they are rarely in sync, removing most of the periodicity of the modulation.

In stereo, the sound is lush and immersive, and most importantly from where I stand, not distracting.  Much the same way that you may only want normal tremolo here and there but don't mind leaving harmonic tremolo on for extended periods, this is also something you don't mind leaving on for a while.

Time Escobedo's "Wobbtron" can do a decent job, though some may prefer using an LDR rather than JFET as the variable resistance. 

I bet that does indeed sound very nice. Is the Panaramic doing a stereo vibrato? I didn't realize that.

I vaguely remember the Wobbletron... that's going back a few years. I think I might play around with that one a little bit, looks interesting.

[Mark Hammer]

Yes.  The Panoramic does do a stereo vibrato.  But it is applied to the two amp channels (which power separate speakers) in reciprocal fashion, due to the use of one LFO.  Here's someone's demo of the 280.

[Paul Marossy]

Yes.  The Panoramic does do a stereo vibrato.  But it is applied to the two amp channels (which power separate speakers) in reciprocal fashion, due to the use of one LFO. 

OK, I get it. That's kind of what I thought as soon as I read "stereo", knowing what the schematic looks like. I kinda didn't follow that through all the way to the output. 

[Danich_ivanov]

Magnavibe is awesome. I would make a two transistor (non-inverting) preamp with some amount of volume, then two of these stages, and you'll be good to go. Last time i messed around with any kind of Lfo, it was a modified lfo from univibe roughly adopted to 9v. Yes, it requires a dual-gang, but i feel like it is totally worth it, especially for this kind'a thing, and fairly easy to set up. I tried it with jfets as varistors, worked just fine.

[Danich_ivanov]

Oh, there's also a fun thing to try with this type of circuit: since you have no dry signal mixed in, you don't have to worry about levels all that much, and what you can do is set emitter resistors lower than collector ones (in case you're going bjt of course), it should get less vibrato like, and more phaser/leslie like.

[teemuk]

Well, like R.G. pointed out previously, Magnatone's vibrato circuit is basically nothing but a variable all-pass filter, which modulates the phase shift with signal from LFO. The Magnatone circuit is excessively complex due to using varistors as variable resistive element in the all-pass filter, and due to introducing the LFO signal in a "balanced" manner. Otherwise its just bread-and-butter all-pass filter with cathodyne phase splitting. Adding more phase shift stages was probably deemed too expensive at the time.

Using "modern" components the circuit can be simplified to a very rudimentary topology, which was actually also discovered by Bonham himself. By that time though, phase shifting effects were already becoming "known art" in the industry. Everyone was making them at that time. To make a case in point, Bonham's later solid-state version of the circuit was basically just a "Uni-Vibe" circuit. (e.g. SS Estey amps employ discrete BJT phase splitting and LDR as resistive element).
The phase shifting effect craze was actually so huge that some sort of phase shift "vibrato" became a "standard" feature of many guitar amps, along with the usual spring reverb and distortion, of course. Solid-state naturally setting the trend, as was customary in those days. Typical feature was to select between former (amplitude-modulated) tremolo effect and (phase shift modulated) vibrato effect. So when you ask if it has been done? Yes, something like 50 years ago!
In just a few years from that, designers started to add more phase shifting stages (it was waaaayyy more economical now) and feedback, and –like pointed out previously - discovered the so-called "phaser" effect.

Actually "solid-state" conversion of the circuit is the most trivial part of the design. Finding a proper resistive element can be problematical. Especially regarding any sort of matching. Varistors are somewhat "obsolete" along with OTAs and those electroluminescent cells, LDRs are "slow" and have no definite "impedance", BJTs and diodes have terrible input signal range characteristics, FETs and MOSFETs suffer from terrible inconsistency and are becoming obsolete anyway. A tube-based variable resistor? Now that would be something Magnatone actually never did. Anyway, trust me, least of your worries is how a vacuum tube "cathodyne" phase splitter converts to a solid-state "emitterdyne" (I made that up but you should get the point).
BTW, One clever trick exploits MOSFETs of CMOS logic chips (such as inverters). These integrated MOSFETs actually seem to be rather "matched" as is.

[Eb7+9]

True hi-fi stereo Vibrato a la Don Bonham Magnatone 480
... circa 2005


www.lynx.net/~jc/stereoVibe1.html

[R.G.]

Hi, J.C.   

Ah, memory lane. I think my first vibrato phase shifter was my clumsy copy of a circuit that I found in one of the giant Markus compendiums of circuits back in about 1972. Your pictures bring up memories. It was on perfboard and had the two LDRs facing a blinky incandescent bulb, much like that.

I think I still have the book. Perhaps I should hunt that up.

Teemuk's right - this stuff is old.

[Eb7+9]

(apologies for un-necessary comments, shoulda taken the time to read RGs more carefully)

The higher voltage class-A stereo version sounds best of all the versions I've tried ... tho for slow speed stuff the stereo op-amp version produces a deeper panoramic sweep - esp good on bass

Matching optics is no biggie and not sure what Teemuk means by "no definite impedance" since biasing cells at a given mid-point is trivial ... what's this cmos trick ?!

[R.G.]

what's this cmos trick ?!

Early CMOS logic chips were un-buffered, and one of them offered access to the gates and drains of the actual inverter transistors. Hooking up the power supply pins (im) properly in activated the opposite polarity P-channel devices and allowed one to treat one chip as an array of N-channel MOSFETs with their sources tied together.  They had the advantages of monolith construction for matching.

I can't remember whether I saw it first from Anderton or Lancaster. I'd have to go do some research to find the source I remember.

[PRR]

> A tube-based variable resistor? Now that would be something Magnatone actually never did.

RCA did. Commercialized around 1950, though probably developed in the late 1930s.

This was for BIG AM radio transmitters. You have a big power oscillator for the RF wave. You then try to modulate that with an audio signal. The classic way was a BIG audio power amplifier swinging the RF power amp's supply rail.

In the Ampliphase transmitter, you had two RF power tubes. Audio modulated a small RF signal which was phase-shifted 135 degrees to two signals which drove the two RF power tubes. When combined, RF amplitude varied from zero to 200%, making AM. This eliminated two big audio tubes and all high-power audio iron. The phase-shift stages used reactance-tube techniques to vary the resistance in a phase-shift circuit.
https://www.americanradiohistory.com/ARCHIVE-RCA/RCA-Broadcast-News/RCA-95.pdf  pg 12
http://www.rossrevenge.co.uk/tx/ampli.htm
http://www.rossrevenge.co.uk/tx/block50.gif
Note that this scheme only aimed for 7.5 deg phase shift and near 1mc.

[Eb7+9]

So, correct me here if I'm wrong, but neither of these techniques can be used to simulate controlled floating resistors (what's required in a Maggie emulator) ... and the later could only produce very limited phase shift ...

so not of any real use here / and not worthy of mention other than vague annectodals

—-

To get back on topic, adapting the Maggie to SS naturally involves either using optics say Univibr style like I did above / ie., bulb and cells or opto-isolators operating differential (using current drive as I did in another couple of projects). or using pwm'd Analogue gates operating in a similar manner - as I'm experimenting with these days using a non-linear CV bender to smooth out the hyper ness of the 1/x transfer function

Again, to recap, there are no other (practical) methods that will provide four or more decades of variable floating redistance // what's necessary for produce a full depth emulation that is plenty adequate speed wise as well

[Danich_ivanov]

My memory made me rediscover this, that you might be interested in:



Pretty much a simplified univibe, i think it's worth a shot.

It is interesting that it has 3 stages, i guess to keep the circuit in phase, but if i were to make it, i would simplify first stage, or simply not modulate it.

On the other hand i don't know, maybe 3 stages will be awesome.

[bool]

That's a cool idea, but what a dump of transistors ....

It would be interesting to rework the circuit using something like mpsa13 darlingtons; that alone would cut the transistor count in half. And all the bootstrapping excess could be imho reduced with a better high-z biasing arrangement, cutting down the cap-count significantly.

Lastly, it would be quite interesting if the circuit could be adapted to use small logic-level mosfets instead of optos ... not jfets.

[R.G.]

So, correct me here if I'm wrong, but neither of these techniques can be used to simulate controlled floating resistors (what's required in a Maggie emulator) ... and the later could only produce very limited phase shift ...

I hesitate to imply correcting you, but it's not clear that the techniques could not produce the ultimate effect of a varying floating resistance, as they were optimized for something else entirely. A solid pass at designing a different end result might be effective. Or not. I think the question is open.

And, if you think about the desired result, it's not clear that only floating resistances are needed. After all the Magnatone stages are just phase shift stages. It's not clear how far from the pure, original circuits one would go to produce the effect.

So one is presented with the dilemma - how much change is allowed by one's own internal "originality meter"? The change being contemplated is already to rip out the varistor setup and replace it with something entirely different. It gets down to some philosophical questions of what, exactly is required in a Magnatone emulator. Just phasing? No problem. Must use tubes? Again, no problem. Must use tubes as all-pass filters? Trickier, depending on the circuit. Must use tubes in a phase-splitter based phaser with only a cap and a variable resistor as phasing elements? Yep, you probably will need a floating resistor for that. Is it OK if the floating resistor is gyrator based? How about doing the obvious and mechanically hooking up a gang of actual pots wobbled by a motor arrangement? True, that has some mechanical and reliabilitiy issues - but the obvious extension to constant-rotation pots is a whole new wrinkle that's tough to reproduce.

I guess the question is then who is appointed to judge how close to the original circuit is required for a Magnatone emulation?

Isn't what is needed really, after all, that it sound like the Magnatone? How's that measured?

so not of any real use here / and not worthy of mention other than vague annectodals

Again, who judges? I actually kind of liked it. It pointed out a different path that exists. Whether it's the only true, correct way to get to the end resuilt depends on who gets to be judge and jury.

And many of us like PRR's references. I would say that it was just fine for PRR's comments to be included, and that it did have worth.

To get back on topic, adapting the Maggie to SS naturally involves either using optics say Univibr style like I did above /

And as the fellow did in that Marcus compendium I mentioned. I really should go dig that out. I think it's in one of the boxes of books in the workshop. As for naturally, "naturally" has historically meant "what's easiest with the parts at hand".

ie., bulb and cells or opto-isolators operating differential (using current drive as I did in another couple of projects). or using pwm'd Analogue gates operating in a similar manner - as I'm experimenting with these days using a non-linear CV bender to smooth out the hyper ness of the 1/x transfer function

Again, to recap, there are no other (practical) methods that will provide four or more decades of variable floating redistance // what's necessary for produce a full depth emulation that is plenty adequate speed wise as well

Again, who judges existence of other methods or their practicality? I'm pretty certain that the designers of the Magnatone amps would think that using several hundred million transistors to implement a DSP processor to do a phaser, let alone coding up the application was rediculously impractical. And they were right - from where they stood. Where you stand and what you have experienced has a powerful effect on what is "practical". Others might have a different viewpoint of "practical".

[PRR]

> what's required in a Maggie emulator

Is the True Maggie plan posted in this thread and I missed it?

There's certainly may ways to skin a phase, but without a goal (proven product) it's all just gas.

[jubal81]

Surfy Bear used a string of zener diodes in place of the varistors in his new vibrato pedal and results sound pretty good.