Add On Vibrato Circuit Inspired From Trem-O-Nator

[ghostinthescope]

Hello, I spend a lot of time reading posts on this site and I have learned so much here. Thank you everyone for all the experience, thoughts and ideas everyone shares here. This is my favorite tube amp related forum by far.

I have been experimenting with the attached circuit and wanted to share it in case someone else would like to experiment with it. I took inspiration from Sluckey’s use of the vactrol in his Tremonator. Please share your thoughts or comments.

I wanted to make a pitch shifting vibrato without varistors. This is probably most akin to the 2 varistor Magnatone vibrato sound. Something that was a small parts count, could be easily added to a Fender circuit, and make little to no change in the stock signal when the effect is switched off.

A modified Princeton Reverb schematic was used to show the add on circuit. It could have easily been an AB763 schematic or similar.

Audio clips are crummy phone recordings with the speed control being adjusted while playing. Intensity control at max setting.

https://soundcloud.com/user-27510026/sets/test-clips/s-7R9NCRXCipF?si=f3912665d3bc4c7b93202d084ab6028d&utm_source=clipboard&utm_medium=text&utm_campaign=social_sharing

notes from the schematic…

NOTES:

*has only been bench tested as of 5/12/24
bench tested on princeton reverb circuit and single channel deluxe reverb 

C2:  3300pf - 0.01uf adjust for bass and effect intensity
Smaller = less intense/ less bass

C3: removes oscillation adjust as needed

R7: smaller value increases intensity at a trade off of volume/tone  when vibrato is
completely bypassed from signal path. 620K was satisfactory for intensity and signal loss

Footswitch: if oscillator is made switchable for both tremolo and vibrato effects a dpdt switch could be used to ground vibrato circuit signal as well as oscillator circuit signal.

I have never tried this idea of a footswitch and not sure how well it would work with a cable over 6’. Has been tested out of amp with a cable of 6’

Also of note... The original tremolo circuit and intensity control have not been eliminated.
the intensity control of the bias vary tremolo circuit will need to be set to minimum in order to
make use of the pitch shifting vibrato or use a switch to select between the two as mentioned
above.

Hopefully I explained things clearly here in this post. Please let me know if I’m using wrong terminology or unclear descriptions.

edits: I changed the note about C2... it originally read smaller = less intense/ more bass
         It's supposed to be less intense/ less bass
         
         Updated notes and schematic reposted

[tdvt]

This is really cool, thanks for sharing!

I think the clips sound great & it has a nice range of speed.

If I am reading it right, you are sort of co-opting the stock vibrato LFO, so it is doing double duty, running one or the other..?

[ghostinthescope]

Thanks tdvt!

Yes, it’s just as you say… co-opting the stock oscillator circuit the oscillator can be used as an “either or” double duty by means of a switch or by turning the intensity knobs. There is room to explore different ways of switching between tremolo and vibrato. If the oscillator controls both the tremolo and the bias vary at the same time it won’t do any pitch shifting.

[Willabe]

Sounds pretty good to me.

Could you run a 2nd of the same circuit in series for a deeper, fuller effect? Would it work? Could you run it in stereo with say 2 single ended Champ power amps?   

I have a new Maggi with stereo pitch shift. That thing is the bomb when run in stereo, and each section is only 1 phase shift. You can run it in mono, there's a 3 position rotary switch, off/mono/stereo. Going from mono to stereo is a huge difference in sound and depth. With stereo there's a 'watery flutter' sound that's added in/on top of the pitch shift that it gets going from speaker to speaker.     

[tdvt]

Part of the reason I asked is I had a similar thought as Willabe, having a second LFO...

Further & to that end, while I am less of a tinkerer than most, I had been fooling with a few of the MOSFET trem drawings found out & about (several here) & I had really good results with the LFO section. Not yet sorted with tying it into the circuit to my satisfaction.

I tried a few variations, the Trem-O-Nator among them, but haven't gotten back to it for a while.

As the LFO isn't carrying any signal anyway, I see no reason not to try SS. Your tube-driven pitch shift with a SS LFO could be a nice tidy package.


Looked through my MOSFET trem files, the one attached below was from here, & is close to where I was, but I had added some zeners & changed a few other values while fooling around. Not sure I have it documented..

I have a new Maggi with stereo pitch shift. That thing is the bomb when run in stereo, and each section is only 1 phase shift.

That is quite an amp, pretty smitten with some of the vids I watched about it.

[ghostinthescope]

Thanks for the comments willabe. I would love to here that maggie you speak of. I'll check out some videos.

Two vibrato circuits in a row might be possible, but does have some obstacles that come to mind. The signal needs to be inverted (180 degrees) and also 2X the gain of the dry signal. I think the vintage magnatone circuits reduce the original signal by half or more before any phase shifting happens. Maybe that would be the way to achieve this. The further down the signal path the vibrato circuit goes the more likely it is to run out of headroom. Also I think it would all need to happen before the reverb. I will definitely be thinking on this puzzle!

cool idea to run it in stereo with two single ended amps. Is there a schematic of your maggie floating around the internet?


tdvt, thanks for sharing this SS tremolo circuit! I agree that since the LFO isn't in the signal path it would be a good pairing with the pitch shift and I would really like to pair these circuits. it would help keep the triode count down.

do you think there is a way to add a second transistor and vactrol to that circuit? could the vactrol LEDs remain in phase with one another? and/or is there a way to make the vactrol LEDs flash out of phase with each other?

[tdvt]

do you think there is a way to add a second transistor and vactrol to that circuit? could the vactrol LEDs remain in phase with one another? and/or is there a way to make the vactrol LEDs flash out of phase with each other?

I am NOT the guy to ask about that, as the SS trem trial was the first I have fooled with anything solid-state in a loooong time. There are many here that have a deep working knowledge of SS, so I hope they will weigh in.


Thinking you would find this interesting; MOSFET FOLLIES, as you can still look at things from a "tube" perspective while the SS thing slowly sinks in.

Anyway, until someone says otherwise, I would think that you could parallel the LED-sides of 2 Vactrols to control 2 otherwise discrete circuits from the same LFO...?

I have also seen a couple designs with people using the same MOSFET in the schematic I linked, in a follower circuit, with the main purpose of flipping phase.

So that might be food for thought towards the second half of your question.

[Willabe]

cool idea to run it in stereo with two single ended amps. Is there a schematic of your maggie floating around the internet?

My Maggie is 2 @ 2 x6V6 22w amps, so 44w. I haven't found a schematic for it. But it's closely based on an old Maggie. I know they changed LFO to SS. They did it in part to get a much slower modulator speed. 

What I've thought of for a long time now is doing the Maggie pre amp in stereo and feeding it into 2 @ 6L6GB/C SE amps. So ~20w? That would be much more manageable volume wise.

The signal needs to be inverted (180 degrees)

do you think there is a way to add a second transistor and vactrol to that circuit? could the vactrol LEDs remain in phase with one another? and/or is there a way to make the vactrol LEDs flash out of phase with each other?

You mean the LFO signal? The old Maggies took the output from the LFO at the K and the plate. Look at the old schematics. There in Doug's library.

I will say that there is NOTHING that sounds like a Maggie pitch shift that I've ever heard, NOTHING! 

[tdvt]

Is there a schematic of your maggie floating around the internet?

I haven't found a schematic for it.

Don't recall exactly where this turned up. Pretty Deluxe Reverb-ish. 

EDIT: Came from HERE

Anyway....

[ghostinthescope]

tdvt, thanks for the schematic, thread and the Mosfet follies link.

I tried tying two LEDs in parallel to the cathode of an oscillator, but the brightness drops with two. I think a second driver for the second vactrol is necessary. Maybe Mosfet Follies holds the key. I did experiment with placing the vibrato circuit earlier in the signal path and it works, but will need some tweaking that I didn't have time for. A deeper vibrato with two stages seems possible, but I won't have time to experiment until this weekend or the next.

You mean the LFO signal? The old Maggies took the output from the LFO at the K and the plate. Look at the old schematics. There in Doug's library.

I have seen how they use the cathodyne phase inverter for the LFO, but I meant the guitar signal circuit. Most of those vintage Magnatones have triode with a cathode resistor twice the size as the plate load resistor. Usually 47k for the cathode and 22k for the plate. That's the triode for the first and second part of the phase shifting circuit with the varistors. Seems that the gain for the signal coming out of the plate is less than 0.5 gain at that point. And the gain coming out of the cathode must be 1 or less.

Could plug it into the tube gain calculator for a more accurate number

https://www.ampbooks.com/mobile/amplifier-calculators/output-impedance/calculator/

My Maggie is 2 @ 2 x6V6 22w amps, so 44w. I haven't found a schematic for it. But it's closely based on an old Maggie. I know they changed LFO to SS. They did it in part to get a much slower modulator speed. 

What I've thought of for a long time now is doing the Maggie pre amp in stereo and feeding it into 2 @ 6L6GB/C SE amps. So ~20w? That would be much more manageable volume wise.

I don't really play anything over 20w anymore... hard on the back and too loud for my ears these days!

[Willabe]

I don't really play anything over 20w anymore... hard on the back and too loud for my ears these days!

I don't play out any more, maybe some day in church again, that would be nice. The Maggie I only use for the vibrato, played clean, so it's not too bad on volume. For distortion I use smaller amps, 20w and under.

That's why I thought a Maggie pre with the vibrato into a couple of SE Champs, 6V6 or 6L6GB/C, something around 20w or so. The stereo vibrato is just killer! (If you like that kind of thing.) 

[J3ona1]

This is an awesome circuit! Did you use terminal strips to add the circuit or fashioned a small/daughter board?

[ghostinthescope]

This is an awesome circuit! Did you use terminal strips to add the circuit or fashioned a small/daughter board?

Glad you like it, J3ona1. I have not installed this in an amp. It's only been tested on the bench. If you think you might try to use it in an amp I'd recommend prototyping it yourself and playing around with it before trying to install it, since I only had a weekend of playing around with it and didn't get to road test it. I used that princeton schematic to show how the circuit was implemented, but I think it might be challenging to squeeze into an actual PR! Maybe more appropriate for a build from scratch? But if you end up messing with it let me know how it goes!


willabe, I'll bet that dual single ended amp would have sound awesome.

[J3ona1]

Folks sorry to resurrect this thread but I LOVE the sound of this vibrato and I have a revibe chassis on hand. Given that I have four triodes to play with  (after the 6g15 circuit) could I add a cathode follower to this addon vibrato to deepen the effect, or would the gain be unnecessary for a standalone unit?

[trobbins]

The vactrol based circuit in this thread doesn't seem to provide pitch shifting vibrato imho, and is more related to tremolo with a modulating high-pass filter characteristic.

The vactrol and R4 work with C2 to provide a high-pass corner at relatively low bass frequency that would shift up to about 1kHz max depending on the LED current swing (abt. 1mA pk) in the Vactrol.

[J3ona1]

I would build a Revibe but it sounds too much like a tremolo circuit mixed with a wah, and I really like the sound of vibrato (with the hope of emulating rotary sounds). I guess I'll have to shoot for building a vibrato only unit based on the AC30.

[ghostinthescope]

Hi J3ona1,
You might be able to get a deeper effect here if you used a cathode follower, but you’d need to spend quality time breadboarding to verify and tweak before you build it into a chassis. I’ve only bench tested this circuit through attaching it to two different amps.

I regret titling this thread as an add on circuit. It should’ve been “please experiment and have fun circuit/share your experience circuit”.

That being said if you experiment with this please share your thoughts.

[ghostinthescope]

Hi Trobbins,
Thanks for looking at this and commenting on it. I appreciate a new set of eyes and new perspectives.

I spent some time with this on a breadboard and will share my observations about this circuit.

Before posting in this thread I used both a stroboscope and oscilloscope I did verify that it indeed shifts pitch. No, it absolutely isn’t as deep as a two stage Maggie, but to me it sounds like a single stage one.

Some important aspects of the circuit may have gone overlooked in the analysis that was given.

This circuit inverts the original signal, amplifies it (at max a gain of 2) and then when the vactrol is off inverted and non-inverted signals are mixed together. The inverted signal at a gain of 2 cancels out the non-inverted signal and remains inverted at a gain of 1. That being said… the inverted signal has some gentle filtering, so those frequencies don’t get completely inverted as they don’t have enough amplitude to cancel out their counterparts in the non-inverted signal.
When the vactrol is on the inverted signal is shunted to ground and only the non-inverting signal remains.

[J3ona1]

Hi J3ona1,
You might be able to get a deeper effect here if you used a cathode follower, but you’d need to spend quality time breadboarding to verify and tweak before you build it into a chassis...

That being said if you experiment with this please share your thoughts.

Thanks ghostinthescope!!! I'll let you know if I give it a shot!

[Willabe]

I would build a Revibe but it sounds too much like a tremolo circuit mixed with a wah, and I really like the sound of vibrato (with the hope of emulating rotary sounds). I guess I'll have to shoot for building a vibrato only unit based on the AC30.

I LOVE pitch shift vibrato!!!!!! 

I don't think the AC30 type will be deep/thick enough of a true pitch vibrato. I played a reissue AC30 and the vibrato was meh. It's only 1 stage of pitch shift.

But, you could build a Warbler, it's true pitch shift vibrato. I built 1, it sounds great! It's all tube, only 3 small bottle tubes, 3 stages of pitch shift, has a proven design and layout with all documentation in the link below from our friend Sluckey. He did a great job on this build, from start to finish, as always. Build version 2.

https://www.sluckeyamps.com/warbler/warbler.htm
 
I love the sound so much, that I bought 1 of the new Magnatone amps with the Maggie vibrato, the Twilighter Stereo. It's killer!!!!!! Nothing like, nothing comes close to it. It's it's own thing. There's a rotary switch on the control panel that you can set for using the vibrato in mono or stereo. It has a 2 x 12" cab. Set for mono it sounds good, real good, but set it for stereo and wham!!!!!           It gets this fluttery/watery sound added on top because of the effect flipping between the 2 speakers back and forth. Man!!!!

Neil Young has had a stereo Maggie on stage for a few songs for decades. I spoke with Larry Cragg, Neil's long time guitar/amp tech about the Maggie amps. Because of Cragg's experience with this/these amps, Magnatone asked him to help them develop the new line of pitch shift Maggies based on the old amps.   

He told me that he told the guys at Magnatone don't even build the smaller cab 2 x 10" Maggie stereo, their too close together, it wont get the watery flutter like the 2 x 12" cab gets. He feels that's a huge part of that amps sound. I agree, I love it! 

I'd like to see Maggie put out just a stereo preamp, with 4 stages of pitch shift, 2 on each side. And a SE stereo just like the Twilighter, but a single 6L6GC on each side, that would get the amp down to ~20w's total, instead of the 44w, 2 x 6V6 per side of the Twilighter. 

I own a real Leslie, with this add on that lets you use a volume pedal/expression pedal so you can get different speeds/more speeds than just the normal 2 speeds, slow/fast. There's different speeds, in between the slow/fast of a Leslie, that to me sounds much better for guitar.

The real Leslie with the extra speeds is unbelievable, if you like that sound. But the Maggie is stunning too. They are both amazing. And both are big, heavy and expensive. But their different animals.

I also have a Diamond Vibrato V1 pedal that's really good sounding. They re-issued it as V2. I don't know if they sound as good.

And there's many Leslie type pedals out there now, and many sound good to very good, but none of these move real air, like a real Leslie. The Maggie moves air, kind of, when in stereo with sound flipping back/forth between the 2 speakers, that's where the watery flutter comes from.

Maggie Stereo Twilighter;

And Lonnie Mack started the Maggie thing, a lot of his old cuts use it, it was his signature sound, just drenched with it.

[trobbins]

Thanks for highlighting your approach.   I can now see the mixing aspect occurring at the 3M3 input node.  The reverb preamp will load that node by 1Meg above a few hundred Hz, and the 10pF across the 3M3 will further load the node above a few kHz. 

It looks like the mid-point of Vactrol LED operation is circa 260Vdc level on the 12AX7 anode, so (420-260)/220k = 0.7mA.  The peak plate voltage swing is likely about 100V away from the 260V mid-point, so down to about 160V anode, which indicates Vactrol current could peak up to (420-160)/220k = 1.2mA. That indicates the phase corner mid-point frequency is about 5n6 and ~45kohm = 600Hz, and the corner frequency could swing up to circa 5n6 and 12kohm (eg. vactrol current peak of 1.2mApk) = 2.4kHz.  The Vactrol characteristic likely has quite some tolerance, and aging, and an asymmetric delay, but seems to be a good means to an end.

Maggie's vibrato method swings the phase sort of symmetrically around a mid-point of 1kHz.  Your phase shift perhaps swings around a somewhat lower mid-point frequency, but yes I can now see how the effect could get close to the maggie's method.

The Vactrol LED on-voltage is ~1.5V, so putting two in series (and then paralleling the LDR's to achieve greater 'swing') may not work even though your amp has quite a high B+ level for the oscillator stage, as the grid leak will push the Vgk much more negative than 1.5V.  It may be a bit too fancy to practically generate a low voltage negative rail (circa -1.5V) that could then allow one or more extra Vactrols to be inserted in series in the cathode.

I'm not sure how high the Vactrol LDR resistance can extend, or whether there is light leakage into the package, but the datasheet does seem to say it could be quite high such that the 1M pot would be the limiter.  I'm not sure if its practical to bleed some dc bias current through the LED to shift it's mid-point resistance slightly lower, and whether that would have any benefit.

Do you notice any LFO frequency noticeably bleeding into the speaker output and causing concern?  Hopefully all the stages, and output transformer, sufficiently attenuate anything coming through below say 20-30Hz.

Ciao, Tim

[J3ona1]

I would build a Revibe but it sounds too much like a tremolo circuit mixed with a wah, and I really like the sound of vibrato (with the hope of emulating rotary sounds). I guess I'll have to shoot for building a vibrato only unit based on the AC30.

I LOVE pitch shift vibrato!!!!!! 

I don't think the AC30 type will be deep/thick enough of a true pitch vibrato. I played a reissue AC30 and the vibrato was meh. It's only 1 stage of pitch shift.

But, you could build a Warbler, it's true pitch shift vibrato. I built 1, it sounds great! It's all tube, only 3 small bottle tubes, 3 stages of pitch shift, has a proven design and layout with all documentation in the link below from our friend Sluckey. He did a great job on this build, from start to finish, as always. Build version 2.

https://www.sluckeyamps.com/warbler/warbler.htm
 
I love the sound so much, that I bought 1 of the new Magnatone amps with the Maggie vibrato, the Twilighter Stereo. It's killer!!!!!! Nothing like, nothing comes close to it. It's it's own thing. There's a rotary switch on the control panel that you can set for using the vibrato in mono or stereo. It has a 2 x 12" cab. Set for mono it sounds good, real good, but set it for stereo and wham!!!!!           It gets this fluttery/watery sound added on top because of the effect flipping between the 2 speakers back and forth. Man!!!!

Brad I wish I could get that stereo Maggie sound. Its to die for. Thank you for mentioning Sluckey's warbler. It does sound divine! Now to find a saturable reactor at low low low prices!

[passaloutre]

There has to be a way to do this without unobtanium parts.