Magnatone Duo-Vibra-Trem: hybrid dual channel stereo project

[Taho]

Hi Folks!

I have a project that's more custom than usual so I thought I'd register to document the process here and hear your feedback and recommendations.

First of all, my sources and inspirations:

• 2020s Magnatone Twilighter (current production) schematic: https://el34world.com/charts/Schematics/files/Magnatone/Magnatone_Twilighter_2020.pdf
• 1950s Magnatone 280b schematic: https://el34world.com/charts/Schematics/files/Magnatone/Magnatone_280b.png
• Sluckey's Maggie M-2: https://sluckeyamps.com/lil_maggie/lil_maggie.htm
• Sluckey's M10A: https://sluckeyamps.com/magnatone/Magnatone_M10A.pdf

The project is a hybrid of 1/current Twilighter Stereo + 2/old school Magnatone 280
I'm gonna use the Twilighter as the base replace the preamp and add all the modulation related function I need:
- preamp: two parallel channels with tone stacks from the 280B (unlike the modern Magnatones that have AB763 preamps basically, as you can see in the Twilighter schematic. And there are two distinct channels -- not one preamp that splits into 2 outputs)
- modulations: option for tremolo or vibrato or dry. Or any combination of two at the same time (independently left and right)
- modulations: option for mono or stereo (i.e. LFO in phase or out between the channels), for both tremolo and vibrato... unlike the modern Magnatones where Tremolo only works in mono
- vibrato: option to have one stage for the pitch shifting vibrato (like Twilighters) or two (like 280B's)

I think it would make much more sense if I show you the front panel and the signal path mock-ups:


https://storage.googleapis.com/thsr-bucket-public/Magnatone-Dual-Channel-Stereo/20250329%20panel%20front2.jpg
- The 280B preamp has a mellow switch on channel 1 and a bright on 2: this adds both on both

I'm leaning towards a Twin Reverb chassis / form factor. Which to scale would look like this very light-weight sparse layout:

https://storage.googleapis.com/thsr-bucket-public/Magnatone-Dual-Channel-Stereo/Screen%20Shot%202025-03-29%20at%209.38.47%20PM.jpg

The signal path:

https://storage.googleapis.com/thsr-bucket-public/Magnatone-Dual-Channel-Stereo/20250329%20signal%20path.jpg
- I'm still puzzling over the choice of LFO generator but I'm leaning more and more towards the high B+ SS option (LND150, etc..)

Anyway that's the general outline. I hope you will appreciate my extremely streamlined, minimalist, accessible and most of all economical build!

So at the moment I'm designing the vibrato stages.
I've redrawn the 2020 Twilighter from V5A (last preamp) until right before the phase inverter:

https://storage.googleapis.com/thsr-bucket-public/Magnatone-Dual-Channel-Stereo/Screen%20Shot%202025-03-29%20at%209.14.44%20PM.jpg
I'm a little puzzled why there is the 1M-1M voltage divider at the end but I think it's to tame the gain before next stage?

Then I added a second vibrato stage by checking how it's done in the 280, 480, M15, M20 etc:

https://storage.googleapis.com/thsr-bucket-public/Magnatone-Dual-Channel-Stereo/Screen%20Shot%202025-03-29%20at%209.21.04%20PM.jpg
Not really sure what to do with R36 and 35, and how to determine them? But I saw on some models that the cathode R of stage 2 is the same value as the plate R of stage 1 and vice-versa so I will try that at first and adjust as needed.
I also need to adjust the divider at the end to yield the same gain wether if one stage is on or two stages are on. If there's no easy way to calculate them mathematically I will put a trim pot and when wiring up

Finally, both options combined could look like this with a bypass 2P2T switch

https://storage.googleapis.com/thsr-bucket-public/Magnatone-Dual-Channel-Stereo/Screen%20Shot%202025-03-29%20at%209.24.48%20PM.jpg
And here I'm not sure if there are ways to simplify the fact that there are two separate caps and two separate dividers.

I keep you posted, Cheers!

[Willabe]

Welcome. 

That's a lot to take on, it's up to you what and how you build this, but;

I hope you will appreciate my extremely streamlined, minimalist, accessible and most of all economical build!

I'm leaning towards a Twin Reverb chassis / form factor. Which to scale would look like this very light-weight sparse layout:

I don't think it's any of the above. You've made it harder to build by adding several extras that you probably don't need. That chassis is full, 12 tubes, PT, choke, 2 OT's! And all that transformer iron and copper is fairly heavy.

The Twilighter schematic you posted is not the stereo Twilighter they make. I own 1. And the stereo vibrato it has sounds fabulous!    Mono vib is good but nowhere near as good as the stereo, I never use it in mono, never.

In stereo it has a fluttery/watery sloshing sound from the signal going back and forth between the 2 speakers that doesn't happen in mono.

I don't know how experienced you are with scratch builds but there's a LOT of circuitry and wiring that needs to be done with what your proposing. And grounding will be a major head scratcher to get right so it's quite.

The new stereo Twilighter and the old Maggy's with stereo vibrato used 1 preamp, with 1 set of controls, volume, treble/bass, the new Twilighter also has a mid control. Then split it to feed the vibrato sections.

Using 2 separate channels with 2 sets of controls, adds an extra 12AX7, mono/stereo switches for both channels, 4 input jacks; left/right/TS/TRS, vibrato/trem/dry switches for both channels, and a mono/stereo switch. That's a lot of added things you probably will not use very much, if at all.

I would not put the B+ filter caps on the back of the chassis in a dog house like Fender. I'd put the filter caps inside the chassis on the eyelet/turret board next to the tube circuitry it feeds. That way less long runs of wire in the chassis and easier/better grounding.

And you don't need a 2nd dry channel, just turn off the vib with the ft sw.

Going with a single channel pre that's then split to feed the 2 vib and power amp circuits, just like Magnatone did and now does again, will cut a lot of circuitry, wiring, grounding, and a 12AX7 tube. It will help it to be a cleaner build.

And especially on a build this large and complex you need to include your grounding in the schematic and layout. Which circuit grounds go to which filter cap ground leads. Has to be drawn out. If you use 'random grounding' it will almost certainly humm.

Read this from Merlin on grounding, use the multi star bussed grounding.

https://www.valvewizard.co.uk/Grounding.html 

[Willabe]

Example of how I draw out my grounding on the schematic and the layout drawing.

[Taho]

Willabe, thank you!

I don't think it's any of the above. You've made it harder to build by adding several extras that you probably don't need. That chassis is full, 12 tubes, PT, choke, 2 OT's! And all that transformer iron and copper is fairly heavy.

I was definitely sarcastic for those parts hehehe!!

The Twilighter schematic you posted is not the stereo Twilighter they make. I own 1. And the stereo vibrato it has sounds fabulous!    Mono vib is good but nowhere near as good as the stereo, I never use it in mono, never.

That could be super interesting if you have a Twilighter Stereo on hand. I studied its schematic but unfortunately I don't think it's publicly available. According to the drawings, it looks like when you switch to Trem mode (AM on the FM/AM switch) the phase of the LFO doesn't flip between the two speakers, in other words AM sounds the same with the rotary selector in Mono, Stereo or anything. Are you able to confirm that?

The new stereo Twilighter and the old Maggy's with stereo vibrato used 1 preamp, with 1 set of controls, volume, treble/bass, the new Twilighter also has a mid control. Then split it to feed the vibrato sections.

In the modern Twilighter schem I was amazed to find an AB763 tone stack! Look at V1a to V1b, reminds you of anything hehe?

And you don't need a 2nd dry channel, just turn off the vib with the ft sw.

Yes but if you wanna have channel 1 = dry and channel 2 = vibrato?
Or 1=vibrato and 2=tremolo.
Or 1=vibrato and 2=tremolo but their LFO out of phase.
I think my layout covers all possibilities.
A Rhodes electric piano will go into this so you understand why I want all these options

Read this from Merlin on grounding, use the multi star bussed grounding.

https://www.valvewizard.co.uk/Grounding.html

Thank you for the grounding resources and recommendations! I will strongly consider going doghouse-less now that you mention it.
The grounding scheme will be top priority when I get to drawing the chassis layout

[Willabe]

You can draw up a nice looking face plate layout for an amp, but it wont always work with the amps internal layout. 

I wouldn't get to attached to it until I fully worked out my chassis circuit layout.

[Willabe]

Here's what happened with the new 'traditional' Magnatone pitch shift vibratos.

Larry Cragg, Neil Youngs guiar/amp tech who was on the road with Neil for several decades was asked by the new Magnatone company if he would help them develop the new traditional Maggie vibrato amps. They asked him because of his experience with Neil's Maggie he has used on stage for decades that has pitch shift vibrato.

Cragg has a pretty/very large amp/guitar rental store in Cali and I think he still does amp/guitar repair.

Cragg said yes to Magnatone  IF  they made the amp also sound great with the vibrato off.

Cragg says the vibrato sounds great but the amp with the vibrato off sounds awful. That's why the preamp is different. Cragg worked with them, circuit, OT's, speakers, etc, and played all the proto types until he liked the amp with vib on/off. There's interviews out there with Cragg about the new vibrato Maggies. 

And yes you do want the mid control.     

I studied its schematic but unfortunately I don't think it's publicly available.

I seem to remember I read that someone said that they called Magnatone and they emailed them a copy of the stereo Twilighter.

According to the drawings, it looks like when you switch to Trem mode (AM on the FM/AM switch) the phase of the LFO doesn't flip between the two speakers, in other words AM sounds the same with the rotary selector in Mono, Stereo or anything. Are you able to confirm that?

That doesn't make any sense. Those are separate switches, they should both do what they do independently of each other. But maybe.   

If I get time I'll plug in and check those switches. I probably tried the trem 1 time. Just to make sure it was working more than wanting to hear it.  LOL

Yes but if you wanna have channel 1 = dry and channel 2 = vibrato?
Or 1=vibrato and 2=tremolo.
Or 1=vibrato and 2=tremolo but their LFO out of phase.
I think my layout covers all possibilities.
A Rhodes electric piano will go into this so you understand why I want all these options.

No, I don't understand.

Why do you want all that stuff? That's what modern pedals are for. Magnatone didn't even try to do all that. There's reasons why they decided not to do all those things.

I love trem, built amps with bias trem, but I bought that Maggie for 1 thing and 1 thing only,  true pitch shift stereo vibrato that only a Maggie can do.       

I never use it in mono, never use the trem, never use the wet/dry, dry/wet reverb. 

They really need the reverb it gives the vibrato depth and smooths it out real nice.

You can buy great sounding pedals with pitch shift vibrato and trem, some can probably do all the things you want, but none sound like a Maggie, even the pedals that are copied to sound like a Maggie. And they have pedals that split the signal so you can run stereo, vib on the left, trem on the right, etc.

A Rhodes electric piano will go into this .....

I've never seen a real Rhodes preamp that can do all those things. A Rhodes Plug In could probably do those things. 

But those are digital or maybe solid state analog IC chips, takes WAY less space, with a PC board almost no wires and small switches, completely different animal. We're talking full grown elephant to tiny mouse.

What you want to do with 12 tubes, PT/choke/2 OT's, all those extra switches for extra options, will be a nightmare to get wired up and grounded right so everything works without any parasitic oscillation, noise and humm/buzz.       

Thank you for the grounding resources and recommendations! I will strongly consider going doghouse-less now that you mention it. The grounding scheme will be top priority when I get to drawing the chassis layout.

They never should have built amps with a dog house for the filter caps. They should have always been local filter caps with a multi star bussed grounding. Especially now with the much smaller caps that are available.

And you never said anything about your scratch layout drawing and scratch build experience.

[Willabe]

You can draw up a nice looking face plate layout for an amp, but it wont always work with the amps internal layout. 

I wouldn't get to attached to it until I fully worked out my chassis circuit layout.

[Taho]

Absolutely, that front plate is just a mockup and bound to evolve. Currently I'm working on the schematic. Still puzzling over my questions form the first post though.

About the FM/AM I think I worded it wrong sorry. I meant that if you are in Stereo mode and you flip FM to AM, the stereo stops. Samely, if you are in AM mode and rotate between Mono and Stereo nothing changes. I might have misread the Twilighter Stereo schematic (yes it's not online but someone sent me a copy) but it strongly looks like it, so that's why I was asking to confirm when you have time.

And you never said anything about your scratch layout drawing and scratch build experience.

So far I've built a Deluxe Reverb, a 50W Hiwatt (mid 70s), a Marshall "Bluesbreaker" (Tremolo) and a 50W Plexi (~67 ~68). I've used existing layouts that I found around the web and frankensteined 2 or 3 together on each project: pictures of original gutshots, Sluckey, Ceriatone, Mojotone, TAD, Tube Town. I don't build the cabs and combos as I don't have wood working tools. This one will be my first original layout as I can't exactly copy the Twilighter or 280: I wanna go with turrets, but the Twilighter has PCBs and 280 has terminal strips.

[Taho]

Cragg says the vibrato sounds great but the amp with the vibrato off sounds awful. That's why the preamp is different. Cragg worked with them, circuit, OT's, speakers, etc, and played all the proto types until he liked the amp with vib on/off

Haha looks like he kept sending them back to the kitchen until they plopped a Twin Reverb front-end in there at which point he said, "yep greenlight to that!" haha
I for one love the Maggie sound without the vibrato and won't replace the tone stack

[Willabe]

No, he was much more involved than that. And it was probably his idea to use an AB763. He also told them to use oversized OT's.

He's a long time amp tech in Cali. He likes over driven amp/distortion. He was probably tweaking the proto types himself.

[Willabe]

Ok, I checked the Maggie trem, vibrato and AM/FM switches.

Yes, I think the trem is only mono. I don't hear it moving back and forth. It's extremely clear with vibrato!

If I had a 2nd speaker cab hooked up I could be 100% sure, but I'm 99% without a separate 2nd speaker cab. If it is flipping side to side, it's not even worth it. And I'm not getting down on my knees and sticking my ear in the speaker banging on a strapped on guitar.  LOL

The trem is pretty weak, doesn't sound good at all, I wouldn't use it. I'd use a little pedal before I'd use that trem. Any tube amp bias trem set up right kills this Maggie trem! 

The 5 position rotary switch, Off/Mono/Stereo/Wet-Dry/Dry-Wet.  The 1st 3 positions; Off/Mono/Stereo, only effects the vibrato/trem, the verb is controlled by the verb pot on the chassis and the ft sw. Positions 4/5; Wet-Dry/Dry-Wet, effects the verb/trem/vibrato. Setting to 4/5 there is no stereo vibrato. I can hear the verb, trem and vibrato switching sides in the cab when flipping the switch from 4 to 5 back to 4. But what it's really for is 2 speaker cabs.

If you had a 2nd cab, you could run 1 cab on each side of the stage in stereo, or run them in Wet-Dry/Dry-Wet. 

But here's the odd thing;

With either the vib/trem both off or on, reverb off or on, there's a boost in volume and fullness/fattness switching from mono to stereo. Very noticeable. Must be kicking in another triode stage or 2 when switched to stereo.

So if you want a fuller/fatter tone even with all the effects off, leave that 5 position rotary switch in stereo. 

This amp set for stereo vibrato with a good amount of reverb, just kills!!!!! Such a beautiful sounding guitar texture. 

This amp and a real Leslie, with a 5 position speed mod, are the best modulated guitar sounds I've heard. The speeds in between the stock Leslie fast/slow sound much better for guitar than the stock speeds. Much better! (Guy in Minneapolis Minn., Bill sells the 5 speed mod for single speed Leslie's, Hammond organ guy.)   

[Willabe]

The build your proposing is extremely complicated.

You said this would be your 1st scratch layout and you haven't seen/read/understand/used bussed ground star layouts. Grounds are going to be a huge thing in a build as large and complicated as this. No exaggeration. 

Because of this it would probably be a very good thing to build the preamp with all the switch options on a large bread board and try each option to see if you even like them. And if they do enough to make them worthwhile to wire them up. Anything that could be eliminated would help a bit to a lot.

I think there's only a few guys to maybe 10 or so, here that could build what your proposing, making the scratch layout, including the grounding layout and make it all work and keep it humm free.

I'b build it as a pre amp FX unit only if I were to build it. Then if it worked I might build it as a full stereo amp, maybe.

I can't imagine having the vibrato and trem flipping back and forth in stereo between 2 speakers. I don't think that would sound good at all. 

And being able to switch between 1 stage or 2 stages of pitch shift on each side, seems a waste of wiring up 2 more switches and more clutter. Just turn down the intensity.

The new stereo Twilighter is only 1 stage of pitch shift on each side. But they're driving them pretty strong. So they get away with it in stereo. Mono's still very good.

I'd love for them to make a 2 stage (each side) pitch shift to hear it. Even just as a preamp, with verb. Then run it through a couple SE 6L6GC/KT66 Champs with full TS, T/M/B. Somewhere ~20w. Nice.

[Taho]

Willabe, thanks a million for testing! This confirms a lot for me regarding the understanding of their circuit, also how good that vibrato sounds hehe!!

And you are actually justly pointing out that I'm proposing functionalities that might not sound good. What are the protocols for bread boarding with 300VDC voltages? I'm comfortable with 9V pedal proto boards but with tubes it seems risky

[Willabe]

There's a few treads here on bread boards guys built. Do a search.

My bread board is here somewhere.

Sivergun has 2 thread on his bread board. And dummyload has a couple boards.

[Willabe]

I found the link to my bread board build thread;

https://el34world.com/Forum/index.php?topic=18354.msg187427#msg187427

In reply #17 some other BB threads are mentioned.

Somewhere in there or in Silverguns thread, is where we got the screw terminal block and that you can get them in 600v. And the din rail for 8 pin octal tube sockets.   

[wsscott]

That Twilighter schematic is the one I got from Magnatone from the phone call to its tech department.  Really nice guy.  I used it with Stuckey's help to do a Mod to that circuit.  I eliminated the Vibrato in my Twilighter build because I had built the ReVibe Mod based on Stuckey's M10A so I could use that for the fantastic pitch shifting effect. So my build just has the Tremolo, which I really like.  Sluckey commented to me at the time that the circuit is very similar to the Deluxe Reverb's circuit.

Anyway that's the back story on that.  Good Luck!  Oh, and Willabe can tell you about his working with me on the grounding hum issues I had with my ReVibe Mod, that we finally got fixed a couple weeks ago.  And the Merlin article was quite helpful.  I have found that doing a Layout is really essential.  Next time I'll do it with Grounding layout in mind.  The DIYLC Layout program is pretty easy and it's free.  It takes a little time to get used to it's features, but pretty simple to use.