magnatone 260 vibrato in fender super 5D4

[dinkotom]

Hi all,
I've recently built a fender super 5d4 with vibrato circuit from Magnatone 260. I wasn't sure how to connect the output from vibrato to the PI. I tried few logical (to me ) points, but the amp squealed or buzzed loudly, so I settled with this circuit (see attachment).
- the amp doesn't squeal, sounds good, but the stock 5D4 circuit (without vibrato, it only takes disconnecting few wires to make it stock 5D4) sounds much fuller and bigger. I guess there must be a better way of connecting these two circuits, I'd appreciate any thoughts on this.
here are the stock schematics for maggie 260 and fender 5D4:
http://www.webphix.com/schematic%20h...atone_260a.pdf
http://www.webphix.com/schematic%20h..._5d4_schem.pdf
Thanks,
Dinko

[HotBluePlates]

Best guess, try ditching the 10M resistor.

The 270kΩ connected to it will still run to ground, with the ungrounded end connected to the 0.001uF cap and the input grid of your phase inverter.

The 10M worked against the 270k in the Magnatone 260 to set the relative levels of the two channels. The 0.001uF cap will likely need to be bumped up to 0.022 to 0.047uF to maintain the same bass response.

So I guess you got your hands on some varistors. I had recently been looking at the Magnatone schematics and patents, and thinking the varistors could be replaced with new vactrol optoisolators.

Guess I need to work that circuit up; if I do it right, I bet Doug would start selling tons of vactrols...

[plexi50]

I have been looking to build some amps with the maggie vibrato for some time. Mostly just gathering any info i could find on how to do it. Just think: Some where in a warehouse there are boxes full of NOS varistors and no one knows what they are.
Nice dream*

[terminalgs]

Best guess, try ditching the 10M resistor.

The 270kΩ connected to it will still run to ground, with the ungrounded end connected to the 0.001uF cap and the input grid of your phase inverter.

yeah, the 10M is a mix resistor for 2 channels in the 260,  you don't need that.   I don't think the 270K to ground is helping you either.  you want a grid reference to ground for the next triode, but  270K might be too small (especially following a .001, maybe change that too, like HotPlateBlues suggests).  maybe 1M? (M9 & M10 used a 1M there) you can stick a pot in there and see what works.

So I guess you got your hands on some varistors. I had recently been looking at the Magnatone schematics and patents, and thinking the varistors could be replaced with new vactrol optoisolators.

From what I've been led to believe,  the problem with optoisolators is the rise and fall is too fast,  its too close to on/off.  the varistor's (accidental?) magic was that  they responded slower and smoother to the rise/fall pulse of the oscillator.   Of course,  the Univibe did it with opto's,  and that's pretty lush. 

[tubeswell]

Hi Dinko,

I remember this project of yours :-)  Thinking about it a bit more, you probably would get lots of squeal with a vibrato circuit like that if you had a lot of gain in front of it. I found this with a stand alone vibrato unit I built - I only needed a gain of about 5 in the input stage, otherwise it squealed like a pig. Each triode in the 12AY7 in V1 has a gain of about 30 (I'd guess) - then you have an unbypassed triode stage recovering the tone stack that has a gain of maybe 40+ (I'd guess) before you hit the vibrato unit . So perhaps ditch this recovery stage and see if it squeals less. If you want more gain boost, you could move that recovery stage to in front of the PI. Then you could experiment with treble boost (or stepped treble boost) cathode bypass for it until the tonal balance was 'right'. Just sayin'.

As of the 'fullness' of the 5D4, its a rather different amp to the Maggie 260; having a simpler/fendery-er arrangement of gain stages, so that you can run the V1 stage with more gain and get a better SN ratio and more touch sensitivity etc. Maybe you have to think about that when you seek to combine the two circuits. (Although it may not be strictly possible to get the ideal sounds of both circuits in the hybrid).

Pete

[HotBluePlates]

...   I don't think the 270K to ground is helping you either.  you want a grid reference to ground for the next triode, but  270K might be too small (especially following a .001, maybe change that too, like HotPlateBlues suggests).  maybe 1M? (M9 & M10 used a 1M there) you can stick a pot in there and see what works.

Good point about the 1M vs 270k. 1M would likely be a better value, because it would load the vibrato circuit less.

From what I've been led to believe,  the problem with optoisolators is the rise and fall is too fast,  its too close to on/off.  the varistor's (accidental?) magic was that  they responded slower and smoother to the rise/fall pulse of the oscillator.   Of course,  the Univibe did it with opto's,  and that's pretty lush.

The problem is the varistors are near-unobtanium.

The varistors were used because they are variable impedances, and will change their apparent resistance with varying applied voltage. An optoisolator can be used to vary its apparent resistance with a varying control voltage.

What I'm thinking is the original Magnatone circuit used a fairly large applied voltage to alter the varistor resistance, but we don't need nearly so much with an optoisolator. The trick might be to adjust the size of the control signal in the Magnatone circuit, while keeping the circuit positions of the varied resistances. So, not a plug-n-play change, but something that needs adjustment for the new parts.

If you have a link for the discussion that showed optoisolators as unsuitable, I'd like to see it to better understand the issues. In the meantime, it's not hard to get ahold of the data sheets for them, and see what we're dealing with. Also, note that I'm not talking about neon opto's, which do have an on-off characteristic, but LED opto's (or something akin to old-fashioned incandescent units). Perhaps use of neon opto's was part of the problem...

[Geezer]

hmmmm, that would be a BIG development if something could be adapted from the old Maggie system/components to use modern replacements!

Maybe Sluckey has some info that would be helpful in seeing what was tried concerning new style opto units(?) 

[sluckey]

Not all optoisolaters are created equal. The type we need have a LDR (light dependant resistor) at the output. LDR output is more suitable for analog circuits.  It's much more common (except in guitar amp applications) to see an opto that uses a photo transistor, SCR, or Triac as the output device. These outputs are more common in digital circuits.

I used a Vactrol VTL5C1 to replace an obsolete optoisolator in my Sunn Sceptre's tremolo circuit. The original opto used an incandescent light source coupled to a LDR. The VTL5C1 uses a LED light source coupled to a LDR. Only had to change one resistor in the original circuit.

While researching the Vactrols I discovered there is a whole family of VTL*** optos. The differences is the characteristics of the LDR. The 5C1 has a fast turn on/turn off characteristics that is well suited for channel switching applications. But others in that family have a much different (longer) turn off characteristics that would probably mimic the varistor characteristics more better. I think it would be easy to modify the Maggie vibrator modulator circuit to work with Vactrols.... IF, you had the circuit on a breadboard or even a turret board. But it would be very difficult to experiment inside my M10A's chassis using real point to point wiring. I've seen some really nice proto/breadboard rigs in this forum. Hint, hint...

[dinkotom]

Best guess, try ditching the 10M resistor.

I did that in one of my previous 260 amps, it sounded like it was turned on to eleven and then some , and thumped like hell with the vibrato on, like the speakers are gonna explode

yeah, the 10M is a mix resistor for 2 channels in the 260,  you don't need that.   I don't think the 270K to ground is helping you either.  you want a grid reference to ground for the next triode, but  270K might be too small (especially following a .001, maybe change that too, like HotPlateBlues suggests).  maybe 1M? (M9 & M10 used a 1M there) you can stick a pot in there and see what works.

i tried 1M, but then the amp produced loud low hum, so I settled for 330K

Hi Dinko,

I remember this project of yours :-)  Thinking about it a bit more, you probably would get lots of squeal with a vibrato circuit like that if you had a lot of gain in front of it. I found this with a stand alone vibrato unit I built - I only needed a gain of about 5 in the input stage, otherwise it squealed like a pig. Each triode in the 12AY7 in V1 has a gain of about 30 (I'd guess) - then you have an unbypassed triode stage recovering the tone stack that has a gain of maybe 40+ (I'd guess) before you hit the vibrato unit . So perhaps ditch this recovery stage and see if it squeals less. If you want more gain boost, you could move that recovery stage to in front of the PI. Then you could experiment with treble boost (or stepped treble boost) cathode bypass for it until the tonal balance was 'right'. Just sayin'.

As of the 'fullness' of the 5D4, its a rather different amp to the Maggie 260; having a simpler/fendery-er arrangement of gain stages, so that you can run the V1 stage with more gain and get a better SN ratio and more touch sensitivity etc. Maybe you have to think about that when you seek to combine the two circuits. (Although it may not be strictly possible to get the ideal sounds of both circuits in the hybrid).

Pete

Hey Pete,
If I understood you correctly, I should connect the output from the volume pot directly to the grid of the first left 6CG7 triode?

and speaking of reproducing maggie vibrato using modern parts, hear this, I can't tell the difference:
Real Magnatone vs. BTMB Maggie Vibrato

[plexi50]

 The maggie box is so dam close to being identical. The magnatone though seems to have just the slightest twinng of clarity. What do you think?

[tubeswell]

Hey Pete,
If I understood you correctly, I should connect the output from the volume pot directly to the grid of the first left 6CG7 triode?

Yep I'd be inclined to try it and see if the squealing reduces (a bit). I'd say you should be able to at least turn the vol up some more without the squeal kicking in without V2 in there. If its still there, then try reducing the gain in V1 a bit more (but maybe just try taking out that V2 stage first). If anything that V2 stage could go back in after the vibrato circuit - before the PI

and speaking of reproducing maggie vibrato using modern parts, hear this, I can't tell the difference:
Real Magnatone vs. BTMB Maggie Vibrato

Yikes! its pretty close. The amp vibrato sounds like its got a slightly different waveform (but it could be that the similarity is just messin' with my head).

[terminalgs]

If you have a link for the discussion that showed optoisolators as unsuitable, I'd like to see it to better understand the issues. In the meantime, it's not hard to get ahold of the data sheets for them, and see what we're dealing with. Also, note that I'm not talking about neon opto's, which do have an on-off characteristic, but LED opto's (or something akin to old-fashioned incandescent units). Perhaps use of neon opto's was part of the problem...

I got that from a conversation I had with Don Garvin,  ex magnatone employee.   There was no in depth conversation about it, other than using LDRs or opto-isolator/photocouplers didn't provide the same effect as the varistors  due to the impedance of the varistors and the quick rise/fall of an LDR.   maybe its just a matter of finding the right LDR.   

again, the univibe uses LDRs , and its as lush as it gets.  but its all solid state.