Magnatone Varistors

[jewishjay]

Ok....I know this has been talked about before....but

1. what were the specs of the now unobtanium Maggie Vibrato varistors?
2. with only 2 varistors, is this Maggie 410 a good example of that "sound" or do you have to pick one that has four or more?
3. there are tons of MOV varistors on mouser....what am I looking for?

[sluckey]

That 410 was an entry level Maggie. A more desirable 2 varistor model was the Troubadour Model 213.

[jewishjay]

That 410 was an entry level Maggie. A more desirable 2 varistor model was the Troubadour Model 213.

Whoa. That 213 is much more complex. I dont understand whats happening in that 12AU7 at all. Might need a separate thread someday.....

For now, I think I'd rather tackle the single ended 410 with that hybrid 12dw7 in the middle. What an interesting tube! Half 12ax, half 12au....neato!

Mouser sells lots varistors...will any of them work? What am I looking for?

[sluckey]

Read this...

     https://ampgarage.com/forum/viewtopic.php?p=288904#p288904

Also search this forum for "magnatone varistor". Lot's of info.

[Willabe]

The 4 x varistors sound much better than the 2x models.

The 2 x doesn't have the depth/lush sound.

[jewishjay]

The 4 x varistors sound much better than the 2x models.

The 2 x doesn't have the depth/lush sound.

Which model would you say is "the one" ?

[Willabe]

Which model would you say is "the one" ?

             I think the Maggie's with 2 stages of pitch shift sound great, 4 x varistor.

 

[sluckey]

This is The One!

[Willabe]

This is The One!

Yeah that's top of their line? Stereo vibrato, 4 x on each channel, 8 varistors total, + reverb. Total of 12 tubes, and that's with a SS B+ rectifier. So 2 stages of phase shift on each channel, in stereo 4 phase shift stages total. 

[Willabe]

There's a member here, can't remember his forum name, that responded to a post on Maggie vibratos. I searched and cant find it. It was not that long ago.

He said he has lots of varistors for sale, disk and dog bone types. I think he's in Washington state or Oregon. He makes a Maggie preamp vibrato using vari's. Robert Cray was using it. I can't find his company either.   

[HotBluePlates]

There's a member here, can't remember his forum name, that responded to a post on Maggie vibratos. I searched and cant find it. It was not that long ago.

He said he has lots of varistors for sale, disk and dog bone types. ...

Zack Engineering

Listing 1

Listing 2

FWIW, the Magnatone circuit is not the only one that does true pitch-variation.  Vox' vib/trem circuit does as well (especially since they swiped Wurlitzer's vibrato circuit), though you will probably want to add a Speed pot in lieu of the switch.

[sluckey]

IMO the Vox circuit doesn't hold a candle to even the single stage Magnatone circuit.

[Blueboozer]

There's a member here, can't remember his forum name, that responded to a post on Maggie vibratos. I searched and cant find it. It was not that long ago.

He said he has lots of varistors for sale, disk and dog bone types. I think he's in Washington state or Oregon. He makes a Maggie preamp vibrato using vari's. Robert Cray was using it. I can't find his company either.   

He was selling them on Amazon. I bought 8 of each. Zack Engineering Vibroworld

Dogbone https://www.amazon.com/dp/B08TGPR8XR

Disc https://www.amazon.com/dp/B08TGV86P3

[Willabe]

Thanks HBP and Blueboozer. 

[jewishjay]

Ok, for now I think I'm gonna move forward on the 410 with this MOV substitute for each of the Varistors. That document made it seem like a really close facsimile, after several experiments. So I'm working on an eyelet board layout....

Any of you ever tried this MOV arrangement?

[trobbins]

I had another go at checking the MOV substitute clone a few months ago and updated my Maggie vibrato doc (https://www.dalmura.com.au/static/Magnatone%20vibrato%20design.pdf).

I could only purchase a few different MOV values to test (Littlefuse ZA V08, V39 and V82), and configured a V-I curve to closely match a real varistor VI curve.  However my VI curve is a smoothed plot from 15 spot measurements so only subtly identifies the regions in between the MOV part voltages where the curve departs from the real varistor characteristic.

A spectrum analyser can more easily identify differences between the clone and the real deal by assessment of the vibrato induced sidebands of a tone being passed.  I came to a view that a clone with a MOV voltage that is centred on the idle operating point is likely to provide a closer alignment of clone to real response.  The varistor idle point in my 213 clone was about 55V and 0.1mA.  My varistor clone had V39 and V82 MOVs, which have 0.1mA operating points below (35V) and above (77V), so the best clone I could configure also used 2x V39 MOVs to get 35V and 70V points at idle as the closest points on either side of 55V.  (I couldn't easily get in a V56 or V68 to check).  Martin used a V56 as the middle MOV, which should align ok. 

Your 410 clone amp should preferably operate at the nominal DC levels on the schematic, and with the same tube types.  From the 410 schematic I can see that the 2 series varistors and 180k have about 172V across them so the varistor idle voltage is likely up around 75V. 

I don't think there have been any detailed comparisons done in real life - as that would be difficult to set up - but cursory tests indicate acceptable clone performance.  So I'd suggest to just go for it, as any tweaking of parts is unlikely to show up noticeable differences.

[dragonworks]

I have two M15s and the Vibrato is working in both. My problem is the reverb in one of them.

[jewishjay]

Ok! And referring back to the schematic I posted in the 1st post of this thread....

I notice there's not a 1Meg input impedance R....should I add one? or not add one? How would it affect...?

1. volume
2. break up
3. current flo

[sluckey]

I would put one between the tip and sleeve of the Hi jack. Otherwise the preamp tube must rely on the stuff in your guitar to provide a grid leak resistance. Adding the resistor is totally transparent and just a good thing to do. Makes your amp more valuable.   

[jojokeo]

I got lucky literally on my birthday a number of years ago and ran into the actual guy that worked for Maggie and if memory serves was the actual designer of the vaunted circuit. He was selling them for a hefty price but if more were bought the prices came down to maybe $40-$50 each? I bought a nice little supply of at least a dozen and have them wrapped up somewhere saving them for a rainy day or when the itch needed to be scratched? I've got various pedals that do this vibrato pretty well. I'm sure Sluckey would scoff but I've even modded quite a few of the old Boss CE-2 chorus pedals where increasing the depth and  decreasing speed along with a modern dual op-amp chip and some other goodies and the pedal goes from chorus into vibrato mode very well. I only like it when running a wet/dry rig where extra juice evens out in the mix. Overall, I'm not a huge trem OR vibe guy...    to each his own. There's only so much a person can utilize this effect for in my opinion then it's on to actual playing chops vs the occasional wet effect. But the effect wears off after a little while. As they say, your mileage may vary, lol. Another pedal that you can get stock is the BBE Mind Bender. You can pick them up for <$100 and see if you really like the effect or not before going to the trouble and expense of trying to recreate the vibrato wheel using inferior parts and circuits ending in dismal failure and much frustration? Just my 2 cents folks...

[jewishjay]

UPDATE: I built it! The amp works and sounds nice...but there's no tremolo (or vibrato) effect. The speed and depth controls do nothing. Here's the voltages I read...perhaps I'm not developing any oscillation? There's zero volts on the grid of the oscillator where there should be 22 volts. Please note, I didn't have an 820k resistor, so I used a 750k ...that's not the problem though...is it? There's also low voltage at the grid and cathode of the phase splitter thing. What do you all think?

[sluckey]

Monitor the dc voltage at pin 6 of the oscillator tube. If it's constantly changing the oscillator is working. If it's never changing, ie, steady dc voltage, the oscillator is not working. Not much to go wrong in that circuit.

[jewishjay]

Monitor the dc voltage at pin 6 of the oscillator tube. If it's constantly changing the oscillator is working. If it's never changing, ie, steady dc voltage, the oscillator is not working. Not much to go wrong in that circuit.

Never changing. Steady DC.

[PRR]

It may not be your problem, but why is 12DW7 pin 3 so very low?

[trobbins]

Do you have any other 12DW7's to try, as grid leak bias is very touchy for operation and 109V on the anode indicates there is not enough voltage on the grid?  What is B3 voltage?  The grid is not appropriate to probe with a meter as it will significantly modify the operation.   You may get better operation by increasing the grid leak 3M3 to say 6M8 and re-checking the anode voltage, and then adjust to try and get closer to 190V (assuming B3 is nominal).

[sluckey]

Monitor the dc voltage at pin 6 of the oscillator tube. If it's constantly changing the oscillator is working. If it's never changing, ie, steady dc voltage, the oscillator is not working. Not much to go wrong in that circuit.

Never changing. Steady DC.

Well, there ya go. Double check your wiring and component values. Be sure you used pins 6, 7, and 8 for the oscillator. Change the tube. If no spare 12DW7, use a 12AX7. If a 12AX7 fixes the oscillator, use it while you order another 12DW7. If tube will not fix change those 3 caps and 3 resistors between the plate and grid.

[jewishjay]

Do you have any other 12DW7's to try, as grid leak bias is very touchy for operation and 109V on the anode indicates there is not enough voltage on the grid?  What is B3 voltage?  The grid is not appropriate to probe with a meter as it will significantly modify the operation.   You may get better operation by increasing the grid leak 3M3 to say 6M8 and re-checking the anode voltage, and then adjust to try and get closer to 190V (assuming B3 is nominal).

WINNER! I changed the 3M to 6M8 and the wiggler started wiggling. Also....I had swapped the AX and the DW when I took those voltages earlier, so now everything is really close to the voltages on the schematic. THANKS SO MUCH FOR ALL YOUR HELP. You guys are the best!

It works, and sounds ...nice. I'm not blown away...I think I like the Vox and Selmer stuff I built previously, better. But let me build a cabinet, and then I'll show you guys a demo....

[jewishjay]

I like the sound, but it's definitly tremolo, not vibrato.
It doesnt sound like clips I've heard of the original.
Maybe it can't? What are your thoughts?

[sluckey]

I definitely hear vibrato. Not as deep sounding as a real 410 I just heard on youtube. And definitely not as lush sounding as the two stage vibratos.

[Willabe]

Agree, it's vibrato, just not as full/lush as a 2 stage Maggi.

I tried to tell you when you started this to go with the 2 stage. 

   

[trobbins]

I would still be concerned that circuitry is not properly biased.  With the low frequency oscillator now working you can't use Vdc of the anode as a good indicator.  And measuring <10Hz with a DMM may be quite inaccurate, so you probably need to measure the Vpp of the anode using a scope with at least a 10:1 probe.  If you set Intensity at min, do you get near 68V and 240Vdc as shown on the schematic for the modulator stage?  Is the output stage cathode at about 14Vdc ?

[jewishjay]

I would still be concerned that circuitry is not properly biased.  With the low frequency oscillator now working you can't use Vdc of the anode as a good indicator.  And measuring <10Hz with a DMM may be quite inaccurate, so you probably need to measure the Vpp of the anode using a scope with at least a 10:1 probe.  If you set Intensity at min, do you get near 68V and 240Vdc as shown on the schematic for the modulator stage?  Is the output stage cathode at about 14Vdc ?

I don't have a scope, so here are my measurements with a DMM. I see what you mean though, probing the grid seems to change the oscillation, but here's what I think I'm seeing....and It seems you're right, the oscillator isn't biased right. Where do I go from here?

[trobbins]

I'd suggest you check the specs on your DMM to see what ACV accuracy is available at low frequency.  If the DMM can measure down to say 10Hz as a spec, then increase the oscillator speed to max to give the meter a good chance of being believable.  The nominal Maggie oscillator signal level is about 66Vrms at the anode (check if your DMM ACV range only applies a 10Meg loading), with about 13% THD (not a big deal and you would need a spectrum analyser to confirm nominal signal harmonics are being generated). 

If you temporarily open the oscillator feedback path (eg. lift a leg on the middle 0.01uF cap) then the plate voltage will only have dc voltage on it - see if that is close to 190Vdc and that you have about 300V feed to that stage.  You may still need to adjust the grid leak resistor and/or swap to another valve.