Varistor Characteristics for Magnatone Vibrato Circuits

[martin manning]

Thanks, guys. I'm pretty confident these will produce a good Maggie vibrato sound, but the proof is in the listening. If it's not quite right, the shape can be tweaked by changing resistances and/or voltage break points, including adding more breaks. Maybe there are some new sounds to be discovered too.

[cbass]

Wow this is very exciting can't wait to see how it turns out.

[Structo]

Yes indeedy.

I would probably like to make a standalone vibrato effect if this works out.

[JazzGuitarGimp]

Yes indeedy.

I would probably like to make a standalone vibrato effect if this works out.

You could tap into the LFO output, invert it, and feed it into a second modulation section for stereo...

[Structo]

In engrish?

[martin manning]

Yes indeedy.

I would probably like to make a standalone vibrato effect if this works out.

You could tap into the LFO output, invert it, and feed it into a second modulation section for stereo...

Here's a block diagram of the Magnatone 480...
http://ampgarage.com/forum/viewtopic.ph ... 545#232545

[JazzGuitarGimp]

In engrish?

See "Stereo Tremolo Example 1" attachment. This is based on the Harmonic Tremolo circuit of the Fender Concert amp. The top half of the schematic is the original Fender design. The upper left box is the LFO circuit. It produces a sign-wave output signal that feeds the Modulation Circuit (the upper right box). I've labeled its output as the Left Output.

The wire that connects to the LFO OUTPUT and extends downward is the LFO tap I referred to. It feeds the inverter circuit in the lower left box. I chose a 12AY7 for the inverter because we need a gain of 1 for this circuit. Its only duty is to invert the LFO output sign wave so that when the LFO output (driving the LEFT modulation circuit) is in the middle of the positive half-cycle, the output of the inverter (driving the RIGHT modulation circuit) is in the middle of its negative half-cycle. The new modulation circuit, in the lower right box is identical to the original mod circuit, and it produces the RIGHT Output, but 180º out of phase with the left circuit. The resistor circled in red in the Inverter box would need to be adjusted so that the amplitude of the inverter output is the same as the amplitude of the LFO output.

The Concert circuit is probably a bad circuit to use for this example, as it's a bit more complex than the bias-vary variety. And in fact, there is a much simpler approach to stereoizing this circuit. Since it already employs a concertina phase inverter following the LFO circuit, the stereo version is as simple as the schematic in the "Stereo Tremolo Example 2" attachment. Here, I duplicated the mod circuit, and am driving its two modulation inputs from the opposite outputs of the concertina phase inverter.

[Reeltarded]

Holy shit Lou. Don't make me screw up my twin amp.

[Smokebreak]

I've also just ordered some NOS Russian SiC varistors that are reported to work, so I'll record a clip with those as well, and send a couple to Martin to throw in the testing mix. It will be a couple of weeks before I get those, though. Really looking forward to trying out the new zener contraption!

[JazzGuitarGimp]

I've also just ordered some NOS Russian SiC varistors that are reported to work, so I'll record a clip with those as well, and send a couple to Martin to throw in the testing mix. It will be a couple of weeks before I get those, though. Really looking forward to trying out the new zener contraption!

Keeping eyes on this one, Smoke - looking forward to the results. Martin's substitute looks very promising from the graph.

Cheers!
Lou

[ubxf]

So i tried it and it works but unfortunately it has a chopper click added to the sound . To be perfectly honest i didn't have 56V diode so i put 62V instead because i couldn't wait and had to try it. The sound is quite nice and may be a little more pronounced than the varistors at the same setting. The varistors produce a very faint click also but you can only hear it if everything is cranked and no signal is going through. Thanks Martin

[JazzGuitarGimp]

So i tried it and it works but unfortunately it has a chopper click added to the sound . To be perfectly honest i didn't have 56V diode so i put 62V instead because i couldn't wait and had to try it. The sound is quite nice and may be a little more pronounced than the varistors at the same setting. The varistors produce a very faint click also but you can only hear it if everything is cranked and no signal is going through. Thanks Martin

Maybe need to add small capacitors across the diodes?

[martin manning]

Thanks for the report ubxf. Using 62V vs. 56V Zeners makes the knee in the curve a little sharper, but I don't think it would make any difference to the click. Interesting that varistors exhibit some of that too. We'll see if Smokebreak's results are similar. Putting small caps across the diodes as Lou suggests might cure it, but that would be four more components. Maybe one small cap across the whole assembly (i.e. across the 1M resistor) would do it. Could you try that?

[ubxf]

Thanks Martin , yes i'll try to put a cap across the 1 Meg what value would you suggest.

[martin manning]

I'm thinking 100-200pF. Larger ones will likely remove too much HF and dull or darken the tone, but it might be interesting to find out. Another way would be to put the pairs of Zeners back-to-back, and then put a cap across each pair. That would be adding only two more parts.