Varistor Characteristics for Magnatone Vibrato Circuits

[andyfromdenver]

Ok i'll wait to see what Smokebreak says but may be i'll try it anyway

He should have the parts in a couple of days, but it would be nice to get more than one opinion on what I'm hoping will be a subtle difference in the vibrato sound ;^)

I can pp ya the shipping and am available to try them in my 2/410.

[trobbins]

Franco, from the operating power supply and circuit resistances, did you work out what the likely voltage-current operating points for the varistors were in that Gulbransen circuit? That would indicate if they were operating in the same region as the maggie operates.

But I don't think that seeking out that type of organ amp for its varistors is going to be very practical - just a novelty if you happened across them.

It certainly is a long windy path to take to get essentially a volume control.

[Kagliostro]

from the operating power supply and circuit resistances, did you work out what the likely voltage-current operating points for the varistors were in that Gulbransen circuit? That would indicate if they were operating in the same region as the maggie operates.

I'm not so skilled

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I don't think that seeking out that type of organ amp for its varistors is going to be very practical

Ok, you are right, I was wondering about because a guy at el34world is gutting one of those organ amp to mod it as guitar amp and the expression circuit is cut out

if those varistors where usable it will be nice to put a varistor vibrato on the derived amp

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It certainly is a long windy path to take to get essentially a volume control

Oh, I was essentially thinking it will be a remoted ccontrolled Tone Control

Thanks for answering

Franco

[trobbins]

Franco - yes it is a tone control due to the coupling caps - but you'd have to check the corner frequencies to get a good idea.

[Kagliostro]

Right

Franco

[Smokebreak]

I have received the subs and will be able to test on Friday.

[johnnyreece]

I have received the subs and will be able to test on Friday.

Yay! Hopefully, it results in something else to be thankful for.

[Smokebreak]

I got the MOV subs in, and I believe they are providing FM, but honestly, I've been going through all the subs I have this morning, and my ears are fried. I'm in wubbawubba land and am having a had time differentiating between FM and AM. At first, I thought we may be getting a little of both here, but some fresh ears are gonna have to tell.

Unfortunately, I'm getting some noise, but not a clicking noise. More like a throbbing thump....reminiscent of tremolo noise...maybe. It's not massive by any means...You can hear it at the beginning and end of the clip. Amp volume does not change the volume of the noise, so by my estimation it would be buried at anything other than bedroom levels, while playing. The depth control does increase the noise.
All in all, I think we're headed in the right direction though. Let me know if you have any questions regarding any variables I may have overlooked.

I rolled a bunch of ax7s in the oscillator, and a bunch of au7s in the varistor circuit and eliminated any tube issues as the cause of the noise.

This clip is with my phone, but my plans are to record the 4 different varistor options I have in a controlled, static, proper recording setting.

Depth is cranked. Speed varies : https://soundcloud.com/jeremyslemenda/mov-sub

Edit : clip above was with tele. Here is a LP :
https://soundcloud.com/jeremyslemenda/m ... humbuckers

[martin manning]

First, thanks for taking the time to do this testing!

The noise is minimal, and this version certainly does not have the pronounced clicking that the Zener version had. What would be best is to have a back-to-back comparison with SiC varistors a you described. I'd say its acceptable, good even, so yes definitely a step in the right direction!

[Smokebreak]

I'm happy to help.
I tried to present an unbiased review, but I agree! They sound quite good, and given the NOS alternative$ it's a no-brainer...especially after the humbucker clips, as that was my point of reference for all the other testing.
Kudos, Martin! I'll get to more clips soon.

[trobbins]

In most of the maggie amps (unsure), there is no volume control after the vibrato circuit. As such, any DC unbalance at the varistor centre-point node will go straight through to the output and speakers as a low frequency thump. That sort of thump has been the bane of many a tremolo type circuit.

In a maggie amp, that thump is minimised by using closely matched varistors, and by the low coupling cap values used between the PI and output stage (typically 10nF). So if your coupling caps are a higher value then the amp will be more prone to thumping.

The maggie vibrato generates both true vibrato and also a fair amount of true tremolo - which is what sets it apart from the effect pedals and VOX style vibrato circuits that are much more just true vibrato with no inherent tremolo,

Ciao, Tim

[martin manning]

We know these MOV units are closely matched. One thing that could be tried is to clip out the 1M5 resistor to see what effect that has. The "off" resistance of a varistor is much higher than that, and there are already three of them in parallel.

[trobbins]

It will be interesting to see what 'adjustment' can be done to minimise such beating. I may get time to check that in a day or two as I have the '33/56/82V zener' clone varistors in my maggie clone on the bench.

The beating should be due to the imbalance of the varistors throughout the LFO signal cycle. The varistors may be balanced at a particular operating voltage-current, but throughout a LFO cycle, it will be the residual signal coming through from the minor imbalances throughout the cycle that make up the beating signal passing through to the output stage.

If so, then an adjustment of one varistor arm at the nominal idle operating level may be able to null the lower frequency component of the beating signal. That could mean a trimpot across say the 56V zener (or MOV) may be of more influence than trying to tweak the varistor at the outer ends of its voltage-current operation.

[Smokebreak]

Reporting back in after further testing.
I removed the 1M5 in parallel with each of Martin's substitutes, and noted no change in the beating/thumping or the vibrato character. It bears repeating the the thump here with these devices is minimal, and nothing like the noise from other subs. It's more like the Fender trem throb we're all accustomed to.

I did notice a couple other things though.
With the MOV, the vibe was only happening at full speed. If I backed it off a little, the effect all but dissappeared. When I turned it back up, it would take about 10-15 seconds before the vibe would start going again. Hopefully this comes across in the clips, and is one of the reasons it's so damn long.
Also, I didn't feel there was enough depth/intensity on tap, so I stuck a 1M between the 1M pot and ground and that did wonders for the effect.
Also, finally I have a back to back comparison between the Workman NOS varistors and the MOV sub. They are very close, with the NOS having a bit more of what I call "the underwater chorus of 'WHHHHAAAAA'".

edit : with the 1M mod to the pot, the effect is still a little noticeable even with both controls turned off, affecting the "core/non-vibe" tone, so a 2M or 3M pot would be best here, and worthwhile IMO. Much better range too.

When I started recording with my phone, I didn't intend to have so many parts, thus the quick and dirty nature of it.

0 - :13 MOVs, 1M5s out

:13 - :37 speed pot turned down and recovery time

:38- 2:14 MOVs, 1M5s in . This section is long because in the room, it seemed as if the lower notes weren't taking as much effect as higher notes, thus my moving around the fretboard

2:15 - 2:55 MOVs, 1M5s in, with 1M added to depth pot

2:55 - 3:17 MOVs , 1M5s out, with 1M added to depth pot. Upon listening to this section, the vibe actually seems a bit more intense( than with the 1M5s), with all other variables static

3:18-end Workman NOS with 1M added to depth pot


https://soundcloud.com/jeremyslemenda/mov-testing

[martin manning]

0 - :13 MOVs, 1M5s out

:13 - :37 speed pot turned down and recovery time

:38- 2:14 MOVs, 1M5s in . This section is long because in the room, it seemed as if the lower notes weren't taking as much effect as higher notes, thus my moving around the fretboard

2:15 - 2:55 MOVs in, with 1M added to depth pot

2:55 - 3:17 MOVs out, with 1M added to depth pot. Upon listening to this section, the vibe actually seems a bit more intense( than with the 1M5s), with all other variables static

3:18-end Workman NOS with 1M added to depth pot

Thanks, Jeremy; this is good stuff.

I'm not clear on what you mean by MOV's out in the 2:55 - 3:17 section, though... What is performing the varistor function?

We can conclude that the 1M5 that was across the terminals is not needed, and the effect is marginally better without it.

Do you think that turning the speed down causes the oscillator to stop? If so it might take some seconds to restart. And this does not happen when using the NOS varistors?

Overall, it sounds like the MOV-Resistor substitute is a viable alternative to the NOS varistors, so we can declare a success here. I do believe I can hear a little richer sound from the NOS parts at the end of the clip, though. There is one more tweak I am considering, which is lowering the lowest voltage MOV to 22V, but that is definitely getting down to fine tuning.