Varistor Characteristics for Magnatone Vibrato Circuits

[Smokebreak]

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?

My bad, I meant the device was your MOV, with the 1M5s out

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

For sure

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?

That would be my guess, but there is very little range. Say, once it's turned down to "8", it's basically gone. So not only does the effect cease, there's not really any speed range, it's basically on or off, at the highest speed, the one in the clip. I haven't fiddled with the circuit around the speed pot yet, and I have a 1M hacked across the 1M pot, and I haven't looked at the scheme lately, but that would lead me to believe it's supposed to be 500K. So at the least, my taper is screwy.
The NOS have a broader range of speed, and don't take time to restart, or rather the oscillator to restart, if that's the case. The latter obviously isn't a major concern, but interesting.

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.

Absolutely a success! Great work, Martin. You just saved everyone $75-$100 a build.

[trobbins]

The varistors (NOS or clone) shouldn't have any impact on the operation of the oscillator !?!? Maybe there is something else going on in the amp that needs to be addressed first.

[Kagliostro]

Seems that a good solution is achieved



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Today I've fever and my brain isn't working at 100%

is this the last and better performing version ?

[img:786:606]http://i.imgur.com/cXesHVT.jpg[/img]

K

[martin manning]

Correct. It's the previous version with the 1M5 clipped out. Below is my proposed "final" version with the lowest MOV's breakdown voltage lowered to 22V.

[Kagliostro]

Many thanks Martin

now seems there is only a problem, where to find the MOV

I've tried at RS that is one of the biggest store we can access here (Italy)

but they say that the V33ZA05 is out of production

and the substitute is rated only for 26v instead of 260v

http://it.rs-online.com/web/cpd/0649100 ... rm=v33za05

they don't store the V56ZA05

the only I can find seems to be the V82ZA05

Franco

[martin manning]

I found them at Mouser. Can you order from them? Note add a "P" to the end of each part number.

http://www.mouser.it/Search/Refine.aspx ... d=V56ZA05P

Please delete that long link in your last post... it seems to mess up the browser view.

[Kagliostro]

Please delete that long link in your last post... it seems to mess up the browser view.

Deleted

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But if I add the P the componentrs has different max DC voltage

http://it.rs-online.com/web/cpd/0649100 ... rm=v33za05

isn't it a problem ?

Franco

[martin manning]

No problem, the ones I have all end in P.

[johnnyreece]

Cool. Thanks, Martin! Next time I place a Mouser order, I'll snag some of these to maybe give 'em a shot in my Maggie clone and see what happens. First I'm going to dial it in to (hopefully) give me a more pronounced vibrato.

[trobbins]

Out of interest, I just quickly looked at the residual LFO signal leakage getting through to a grid of the output stage when there is no instrument signal. Depth was turned up to max. The result was that leakage was negligible and a clone varistor would perform fine, if not better than NOS in this respect.

I used the varistor clone with a 3 zener circuit [2M2 : 33V/560k : 56V/150k : 82V/68k].

I was using TrueRTA to view the spectral frequencies down to 10Hz. I could discern the 2nd and above harmonics (20, 30, 40 Hz) of the 10Hz LFO signal, but higher frequencies were polluted with 50Hz mains signals in my test amp. I couldn't properly see the fundamental due to the lower limit of TrueRTA. The LFO harmonics are leaking through the vibrato varistor modulator and passing through the following PI to the grid of an output PP valve.

The leakage level was pretty damn low in comparison to mains hum and other noise floor in the test amp, and the harmonic levels were reducing.

I then modified one clone varistor by adding 1M5 across it, so as to upset the characteristic of that varistor at low applied voltage levels, in comparison to the other clone varistor. I then did the same to the other clone varistor. For both those changes, the 20Hz leakage signal increased, although the 30Hz went down.

To my mind, the observations show that cloned varistors in a maggie vibrato circuit should give negligible speaker thumping.

It would also be difficult to try and trim out any leakage signal, as trimming resistor levels in the clone would likely move harmonic levels up and down, and any mains ripple from the power circuit would likely pollute the assessment.

[Structo]

Could you use a high pass filter to kill the lower frequency?

[trobbins]

The typical output of a maggie vibrato stage goes through an indistinct CR filter at the input of the next concertina PI stage, and then the CR coupling to the PP output stage provides pretty much the last high-pass filtering action before the speakers themselves.

I haven't had a problem myself with vibrato effect thumping, but it came up as an issue - perhaps from something NQR in the clone amp.

[Diablo1]

I was wasting some time today in a small city in the Midwest, after I visited a customer. I stumbled across an old electronics shop, and wandered through their aisles. Then I saw the varistor section! Lo and behold, original NOS Workman FS1205 still in the package with dust on it from the 1960s. I paid full price as marked - $1.05 each. Bought twelve and that's all they had on the hook.....

So, anyone have any other varistor part numbers that work in the Magnatone circuit? I think I have to make a return trip.

[trobbins]

As I understand it, the original parts were identified as:
Carborundum 233BNR-32
Workman FS1203 : 0.05mA @ 49V (Zenith 63-4906)
Workman FS605

New part is I believe:
Metrosil 100-P/W/921 : 1mA @ 100V; 0.1mA @ 55V

Others that have come up in discussions:
FR1039
FS-308
FS1211 : 0.05mA @ 61V (Zenith 63-5327)
FS1205 : 0.05mA @ 80V (Zenith 63-5058)

The original maggie circuit appears to be designed for a varistor conducting about 1mA at 100V, and about 45V at 0.05mA. Given that, I'm pretty sure the resulting effect with a slightly different varistor will be equivalent.

[Diablo1]

As I understand it, the original parts were identified as:
Carborundum 233BNR-32
Workman FS1203 : 0.05mA @ 49V (Zenith 63-4906)
Workman FS605

New part is I believe:
Metrosil 100-P/W/921 : 1mA @ 100V; 0.1mA @ 55V

Others that have come up in discussions:
FR1039
FS-308
FS1211 : 0.05mA @ 61V (Zenith 63-5327)
FS1205 : 0.05mA @ 80V (Zenith 63-5058)

The original maggie circuit appears to be designed for a varistor conducting about 1mA at 100V, and about 45V at 0.05mA. Given that, I'm pretty sure the resulting effect with a slightly different varistor will be equivalent.

Thanks much, Trobbins.
This will help greatly with the treasure hunt.