# Green Meanie (Little Green Scream Machine) Build Out



## Cybercow (Aug 16, 2022)

The Little Green Scream Machine is an excellent Tube Screamer circuit and modding it is a treat. All I did was add some clipping element switching with a 6-way rotary switch using mostly not-so-standard clipping devices.

Found these nifty little 9mm sized 6-way rotary switches on SmallBear and they had a link to a shared OSH Park PCB for mounting switching option components. Wanted to have a means of ear-balling the sonic difference between 6 different diode clipping options.






In this compound clipping configuration are the following diodes\LEDs used:

*LM313H* Reference Diode (Fvd ~0.67v symmetrical)
*MA856* (Fvd ~0.7v to ~1.0v symmetrical - “King of Tone” fame)
*3mm Red/Green* 2-pin LED (Fvd ~2v & 1.7v asymmetrical)
*5mm Red/Blue* 2-pin LED (Fvd ~2.7v & 1.9v asymmetrical)
*Nanolog N2* (Fvd ~0.7v symmetrical)
*Nanolog N3* (Fvd ~1v symmetrical)

(That's about $60 worth of clipping components alone. The LM313H reference diode is a special little device that basically has two diodes in a single 2-pin can. The LM313H  cannot be subjected to any DC voltage - much like the Nanolog devices cannot be exposed to DC voltages. And the LM313H can are about twice as expensive as the Nanolog devices.)





After getting the circuit completed and enclosed, I got it fired up and started checking out the six different clipping options. At first, I quickly ran thru the selections to listen for any significant differences. Obviously the Fvd differences were most noticeable volume-wise and the LEDs were the loudest. All Fvd values were acquired from the respective datasheets where testing was not possible with a Peak Atlas DCA75. (The LM313H, Nanolog N2 & N3 devices cannot withstand direct DC voltages.)

With no sine wave generator at the time, I was unable to provide the circuit with a pure sine wave to get any scope readings of the clipped wave forms. So this dissertation is an aural exploration only. I used a ’79 Strat with switchable dual hum-bucker & single bridge pickup (with the Volume set to max) thru a Vox MV50 AC set mostly to a clean signal with the gain set to 1/3 and the Volume set to 1/2 - pumping a pair of 12’ Marshall speakers.

The *LM313* reference diode has the typical clipping harshness of silicon diodes with what sounds like a bit more bottom end available than the other silicon devices in this selection group. An unusual device in that it is ridiculously expensive for use as a clipping diode configuration. One device has several components, sounding much like two diodes, back-to-back like a standard symmetrical clipping diode pair - only one device is required for clipping. (I didn't test for any actual "symmetry".) Sound-wise, it’s nothing to write home about and are a prohibitive choice due to their expense. (When I found mine over a year ago, they were about $25 each. They are bit less expensive now, but a bit harder to find.)
*MA856* diodes are known for their explicit use in genuine “King of Tone” pedals and are supposed to be “rare”. However I found 10 of them at an easy price (about $1 each) and tried them out in this build. This clipping option (MA856) is the only one that required 2 elements to comprise the back-to-back diodes to form a symmetrical clipping option. To my ears, with the given setup in this Tube Screamer circuit, they are also standard silicon sounding clipping devices that seem to exhibit nothing special about them. Still, they do their job well.
The *3mm red\green 2-pin dual-color LED* has a measured Fvd of 2.0v and 1.7v respectively for each color. This provides an instant asymmetrical clipping diode configuration with a single element. Two diodes (LEDs) in a single 2-pin package. Louder than than four of the other clipping options in this build, these provide a slightly different set of harmonics in their clipped result. About the same price as regular LEDs, but only one is required.
The *5mm red\blue 2-pin dual-color LED* has a measured Fvd of 2.7v and 1.9v respectively for each color. These also provide an instant asymmetrical clipping diode configuration with a single element. Two diodes (LEDs) in a single 2-pin package. With the highest Fvd of all the devices in this clipping selection group, this clipping device selection is the loudest and the volume difference between this and the *3mm red\green 2-pin dual-color LED* is clearly audible. I find this another good asymmetrical clipping device for use for clipping in a feedback loop with an opamp. About the same price as regular LEDs, but only one is required.
The *Nanolog N2* dedicated clipping device has a specified Fvd of ~0.7v and was (aurally) noticed immediately when switching from the *5mm red\blue 2-pin dual-color LED*. Also immediately aurally noted was a drop in the noise floor. While not playing anything thru the circuit with the *N2* selected, I noticed a drop in the slight noise floor of the Tube Screamer; as if the highs were turned way down - or that a low-pass filter was suddenly engaged. While playing my Strat thru the circuit with this clipping option selected, the rich harmonics of clipping were still apparent, but also with a somewhat diminished extreme upper harmonic content - AND - a notable clarity of of the bottom end. While this circuit build does not include any germanium clipping device options, the *N2* tends to be very “germanium” sounding, with a softer clipping knee. (Can I “hear” a ‘clipping knee’? - No. I cannot. But I’ve heard enough Tube Screamers with Ge and Si clipping to know the difference.) Again, this is an expensive device for clipping - coming in at ~ $16 per device.
The *Nanolog N3* dedicated clipping device has a specified Fvd of ~1.0v and is noticeably a little bit louder than the *N2*. The *N3* also exhibits a definite drop in the noise floor in the highs, but still renders some sweet clipping harmonics while playing. Sonically similar to the *N2*, the *N3* is just a little bit louder, presumably due to its higher Fvd. This Nanolog device is priced the same as the *N2*.
After listening to each clipping option in the order of the switching arrangement, I started to jump around for some random comparisons and found good points for each - sonically speaking. Most notable was the noise floor drop when engaging either of the Nanolog devices and find them “warmer” than any of the other clipping devices described herein.

I found nothing extremely “special” about ANY of the devices described here. Even the Nanaolog N2 & N3 devices, with their crazy high price and “mojo”, super-secret quantum tunneling physics marketing-speak held nothing sonically spectacular for me that couldn’t be achieved with some creative pre and\or post filtering in most any dirt circuit. The LM313H was a fluke find almost 3 years ago and only recently did I apply it in a clipping circuit to see how they sound (behave). They are a super-specialized reference diode. And like the Nanolog devices, are just price prohibitive.

In conclusion . . . .  I recommend experimenting, use sockets in testing, and settling on using what sounds good to you. For me, this experiment taught me little more than how throw money at a question instead of investing in a solution. What I take away from this little project is keep it simple and do a little testing before wrapping up a build; as I found no point trying to get fancy with regard to signal clipping with diodes. There are already plenty of inexpensive yet creative ways to provide a nice assortment of clipping options for most any circuit.

Meanwhile, the following images are the photojournalist of the complete build . . . .





























Thanks for looking. Happy building!


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## DAJE (Aug 16, 2022)

I found that very interesting and I appreciate the effort you took in writing it up.


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## fig (Aug 16, 2022)

I’ve always enjoyed your write-ups and this one is no exception, however the build itself _is. _

Green Meanies indeed!


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## Robert (Aug 16, 2022)

Nice build and write-up!  

That's a cool little switching board.  I might have to work something up like that for those mini rotary switches, I forget they exist.


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## spi (Aug 16, 2022)

I never thought of using a dual color LED for a pair of clipping diodes--that's pretty clever.


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## Robert (Aug 16, 2022)

spi said:


> I never thought of using a dual color LED for a pair of clipping diodes--that's pretty clever.



Yep, I bought a bunch from Tayda to play around with.    Saves space too since one covers both directions.


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## chongmagic (Aug 16, 2022)

Nice cardboard jig too!


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## jimilee (Aug 16, 2022)

Nicely executed, I’m jealous.


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## Route14 (Aug 17, 2022)

Looks great! Did you measure the FVD Voltages of the MA856s? Most I've found online are really 1SS133 diodes that people are trying to pass off as MA856s. I bought a bunch a while back from PedalHackers that they said were MA856s but were actually 1SS133s. The 1SS133s are very similar to 914s or 4148s in sound. Took me sometime to figure that out and explained why they didn't seem to offer anything more than other diodes I was using.


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## Cybercow (Aug 17, 2022)

Route14 said:


> Looks great! Did you measure the FVD Voltages of the MA856s? Most I've found online are really 1SS133 diodes that people are trying to pass off as MA856s. I bought a bunch a while back from PedalHackers that they said were MA856s but were actually 1SS133s. The 1SS133s are very similar to 914s or 4148s in sound. Took me sometime to figure that out and explained why they didn't seem to offer anything more than other diodes I was using.


I didn't measure them. I got them from an estate sale a few years back where an old fella was deeply into electronics and had a LOT of great NOS components. The MA856 diodes were separate from the large box of stuff where the real gold was. I'm guessing the estate broker didn't know what the had and anything that looked "newer" was priced individually. I left the $1 (apiece) TL0xx and CD4xxx ICs there.

I did find & test a LOT of the NOS Ge BJTs and about 90% of them measured perfect. A butt-load of genuine 1N34As too. I was surprised to also find a lot of over 100 Mullard OC44 black glass Ge PNP BJTs in the box. It's amazing what one can find at an estate sale in a large box of electronic components with a $20 price tag on it. Lots of various pedal friendly Radio Shack IC still in their blister packs too.


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## Route14 (Aug 17, 2022)

Cybercow said:


> I didn't measure them. I got them from an estate sale a few years back where an old fella was deeply into electronics and had a LOT of great NOS components. The MA856 diodes were separate from the large box of stuff where the real gold was. I'm guessing the estate broker didn't know what the had and anything that looked "newer" was priced individually. I left the $1 (apiece) TL0xx and CD4xxx ICs there.
> 
> I did find & test a LOT of the NOS Ge BJTs and about 90% of them measured perfect. A butt-load of genuine 1N34As too. I was surprised to also find a lot of over 100 Mullard OC44 black glass Ge PNP BJTs in the box. It's amazing what one can find at an estate sale in a large box of electronic components with a $20 price tag on it. Lots of various pedal friendly Radio Shack IC still in their blister packs too.


That's awesome!


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## Chuck D. Bones (Aug 31, 2022)

Great write-up!

The LM313 is not a pair of back-to-back diodes.  It will conduct in both directions, asymmetrically.

This is from the datasheet:





The Nanolog parts are quite leaky, something the mfgr fails to disclose in the datasheet.  Jubal81 has done as fair amount of research & testing with the Nanolog parts and has found that subbing a back-to-back Ge diode pair + a back-to-back Si diode pair with a 4.7K series resistor gets very close to the same transfer function.


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## Cybercow (Aug 31, 2022)

Chuck D. Bones said:


> Great write-up!
> 
> The LM313 is not a pair of back-to-back diodes.  It will conduct in both directions, asymmetrically.
> 
> The Nanolog parts are quite leaky, something the mfgr fails to disclose in the datasheet.  Jubal81 has done as fair amount of research & testing with the Nanolog parts and has found that subbing a back-to-back Ge diode pair + a back-to-back Si diode pair with a 4.7K series resistor gets very close to the same transfer function.


Good stuff to know. Thanks!

On a side note about the Nanolog devices . . . . . they apparently have a higher clipping threshold, akin to a higher Fvd, because they are certainly a bit louder than Ge elements. I replaced the Ge diodes I had where the MA856 diodes are. The volume drop was unacceptable in my testing/auditing.


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## Chuck D. Bones (Aug 31, 2022)

Yeah, big volume drop with Ge due to lower Vf.  Can be mitigated by stacking Ge diodes in series to get to a higher Vf.  Big gain drop too due to Ge leakage.  This too can be mitigated by stacking Ge diodes in series to get lower leakage.  In my book, Ge diodes are good for hard clippers, like the Distortion +.  Some kind of boost after the Ge clippers is necessary if you want to get above unity.  Klon is an example of boost after clipping.

The nanolog devices have a high series resistance, a few KΩ, and that adds to the effective Vf.


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## hamerfan (Sep 8, 2022)

Nice build, cybercow. To avoid trouble with feedback, do not cross the input and output wire.


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## Cybercow (Sep 8, 2022)

hamerfan said:


> Nice build, cybercow. To avoid trouble with feedback, do not cross the input and output wire.


Thanks hamerfan! So far, zero issues with feedback. It's easy enough to pop out the stomp and LED to move that output wire to the underside of the stomp assembly to uncross them.


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## hamerfan (Sep 8, 2022)

Good move!


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## equinox (Sep 30, 2022)

Cybercow said:


> The Little Green Scream Machine is an excellent Tube Screamer circuit and modding it is a treat. All I did was add some clipping element switching with a 6-way rotary switch using mostly not-so-standard clipping devices.
> 
> Found these nifty little 9mm sized 6-way rotary switches on SmallBear and they had a link to a shared OSH Park PCB for mounting switching option components. Wanted to have a means of ear-balling the sonic difference between 6 different diode clipping options.
> 
> ...


From Michigan?!??
Uses a an LM313H and MA856, but on the graphics, you put "313" and "586", which the 586 would be in err and BOTH are Area Codes in Michigan metro Detroit!! :O  lol


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## Cybercow (Sep 30, 2022)

equinox said:


> From Michigan?!??
> Uses a an LM313H and MA856, but on the graphics, you put "313" and "586", which the 586 would be in err and BOTH are Area Codes in Michigan metro Detroit!! :O  lol


I don't suffer from it, I enjoy my lysdexia . . . . . . what can I say? LOL!


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