"Superheated SHO" - asbestos gloves warning!

[frequencycentral]

There was an error in the earlier layout, here's a corrected layout:

[StereoKills]

Awesome, thanks Rick. This is making it's way up my list.

[askwho69]

Gi want to build this now! can 5457 replace by j201 here?

[WGTP]

I never have been as fond of Jfets as I am Mosfets.  So guess which gain pig is going to put several of these on his bread board.  Super Heated Obsidian or Box of Rock/Metal. 

[Cardboard Tube Samurai]

After many, many hours of frustration, I have discovered that there is an error in the original schematic/s posted in this thread. I hunted around to find the original SHO schem and that was when I found it. In this schem, the zener runs between Source and Gate, where it should actually run between Gate and Drain (cathode to Drain).

After getting it working, it sounds amazing!!! Problem is, with the 'superheated' section engaged, I get a lot of compression on lower notes. Any suggestions for remedies to this? Lower input impedance?

Sorry, had a few drinks so this may be a blazingly obvious answer that I'm missing  

*edit* Should probably mention that I also substituted 2N7000 and MPF102 due to lack of availability of genuine parts. Shouldn't make a massive difference AFAIK, but just in case I'm missing something again...

[frequencycentral]

After many, many hours of frustration, I have discovered that there is an error in the original schematic/s posted in this thread. I hunted around to find the original SHO schem and that was when I found it. In this schem, the zener runs between Source and Gate, where it should actually run between Gate and Drain (cathode to Drain).

How so?

Horse's mouth: http://www.zvex.com/module_instructions.pdf
BS170 datasheet: http://www.fairchildsemi.com/ds/BS/BS170.pdf
2n7000 datasheet: http://www.fairchildsemi.com/ds/2N/2N7000.pdf

The 2n7000 has the opposite pinout to the BS170 though........confusion may lie there.

[Cardboard Tube Samurai]

As seen here:



The zener in the original schems are substitutes for the combination of D1 and R3 in the schem shown here. These run between G and D in this schem, whereas the zener runs between S and G in your schem.

[Cardboard Tube Samurai]

Woah, that pic is big!!! Sorry.

[frequencycentral]

^^^ That huge pic is of the SHO MkI, which was superceded by the SHO MkII as in my link here (of which you will note the URL):

Horse's mouth: http://www.zvex.com/module_instructions.pdf

[Cardboard Tube Samurai]

Yep, just saying that if anyone is following the original schematic posted in this thread, they're going to run into trouble, that's all. I am aware that there are plenty of schems floating around the internet, but in case someone decides to make that one, just like I did, they will find that it doesn't work straight away. Young players may never get it to work.

[frequencycentral]

Yep, just saying that if anyone is following the original schematic posted in this thread, they're going to run into trouble, that's all. I am aware that there are plenty of schems floating around the internet, but in case someone decides to make that one, just like I did, they will find that it doesn't work straight away. Young players may never get it to work.

I'm still mystified as to what the error is, as far as I'm concerned it's correct.

[Cardboard Tube Samurai]

Yep, just saying that if anyone is following the original schematic posted in this thread, they're going to run into trouble, that's all. I am aware that there are plenty of schems floating around the internet, but in case someone decides to make that one, just like I did, they will find that it doesn't work straight away. Young players may never get it to work.

I'm still mystified as to what the error is, as far as I'm concerned it's correct.

"run between G and D in this schem, whereas the zener runs between S and G in your schem"

[frequencycentral]

Yep, just saying that if anyone is following the original schematic posted in this thread, they're going to run into trouble, that's all. I am aware that there are plenty of schems floating around the internet, but in case someone decides to make that one, just like I did, they will find that it doesn't work straight away. Young players may never get it to work.

I'm still mystified as to what the error is, as far as I'm concerned it's correct.

.....whereas the zener runs between S and G in your schem

...and in the original Zvex schematic too. And ROG's Peppermill.

[Cardboard Tube Samurai]

Ok, we're not going to get anywhere with this. You're right. I'm right. Or so we both believe. All I know is, I followed your schematic and I couldn't get it to work. I've been doing this for a while now and whilst I'm certainly far from an expert, I understand the concept of different components having different pinouts, please don't insult me. I even went so far as to replace the 2N7000 at one stage, thinking that perhaps I might have stuffed it. I am more than willing to eat a massive chunk of humble pie if I am in fact wrong here but for the life of me, it still looks wrong, sorry. In fact, I'm going to go and study what I built just to triple check. Sorry, I'm not trying to be a sh!thead.

[Mike Burgundy]

The thin insulating layer between channel and gate breaks down due to excessive voltage between gate and source.  As far as I know that is always remedied by adding a zener between G and S, not G and D. Example:
"Direct protection of the MOSFET could involve methods such as shorting the gate
to the source, or applying zener protection gate-to-source."
From International Rectifier application note AN955 http://www.irf.com/technical-info/appnotes/an-955.pdf page 5.

Now I'm curious why yours didn't work.

[Cardboard Tube Samurai]

 

[earthtonesaudio]

Acceptable ESD-protection schemes include:

-Zener cathode to gate, anode to source
-Zener cathode to gate, anode to V- (ground) is also fine (provided: gate bias voltage < Vzener < 20 volts)
-Reverse biased, regular, non-zener diodes from gate to supply voltages is also fine, provided the supply voltage is kept lower than 19.3V

Zener from gate to drain is incorrect.  I mean, it won't break anything to hook it up that way, but it will potentially mess with the bias and won't protect the gate from ESD.

[StereoKills]

I finally got around to building this up, and I'm getting absolutely no volume control from the bias knob. It's as if the knob was stuck at full on 10 the whole time. I've checked all wiring and the perf and everything seems kosher. The volume boost (non superheated) is incredible, bringing my 5w tube amp to an almost unusable volume when set at 1 (for stage volume, I normally need my amp set at 4 - this was more like a 7!) and hits my Echo Base so hard that it distorts. Superheated mode just about doubles the amount of volume boost!

Examination of the circuit brings a few questions to mind.

-Between version 1 (crackle ok) and 2 (no crackle), why the change in values for the drain resistor and source potentiometer?
-Why the inclusion of the 22K input resistor on your layout? It's not on the schematics.

I hope I can tame this beast down to usable levels, because from what little playing I got to do with it, the sound was fantastic.

[StereoKills]

Bump?

[StereoKills]

So, reducing the size of the potentiometer from 5k to 1k in the crackle-less version changes the gain of the BS170 from unity to something like +20db?

I'll try going to the 5k pot to tame the beast.

Still curious about why you made those changes Rick!