18V Cracklejack/SHO?

genXslacker

genXslacker

Well-known member
Assuming appropriately rated capacitors, is there any reason not to run this circuit at 12 or 18V?

The BS170 datasheet shows an absolute max of 60V for Vds and a max Id of 270 mA.

That seems like a lot of breathing room to me, but I'm just a dipshit with a soldering iron.
 
It doesn't get you anything to run it at higher voltages but ostensibly you can ... I say that with the meaning that just guitar input isn't really enough to get grit out of it thus necessitating 18V for the additional headroom.
 
genXslacker said:
Gracias. If I'm understanding you correctly, I should cascade four of them together at 36V each.
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While it can handle a supply voltage of 36V, it can only handle an input transient at the gate of 20V before it blows up. That's why you commonly see zener diodes strapped from the gate to the source when a mosfet is used as the first stage.
 
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Aleph Null said:
While it can handle a supply voltage of 36V, it can only handle an input transient at the gate of 20V before it blows up. That's why you commonly see zener diods strapped from the gate to the sourse when a mosfet is used as the first stage.
Click to expand...

That is good to know. I do always put the Zener in there. My understanding was that he got a lot of returns before including them in the circuit, and I don't need any help blowing transistors.
 
There are like three "ifs" you have to meet before it's a problem. If you power on the pedal before connecting your guitar and if the pedal is engaged when you plug the guitar in and if there is a large enough static discharge, then you'll blow the mosfet. Probably hard to do, but if you have enough units out in the world (like Vex), sooner or later, it'll happen. A zener is pretty cheap insurance.
 
Aleph Null said:
While it can handle a supply voltage of 36V, it can only handle an input transient at the gate of 20V before it blows up. That's why you commonly see zener diodes strapped from the gate to the source when a mosfet is used as the first stage.
Click to expand...

It seems like I commonly see the Zener implemented from gate to ground, which means there's frequently a resistor (variable or otherwise) between the source and the Zener.

Just for the sake of my curiosity, what would be the effect of tying the Zener directly to the source there — i.e., on the source side of the resistor instead of the ground side? If a VR is dimed, it doesn't seem like it should matter much... but again, I'm a dipshit who happens to own some soldering equipment.
 
genXslacker said:
It seems like I commonly see the Zener implemented from gate to ground, which means there's frequently a resistor (variable or otherwise) between the source and the Zener.

Just for the sake of my curiosity, what would be the effect of tying the Zener directly to the source there — i.e., on the source side of the resistor instead of the ground side? If a VR is dimed, it doesn't seem like it should matter much... but again, I'm a dipshit who happens to own some soldering equipment.
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I don't think I understand zener break down mode well enough to be able to answer that. Based on the examples I could find, it looks like either arrangement should work. I believe early schematics of the SHO do actually have a diode to ground while newer ones tie the diode between the drain and gate. I would expect both arrangements to protect the gate. Maybe there's a forward bias thing happening in the latter? Maybe one arrangement is better suited for AC applications and the other DC? I don't know.
 
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