Zener diode in the Mosfet booster

[Bill_F]

I want to build the Mosfet booster to put in the same box with my Small Stone I've just finished. It calls for a 9.1V Zener diode, all I have is 8.2V is that close enough.

Also do you think it would be better to place this boost before or after the Small stone?

One more. Is there a better boost I could use to take care of the volume drop. I also think it would be cool to have a small stone with an external volume control.  

Thanks all,
Bill

[jimbob]

Im in the same boat! Anyone? In fact the rat shack said the dont stock those--went there last night. Any substitutes? Diff voltages? Ideas?

[Russ]

Hi Bill, Jack Orman addressed this subject in a recent post on this site, but I can't recall the info right off the top of my head. Do a search on this site and it will turn up. Try "mosfet booster zener diode" or just "mosfet booster diode".
Hope this helps,
Russ

[Russ]

Sorry Bill, I ran a search myself but I can't find the thread now.  Hopefully one of the other guys can help out.
Sorry about that,
Russ

[bwanasonic]

About the zener diode:

http://www.diystompboxes.com/sboxforum/viewtopic.php?p=99254&highlight=#99254

As far as making a 2-in-1, I would put the boost after the phaser to recover lost volume, but avoid overloading. Keep in mind that the Mosfet boost is a very versatile pedal, and you may not want to *marry* it to the small stone. It can easily become an *always on* effect. A more useful 2-in-1 might be two Mosfet boosts in one pedal.

Kerry M

[Russ]

Thanks Kerry, that's the thread I was looking for.
Russ

[Thomas P.]

I think you could also use a red LED to protect the Mosfet.

[amz-fx]

I think you could also use a red LED to protect the Mosfet.

You would have to use a pair of them...

Any value zener from 7v to 12v work....

-Jack

[petemoore]

I like having a Mosfet Boost on the 'other' [what would normally be bridged for bypass] side of the DPDT TB Switch.
 I found that I wanted to do multiple switching/dialing to go from 'dirt' clean to fuzz before I started putting a boost on the other side of the switch from fuzz.
 This way I can dial in my favorite clean Booster that moves dirt in the amp on the one side, and the Fuzz all dialed in on the other side, can go perfectly from boosted to boosted with Fuzz that way with both sides dialed in.
 So instead of switching from Fuzz to a totally clean [weak] signal I get right back to the 'Muscle' sound of a 'pushed' front end on the amp with one quick switch move...I was leaavin the Booster on the whole time and found it hard to get from where I liked the booster set to where I like my Fuzz setup [dialed in].

[R.G.]

The base-emitter junction of most common NPN silicon transistors makes a really good 6-8V zener. This characteristic is not specified, because it is not something that most people ever need from transistors, but it's a perfectly good misuse of a cheap transistor.

Be sure that you use a transistor that you will never use for an amplifier again, because reverse-breaking the base-emitter of a silicon bipolar transistor **even once** permanently degrades the noise characteristics of the device. Maybe clip off the collector lead of the transistors you try.

A 1K resistor and a 9V battery will usually let you test the zener voltage, as almost all NPNs break over between 6 and 8V.

[Bill_F]

Thank you everybody for coming to the rescue once again.

Bill

[jimbob]

Rg-very impressive and creative!

[B Tremblay]

The base-emitter junction of most common NPN silicon transistors makes a really good 6-8V zener.

Which pin serves as the cathode, base or emitter?

[R.G.]

Which pin serves as the cathode, base or emitter?

Depends on what polarity device you use.

If you use an NPN device, the base is p-type and the emitter is n-type, so current normally flows when the base is positive, emitter is (more) negative.

If you convert that to diode-speak, that means the base is the anode (arrow) and emitter is the cathode (bar).

You can do much the same trick with a PNP, of course, using the emitter as the p-type/anode/arrow and base as the n-type/cathode/bar. However, PNP devices are not necessarily going to have as tight a range on the breakover/zener voltage.

You can also do this with the base-collector junction, of course. However, that one breaks over at the BVcbo voltage for the device, which is much higher than BVceo usually.

All modern silicon bipolar devices have this asymmetrical doping profile. Early germanium devices had symmetrically doped junctions, reflecting the primitive manufacturing processes.

[Jay Doyle]

You can also do this with the base-collector junction, of course. However, that one breaks over at the BVcbo voltage for the device, which is much higher than BVceo usually.

R.G.,

Wouldn't the higher break over voltage of the b-c junction be more useful in this situation than the b-e? My reasoning is that because we are noramlly using 9 Volts as the V+, a break over voltage of 6-8 volts will allow for the possibility of a signal at the gate being large enough that it breaks over that zener voltage of the b-e (or e-b in reality), thereby clipping it.

Because the zener is in there for static protection, the larger voltage of the b-c junction will allow all signals to hit the gate without breaking it over while still protecting against the static charge that may also hit the gate.

Great idea by the way, much cheaper and easier than finding the right zener.

Jay

[R.G.]

Wouldn't the higher break over voltage of the b-c junction be more useful in this situation than the b-e?

Maybe. It's not a critical application. Gate protection zeners just have to break over at voltages higher than the actual circuit operation voltages. I don't have the mosfet booster committed to memory, but if it's a MOSFET common-source amplifier with outputs from drain and/or source running from 9V, it would be hard to get the gate more than about 3.5V above the source for any reasonable current levels, so I actually think anything over maybe 4V would work for many or most of the situations. There's probably a boundary condition I'm not taking into account somewhere, but I'd guess that getting up to over 6V is probably not going to give you any real advantage.

The standard of using a 12V zener for gate protection comes from the power MOSFET world where transcounductances run to about 1A/V and gate oxides are pretty uniformly 20V thick. In those cases, you really want a zener that gives you all the enhancement possible, but that keeps you from overstressing the gate oxide.

The BS170's gate is 20V thick and so is the 2N7000's. The 2N7000's transconductance is 320ma/V and its max drain current is 200ma, so something like Vt+1.0V is max for the device anyway. Four or five volts would work fine there.

The BS 170 is rated at 1.2A pulsed, but to get it to there you have to put 6-7V across gate-source. For any reasonable amplifier, you're just not going to get signal voltages larger than that. In fact, along the lines of the reasoning I went through in "Designing MOSFET Boosters" you are unlikely to get more than Vt+50mv.

So - sure, a higher voltage zener really opens up how much voltage you can put on the gate, but you may or may not need that much.

[Jay Doyle]

Thanks R.G., that made perfect sense!

[jimbob]

I found a 1nt4742a 12v zener at the ratshack..will that work? For the Amz mosfet booster?

[R.G.]

Sure.