Noob Question : Diodes in a JFET Preamp

[bonaventura]

Hi,

been lurking here for a while and decided to give it a try.

so i started building a JFET preamp based on the circuit here : www.hawestv.com/amp_projects/fet_preamp/fetpreamp2.htm

all components in place and i (think) got the correct source resistor value for the FET. pretty rough solder work, though.

that is, all components except the diodes. the original design uses 2 diodes to protect the FET from static surges. however, the schematic indicates that the cathodes (striped sides) should be side by side.

question is : to serve the above purpose, shouldnt one striped side connects to the gate and the other connects to the ground? this way when the voltage goes over the diode's threshold, the current will be grounded.

appreciate your advise.

thanks.

[DavenPaget]

It was used in the same method as ZVEX's SHO . To prevent ESD from destroying FETs ( no , even BJT can be susceptible to this as well )
Found on page 3 :

Some builders like to use a transistor socket for the FET. Obviously these builders expect the FET to blow up. And they're right. If you don't protect a JFET from voltage transients, eventually you'll blow it. All you need to do is plug in your guitar on a dry day. One spark destroys the JFET. Yet I don't use transistor sockets, and don't have to. Why? Because zener diodes protect my JFET from most static discharges.

Sockets are left for trying out different transistors , use Zeners .

[bonaventura]

hi,

thanks for the reply.

have looked at the ZVEX SHO circuit,so basically i was right,right?

striped sides away fm each other?

thanks.

[Johan]

use google to read up on the differance between MOS-FET and J-FET
J

[bonaventura]

hi johan,

i think i got my the transistor right. the jfet.

bought the exact part listed in the site, the MPF102 and installed it already.

the question is which way the diodes go?

striped sides next to each other or the other way around?

[aron]

Check out this page:

http://www.diystompboxes.com/wiki/index.php?title=Frequently_Asked_Questions_(DIY_FAQ)#DIODES

Tell us your answer.

[bonaventura]

hm. . .

now i think it doesnt really matter.as long as the zeners are in series and wired opposite to each other,they are going to send any surges to ground.

right?

[aron]

Yes, but in your schematic, the zeners are shown with the stripes close together as in the wiki. Good Luck with the project!!!!

[bonaventura]

hello,

just an update, the circuit is finished and works properly. it does, however, gave a massive hum when tested with an undersaddle piezo. i figure, its a shielding issue since im still testing on my spaghetti board.

been spending my time reading re: this subject, trying to get my head around this thing and frankly speaking, i think im still a bit fuzzy about some stuff. think i'll just move forward and catch up with the reading later

thanks for the help.

[aron]

Cool! Yeah it's probably just not shielded enough.