embarking on a humble od again maybe, JFet question

[dwp]

Question: what the hey is it?......
But seriously, I have never messed with a transistor, yet, I'm looking at building the 1997 1.2 version OD specimen in Doug's scematic library, then I'm thinking since I have room in the chassis, add the Jfet input. I see the board on Ctone's layout, easy enough,, but right off the bat, the only Jfet that comes up on AES is #4391, appears to be three leads, Ctones board shows 4 leads. This the wrong part? or does it really have 4 leads, just can't see in the pic. the only numbers on the AES site are: Jfet PN 4391, then P-Q4391, pretty sure that's AES stock number only.
All input and advice appreciated as always, like what is the benefit here? what's it do? and if 4391 is the wrong part, what and where is the right one?
Is this a worthy add? Am after versatility, but I always scream in short bursts only, then back to Fender clean. Last humble build, the D-lite, I ended up changing the values back to Twin Reverb, had a hard time taming the OD circuit, the guy who has the amp now still hasn't come to terms with it either. Maybe I need to give him the Jfet add too.....
thanks in advance, dwp

[sluckey]

Can you provide a direct link to the schematic and also the board on Ctone's (Ceriatone???) layout?

[tubenit]

I don't really know for sure ............. but I was thinking the JFET thing was to be able to plug in an acoustic guitar???

I heard a U-toob where the guy was playing an electric using it, but my gut level feeling is that a good OD effects pedal would sound equally good, IMO.

With respect, Tubenit

[dwp]

http://www.ceriatone.com/images/layoutPic/OvertoneLayout/Overtone-Special-V2.jpg

thanks guys, here's the link to the ctone build, I get the way it's going in to the normal channel, note the 4 poles on the jfet. Maybe that's just in the case you get one with a metal shield? I just noticed that there's a jfet arrangement in doug's humble library, shows 3 wire only. didn't find the way it enters the amp though. this is my base, not all the sectionals:

http://www.el34world.com/charts/Schematics/files/dumble/97fullschem.pdf

if it's increased sensitivity for acoustic pickups, I wouldn't want it. Thanks again for your replies.
anyone know where I can get Dale RN65D's without negotiating Mouser or the other large whse site?
will keep posting, parts arrive monday......

[38Super]

Question: what the hey is it?......
But seriously, I have never messed with a transistor, yet, I'm looking at building the 1997 1.2 version OD specimen in Doug's scematic library, then I'm thinking since I have room in the chassis, add the Jfet input. I see the board on Ctone's layout, easy enough,, but right off the bat, the only Jfet that comes up on AES is #4391, appears to be three leads, Ctones board shows 4 leads. This the wrong part? or does it really have 4 leads, just can't see in the pic. the only numbers on the AES site are: Jfet PN 4391, then P-Q4391, pretty sure that's AES stock number only.

JFETS work similarly to tubes in that they are on, until you turn them off with appropriate gate-source voltage - typically three terminal devices.  Most MOSFETs are enhancement mode, meaning you have to turn them on with 100s of mV or volts gate-source.  Mosfets' fourth terminal is the substrate connection.  Sometimes it's internally connected yielding a 3 terminal device.  MOSFETs are essentially voltage controlled resistors and don't have the high transconductance of bipolar (NPN, PNP) devices - meaning you can't squeeze as much voltage gain and drive out of them.


Here's some info on FETs.
http://www.williamson-labs.com/480_fet.htm

Cheers,

rob

[tubenit]

Found this.

Tubenit

[LooseChange]

Just eliminate the entire ss preamp.  The word on the street is no one ever uses it.

[sluckey]

note the 4 poles on the jfet. Maybe that's just in the case you get one with a metal shield?

RE: ceriatone layout... Also note that only 3 legs have anything connected. It is possible that the fourth leg is connected to the can if it is a TO-18 case.

[dwp]

hey thanks for the replies. Yes I found the 4 wire Jfet at Mojo, TO-72, metal cap, 4th must be a ground/bond. The NTE452 is plastic. looks like the ground goes past the thing on the Ctone drawing. Yes a probably worthless add-on, but in the name of go forth and conquer, am going to assemble so I can see what it do, or doesn't do. The OD channel on the 1997 version alittle different than the D-lite, looking forward to experimenting.
thanks again, dwp

[foghornleghorn]

Last year I built three Dumble type circuits, two of which had the FET input circuit. It is actually quite useful, although it was designed to work with the early piezo-electric pickups on acoustic guitars. I built a 100W #124, a '70s OD 50W with '67 Bassman iron, a combination of #13 and #40, and a Bluesmaster HRM with separate masters for clean and OD. All sound great, having separate masters is really nice.

I set the gain up pretty high, that way I have the regular OD channel, which is set lower gain, a bit crunchier, whereas the FET input then becomes very high gain, "violin style" overdrive. With the footswitch addition of PAB (preamp boost) and mid boost, you have quite a variety of overdriven tones.

These amps are potentially very versatile and need careful adjustment to optimize "your" tone, but it's in there all right!

The FET schematic shown is authentic and the NTE452 is "Dumble correct". However there are a number of excellent, and cheaper, substitutes. If anyone needs to know I can dig that up, I just don't recall the numbers off the top of my head as I used NTE 452s in mine.

[38Super]

Fourth terminal on the metal can NTE 452 is connected to metal can (for shielding IMO).  IT's funny, Mouser tagged this part for use in VHF amplifier/mixer.

http://www.nteinc.com/specs/400to499/pdf/nte452.pdf

cheers,

rob

[jerrydyer]

the JFET input actually sounds pretty good with electric and the overdrive channel when dialed in just right. A little sparkle and drive. kinda cool.