FET-2B Bass Preamp, my first "design"

[Renegadrian]

Built a couple of tube version in the last weeeks, with 70V for the plates. great ckt, I love its tone stack and its sound..
Now I can finally use my layout of the tube version...is it better with the same tone stack values or shall I go with the reduced values as suggested!?

[garfo]

ei there. I've built this layout but I'm getting no sound out of it. actually my dmm shows no continuity after the first resistor. I've got voltage going through all of the circuit. 've used this one:
http://effectslayouts.blogspot.pt/2015/01/ripthorn-fet-2b-bass-preamp.html

[mth5044]

You shouldn't have continuity after the first resistor, you should have resistance. An audio probe will help you figure out where you are losing signal. Triple check the orientation of caps and transistors and the value of your resistors and especially your wiring.

[garfo]

I've built the layout from Ripthorn and I've slaed down the tonestack. The circuit works and sounds good but there is some noise on the background.

oh, ok. There is an (already reported) error in the Guero's layout, the 25n cap should be 2n5. My mistake. Now they are basically the same tonestack, but the Guero one is rescaled to try to lower the noise. Any report if this was sucessful?

[Ripthorn]

Sorry to necrobump this thing, but as I have been documenting many of my other designs on the webpage, I decided I would do a quick little page for this circuit as well. Upon coming to this thread, I see that there are several things missing, particularly the schematic. I have re-attached it. The project webpage is https://scientificguitarist.wixsite.com/home/fet-2b, if you want to go there for a brief description, build documents, etc.

[duck_arse]

D2 on that circuit is backwards, and must have a CLR fitted. what's the deal with R1/R2/R3?

[antonis]

R1 is a pull-down resistor for non-existent input cap..
R2 is a "blocking distortion" resistor for non-existent grid..
R3..?? I dunno..
(maybe it has something to do with FET bias or something similarly useless configuration..)

[Ripthorn]

So I did this 13 years ago when I didn't understand electronics as well, so there are some "peculiar" things going on. Just look at the F2B schematic and you'll see very few differences.

R1 - Pull down
R2/R3 - Voltage divider that mimics the "Lo" input of old Fender and Marshall amps
LED - I caught and fixed that shortly after posting this

I have thought of revising/updating this with a few things to help with noise and get rid of some of the "peculiarities".

[antonis]

R1 - Pull down

Pull-down what exactly..??

R2/R3 - Voltage divider that mimics the "Lo" input of old Fender and Marshall amps

Pardon me but I have to disagree..!! 
Hi-Low input selection is made by a couple of 68k resistors and has nothing to do with 1M bias resistor..
http://ampgarage.com/forum/files/input_jack_533.pdf

[anotherjim]

Note the Hi/Lo Fender inputs parallel the 68k resistors when the Hi or "1" input is used so the correct simulation is 1M from jack tip to ground followed by 34k (33k near enough) to the grid or gate.

[antonis]

So, where is the voltage dividing effect, in relation to 1M resistor....??
(in case you are addressed to me, Jim..)

[Ripthorn]

I certainly don't claim this to be an optimized design. It may be funky, but it works well enough. Does R1 strictly need to be there? Probably not, but I don't know for certain. I'll probably breadboard this up again and dial it in better, but not sure when I'll get around to it.

[anotherjim]

The linked .pdf does explain things.
http://ampgarage.com/forum/files/input_jack_533.pdf
Also, see Fender amp schematics.
Potential confusion is there due to the way it's drawn for the normal-closed input jack tip switch contacts and the position of the 1M grid bias resistor.
It isn't R1 that's wrong, it's R3! The voltage divider then is an AC one. R2 in series and the amplifying device input capacitance making a low-pass filter. It should only affect radio frequencies.
Even if you wanted a resistive voltage divider, 68k//1M is a negligible cut.

The Fender 2-jack input is actually very clever. By moving the grid bias resistor away to one of the jacks and adding 2 more resistors it gets Rf rejection and 2 input sensitivities. Just for fun, work out what happens if a couple of guitarists want to share the amp and plug into a jack each.

[Ben N]

The Fender 2-jack input is actually very clever.

Thankee.

Sincerely,
Leo

[Ripthorn]

Well, after much moving it to the back burner, I have finally done a MK II of the FET-2B that makes things a little nicer for a stompbox format. These include addressing the weird input resistor network, adding an input cap, and scaling the tonestack for noise. I did adjust the tone stack balances slightly, as it was pretty crazy bright. It's a very simple thing, but figured I would follow up for the sake of completeness.

Note that it looks like four JFET's, but it's only two. This schematic is from an Eagle file where I wanted the option of SMD or through hole JFET.

[niektb]

ha you got me their for a second using MOSFET symbols for a JFET haha 

[Ripthorn]

That is unfortunately what the Mouser-supplied Eagle part uses. I know, it's dumb.

[dschwartz]

May i suggest using a higher load resistor for the tonestack..100k seems a bit low and will load down the stack.

[Fancy Lime]

Funny that this thread pops up again just now. I've been tinkering with a bass preamp design recently and decided to go with the tone stack from my trusty old Trace Elliott Commando combo, which, unbeknownst to me until stumbling across this here thread, is also just the same old Blackface three band stack plus a mid shift and a deep (=mid cut) switch. Sounds fantastic and is also a pure FET preamp. The same thing in tube, or shall I say valve, was used in the V-Type series. Among the best sounding all-tube bass amps I ever heard. Schematics to both preamps can be found here:
https://www.schematicsunlimited.com/t/trace-elliot

Cheers,
Andy

[POTL]

Try replacing the j201 with a 2n7000, I tried it a couple of years ago and I liked the sound, it was much better than the j201. but you need to properly bias the transistors and decouple the tone stack, volume, and gain from ground, using a virtual ground instead.