Single channel, clean/crunch/distortion amp run by JFETs !
This project describes a small guitar power amp, built around four J112 JFET's.
(https://www.conrad.be/nl/p/on-semiconductor-transistor-bjt-discreet-j112-to-92-tape-3027573.html)
JFETs are well known to produce a very nice "tube" like distortion, so they are perfect to play around with.
My goal is to produce a variety of "sounds" with this amp: from "clean" trough "crunch" all the way to "distortion". I choose for a single channel approach.
See my schematic "JFET guitar amp V1.0.pdf"
Q1 is the input amplifier, which amplifies the guitar signal about 10 times.
RV1 is the input gain volume control.
Q2 amplifies the signal around 2.5 times, and is needed to drive the tone control. This is a one knob tone control, built around a lowpass filter (R8 C7) and a highpass filter (C6 R9) that are mixed with RV2. Setting the potmeter more towards "lowpass" gives a less high tone and vice versa.
Q3 and Q4 are two identical tone shaping stages. SW2a (SW3a) adds a boost in the gain by adding source capacitor C10 (C16). To balance the signal level compared to "non boost" mode I use SW2b (SW3b) to add extra attenuation resistors R17 (R18) in the signal path.
Q3 (Q4) have also a BIAS potmeter to adjust the working point of each JFET. With this control you can adjust the symetry of the signal, producing a more "crunchy" type of sound when set off centre.
With these two stages, you can play around and try and find the right tone you like!
As power amplifier, I use a standard KEMO module: this time the KEMO M033N. (https://www.conrad.be/nl/p/kemo-m03...dc-12-v-dc-18-v-dc-20-v-dc-18-w-4-130082.html)
They are cheap, reliable, and if you don't overdrive them, they work perfect!
RV5 is the output volume control. As you can see, the signal is well attenuated, because the KEMO module has an input sensitivity of 100mV max !!
I use 2 Celestion Ten-30 speakers of 8ohm in parallel, they sound great and are pretty cheap!
The powersupply is pretty straight forward, transformer/rectifier and 15V regulator.
I think this is a nice sounding amplifier, with plenty of power due to the 4 ohm dual speaker setup.
The tone shaping is a non standard approach, that gives a lot of options to find your perfect flavour.
The use of JFETS avoids the complexity of tubes with an equal (or even better) sound quality !!
I have add a few scope images of different signal shapes that are possible with both tone shaping stages.
Also some images of my prototype amp!
Have fun!!
This project describes a small guitar power amp, built around four J112 JFET's.
(https://www.conrad.be/nl/p/on-semiconductor-transistor-bjt-discreet-j112-to-92-tape-3027573.html)
JFETs are well known to produce a very nice "tube" like distortion, so they are perfect to play around with.
My goal is to produce a variety of "sounds" with this amp: from "clean" trough "crunch" all the way to "distortion". I choose for a single channel approach.
See my schematic "JFET guitar amp V1.0.pdf"
Q1 is the input amplifier, which amplifies the guitar signal about 10 times.
RV1 is the input gain volume control.
Q2 amplifies the signal around 2.5 times, and is needed to drive the tone control. This is a one knob tone control, built around a lowpass filter (R8 C7) and a highpass filter (C6 R9) that are mixed with RV2. Setting the potmeter more towards "lowpass" gives a less high tone and vice versa.
Q3 and Q4 are two identical tone shaping stages. SW2a (SW3a) adds a boost in the gain by adding source capacitor C10 (C16). To balance the signal level compared to "non boost" mode I use SW2b (SW3b) to add extra attenuation resistors R17 (R18) in the signal path.
Q3 (Q4) have also a BIAS potmeter to adjust the working point of each JFET. With this control you can adjust the symetry of the signal, producing a more "crunchy" type of sound when set off centre.
With these two stages, you can play around and try and find the right tone you like!
As power amplifier, I use a standard KEMO module: this time the KEMO M033N. (https://www.conrad.be/nl/p/kemo-m03...dc-12-v-dc-18-v-dc-20-v-dc-18-w-4-130082.html)
They are cheap, reliable, and if you don't overdrive them, they work perfect!
RV5 is the output volume control. As you can see, the signal is well attenuated, because the KEMO module has an input sensitivity of 100mV max !!
I use 2 Celestion Ten-30 speakers of 8ohm in parallel, they sound great and are pretty cheap!
The powersupply is pretty straight forward, transformer/rectifier and 15V regulator.
I think this is a nice sounding amplifier, with plenty of power due to the 4 ohm dual speaker setup.
The tone shaping is a non standard approach, that gives a lot of options to find your perfect flavour.
The use of JFETS avoids the complexity of tubes with an equal (or even better) sound quality !!
I have add a few scope images of different signal shapes that are possible with both tone shaping stages.
Also some images of my prototype amp!
Have fun!!
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Hello Bitstream and congratulations on your amp build! I haven't seen too much point to point wiring like that on solid state amps.
Not trying to be critical on that approach, as it's DIY after all. But I would suggest adding a mains fuse on the power supply as a minimum
safety. I did not see one in the schematic or the pictures, but maybe you have already added one by now? Also maybe adding some insulation around the leads connections to the bridge rectifier and DC regulator, it's a little close to the chassis..a chance it could touch and short out..
Other than that , I like JFET preamps as well, they do have a good progression of sounds from clean to overdriven to crunch. Maybe not exactly as a valve/tube, but similar in the way it responds, with gradual progression of low order harmonics with input amplitude . Cheers~
Not trying to be critical on that approach, as it's DIY after all. But I would suggest adding a mains fuse on the power supply as a minimum
safety. I did not see one in the schematic or the pictures, but maybe you have already added one by now? Also maybe adding some insulation around the leads connections to the bridge rectifier and DC regulator, it's a little close to the chassis..a chance it could touch and short out..
Other than that , I like JFET preamps as well, they do have a good progression of sounds from clean to overdriven to crunch. Maybe not exactly as a valve/tube, but similar in the way it responds, with gradual progression of low order harmonics with input amplitude . Cheers~
And the incoming mains earth connection must go straight to nearby metal work with a substantial brass earth post/bolt, and this earth connection must not be used for anything else than safety protective earth (as happens with your bridge rectifier (*)). Signal ground has to be a separate chassis connection. Ensure the separate parts of the metal case are securely earth bonded to each other (crinkle washers ensure good electrical connection, remove any paint of anodizing layers). To save your future self (or others) from the risk of electrocution you should shroud all live and neutral connections within the box from accidental finger contact.
(*) Should your bridge rectifier case shatter or soften/melt, you will be left without a secure earth connection - this is why earth connections are required to be dedicated.
Very good and important point! We just want to ensure your electrical safety, Bitstream. DIY is great fun, if you have a bit of time to add in those features the risk of you or others getting zapped, or the amp shorting and causing an electrical fire, go down considerably.
There's always the battery option, which pretty much eliminates any possibility of AC mains shock hazard. 8-20V DC - more toward the 20V side of course - of battery power shouldnt be so difficult to come up with (3, 6V LA cells?).
Then you'd have something like a "Pignose", which I've seen a guitarists use as their main amp (actually 2, miked) for a show, back in the mid 70's.
Yes, remembering to charge is a PITA, as evidenced by so many Pignose with dead Lead Acid batteries seen on auction sites. With the explosion of better battery chemistry these days, you can design and purchase components for a better performing energy source. 18V drill battery?
AC mains power, with all its problems, including hum and noise; it's amazing how so many are willing to "die for" the convenience of it, should there be a single mistake or failure point. Not like you're burning power just to heat cathodes with this design.
Its connected to ground.
Put continuity meter to plate then bolt... yup beeeeeepp metal thingies
I remember looking at these same modules or similar long long time ago
and all the tube jfet thing circuits.
Basic thought to build something similar when first starting.
Never did of course, went to more power for Bass.
But here it is, you built one of those ideas.
Tested the jfet pre and many many others, led to the distortion wormhole.
Good learning process. What is next on your list
Trying to think of the website that was rather popular for these jfet circuits.
Had numerous filter projects for old simple cabinet simulators too.
Peaked my interest in what and why things did what they did in guitar land.
Definitely the beginning before full on VST went wild and analog effects.
Interesting, way back we could buy used tube fryers for 45$ nobody wanted them at the music store.
Stacks of Traynor MarkII or the way way to high voltage Music Man tube amps.
Toss in chinese EL34's till they popped again.
Basically transformers, sockets chassis etc etc for modding or builds, dirt cheap
Wont see those prices anymore. Before the internet, so no downloading circuits.
Put continuity meter to plate then bolt... yup beeeeeepp metal thingies
I remember looking at these same modules or similar long long time ago
and all the tube jfet thing circuits.
Basic thought to build something similar when first starting.
Never did of course, went to more power for Bass.
But here it is, you built one of those ideas.
Tested the jfet pre and many many others, led to the distortion wormhole.
Good learning process. What is next on your list
Trying to think of the website that was rather popular for these jfet circuits.
Had numerous filter projects for old simple cabinet simulators too.
Peaked my interest in what and why things did what they did in guitar land.
Definitely the beginning before full on VST went wild and analog effects.
Interesting, way back we could buy used tube fryers for 45$ nobody wanted them at the music store.
Stacks of Traynor MarkII or the way way to high voltage Music Man tube amps.
Toss in chinese EL34's till they popped again.
Basically transformers, sockets chassis etc etc for modding or builds, dirt cheap
Wont see those prices anymore. Before the internet, so no downloading circuits.
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Sometimes, you get lucky these days being in the right place at the right time. Picked a working Bogen chassis with good output tubes for that price at a local ham radio flea market. Toward the end of the show, nobody wanted all day and the fellow apparently didnt want to carry back home...
My haul from the Orlando Hamfest back in 2010. I bought all of the guitar amps from one guy for $50 at closing time on Sunday along with a bunch of other random musical instrument related stuff. All of the NOS resistors are Allen Bradley carbon comps. The large bag full of "factory floor sweepings" had lots of useful parts. All were cheap. The tubes were bought onsey-twosey over the weekend for $1 or $2 each. The cardboard box is full of loose tubes that were under $1 each. Oddly the Fender "G-DEC" (Guitar - Digital Entertainment Center) worked. It is a low powered SS guitar amp with a built in digital "accompaniment" section. It would be useful for a beginner to learn how to play along with others. After getting bored with the canned programs, I sold it for $50 making all the other amps free. The Crate had two blown diodes in the power supply causing a blown fuse. I fixed it and used off an on for years before giving it to my oldest grandson about a month ago. My initial plan was to toss the chassis and build a tube amp on a Bud or Hammond box to take its place. Unfortunately, when I actually tried to do that there was no standard pre-made box that would fit in that cabinet. I got the reverb box working, loaned it to a friend, and never saw it again. The Line 6 had been trashed internally and was not complete. Fortunately, it came in a nice pretty black box that DID fit a common chassis. The SS powered Fender Frontman 15 became a vacuum tube powered Champ clone.
