You love to play guitar and you love your amplifier collection.
But, what if I told you that you could make the most unique guitar amplifier known to modern man. From the hills of North Georgia comes the Jelly Amp. Sure there may be some commercial examples of this available but by following this Instructable, you can easily make your own. Not from a kit, but from scratch. Just like MeeMaw makes her prize pies: not from a Betty Crocker kit from the Piggly Wiggly like those city folk, but from the sweat of her dag-nammed blessed little wrinkled brow.
This project has a very simple design. The amplifier is a version of a "Little Gem" amplifier from the brilliant Runoffgrove's runoffgrove.com site. It is based on a 1/2W LM386 audio amplifier with a fixed gain and full output volume. There are no on board controls. The volume and overdrive are controlled by your guitar's volume and tone controls. There is no on/off switch neither. The amplifier turns on when you plug your guitar cord into the input jack. It's powered by a 9V alkaline battery that sits inside the jar.
The sound comes out of a mylar speaker in the lid. Also there is an output jack that will allow you to connect your Jelly Amp into your favorite "big boy" speaker cabinet. It will drive anything from 4 ohms to 16 ohms. Will it blow out the stadium? No ya big dummy its only half a watt. Is it great to practice with? You're darned right it is. At full volume it has just the right amount of distortion. It's a great little practice amp and a great conversation starter.
So lets get ready to build one.......
Step 1: Safety and Parts List
First a word about safety. Don't be an idiot. This project involves drilling holes in a glass jelly jar. You can get hurt. You can put your eye out. If you're an idiot, don't attempt this. I can't be responsible if you get hurt. Wear safety goggles. When you're working in the shop leave PeePaw's other mason jars alone. This ain't the time to be fooling with his "medicine".
So here is the parts list:
1 8oz (half pint) regular mouth mason jar, or jelly jar. You can get these from Walmart or any other place that sells canning supplies. You want to get a plain jar. No fancy quilting or writing on the glass. Take MeeMaw with ya. She'll know what you need.
1 3/8" diamond hole bit
1 9V alkaline battery
1 9v battery snap connector cable
1 mono 1/4 inch jack with nc (shunted) switch ( I like the Switchcraft 12A)
1 stereo 1/4 inch jack ( Switchcraft 12b)
1 LM386 amplifier. Any version is fine, I like the LM386N-4, but I'm kinda funny that way.
1 66mm Mylar Cone Speaker (MCM #SR00187)
1 Red LED
1 470 Ohm
1 1K Ohm
1 10 Ohm
Protoboard. I use a Radio Shack DIP protoboard (276-159B). It keeps things neat. But any type of protoboard is fine as long as it fits in the jelly jar.
Soldering iron and solder
Double sided tape
Step 2: Schematic and Amp Board Fabrication
Assemble the amp board according to the above schematic. I have attached a photo of my protoboard build so that you have an example of a component layout that works. Any protoboard will work. If you want to get fancy, you can design a fancy board on the computer and send it to a foreign land to be magically fabricated.
Lets explore the pinout of the LM386 a bit more shall we?.....
Pin 2. Inverting input. The signal from the guitar is fed into pin 2 through a 100nF cap that eliminates any DC signal. You can play around with the capacitor value. 100nF works well for an electric guitar. The original Runoffgrove schematic used a 10nF cap. I like the sound of the 100nF better. But play around and see what you like. Some of the later mods used a 47nF cap just to confuse the issue more. But dammit I like the 100nF.
Pin 3: Non inverting pickup. This pin is grounded
Pin 4: Ground. This pin is grounded
Pin 6: V+. We are using 9V for this project. In order to reduce oscillation in the amplifier, a 100uF or 220uF capacitor is placed across the negative and positive supply voltage. Ideally this should be as close to pin 6 as possible.
Pin 7: This pin is used to reduce power supply noise. I like to live on the edge. Do you? Go ahead. Leave this pin unconnected. I did, but thats just how I roll. If you want to be a baby about things, put a 100nF cap from pin 7 to ground. Wimp.
Pin 5: Output. The output of the amplifier goes though a Zobel RC network for stabilization. Don't mess with it. Greater minds than ours have worked this out.
Pin 1 and 8: Gain. Leave these pins disconnected for minimum gain. Connect a 10uF cap between the pins for max gain. Mid level gains can be had by placing various resistor values between the pins. The art in choosing the right resistor values is to have enough gain to produce a pleasant overdrive but not make this amp into a screeching harpy. I like using a 1K resistor. When the guitar's volume control is turned down, a fairly clean sound will be produced. With the volume all the way up, the amplifier breaks up nicely. Experiment to find out what you like.
Step 3: Marking and Drilling the Jar
Now is the time to drill the holes for the input jack and output jack.
Let's measure and mark the places where you will drill the holes. My jar had a circumference of 25cm. That means the holes should be 12.5cm apart. Also they worked best about 3.5 cm from the bottom. I wrapped the jar in masking tape and marked the drill sites.
I used a 3/8 (10mm) diamond glass hole bit to drill the holes. Don't scrimp on this. Get a good one. You can use it in the future to drill holes in Bud bottles to make you some Christmas lights for PeePaw's porch. He'll love them. Believe me.
When you drill into glass you have to keep the bit cool by submersing it in water. I use plumber's putty to make a well around drill bit. Go slowly. Use light pressure and clear the glass dust frequently. A drill press is best for doing this.
When you are done you will have a nice clean hole as shown above.
Step 4: Connecting the Input and Output Jacks
The input and output jacks are wired as shown above
Step 5: Assembly and Finishing.
Once you have everything assembled, use double sided tape to mount the amp board to the bottom of the jar.
Feed the input and output jacks into their holes. You can use some epoxy to help secure them and ensure that the nuts don't loosen with use.
Snap the 9V battery into its strap and place it beside the board in the jar. The battery will wedge in the jar and won't flop around. If it flops in your jar, you can use a little double sided tape to secure it to the jar wall.
Solder the speaker to the output jack and put it on top of the jar. Use the jars retaining ring to screw the speaker in place. You won't need the jar's metal lid. Give it to MeeMaw. She'll thank you.
If you want to mark the input jack, borrow MeeMaw's red nail polish. You know the one that PeePaw says makes her look like a Paris Street Walker (the kind he likes). Put a red dot just below the jack.
If you've done everything right when you plug a guitar into the input jack the red LED will light up and sweet sweet music will come out of the speaker. If you haven't done everything right then it'll be time for some troubleshooting. Read this Instructable over and over again until you find the errors of your ways and you're set back on the path of righteousness.
When you plug a speaker cabinet into the output jack, the built in speaker will turn off and sound will now be coming out of the external speakers.
Have fun building this one guys!
And now for your enjoyment, above is a link to a short video of a guy playing this very Jelly Amp to the admiration of all those around him.
****** UPDATE ********
For this of you with build questions, I have made a new Instructable showing how to build this circuit on a protoboard. The link is here:
There is a companion Youtube video showing the entire build. The link is above.
Good Luck and get building!!!