Installing The Tillman JFet Preamp
The Tillman JFet preamp is an amazing little circuit. It's only seven components and yet it has a wonderful effect on any guitars sound, in my opinion. It adds a bit of warmth and roundness to the tone as well as a 3db boost to the signal. I think it also slightly compresses the signal because a chord played through it sounds more balanced than without. In this tutorial I add the Tillman jfet preamp to a Stratocaster using a push/pull potentiometer that allows you to bypass the preamp in the down position (step 4 mentions some other wiring variations possible). Although the steps shown are demonstrating how to add the preamp to a Stratocaster, you can just as easily add it to any guitar that has the space for the preamp and the battery because the preamp is added at the very end of the signal chain. After building the Tillman preamp you can refer to the first picture of step four as a guide to how to install it in any guitar.
Click here to visit Don Tillman's Site, the creator of the circuit. There is a lot of good information about the circuit on there.
• J201 Fet (field effect transistor)
• (1)3.0M, (1)51k, (1)6.8k, (1)2.2k Resistors (1/4-watt 5% tolerance)
• (1)10uf, (1)4.7uf Electrolytic Capacitors (mf and uf both stand for microfarad)
• 9 Volt Battery
Forewarnings and General Info...
The first few times I built it it did not work because I had the Jfet installed incorrectly. It resulted in the preamp actually making the signal quieter with no improvement to the sound. I nearly gave up thinking that I had built it correctly but that it was a bad design. Luckily I found out that I was looking at the pin outs from an incorrect diagram. Aside from confusing the Jfets pins, it is pretty hard to mess up and a great circuit to build to get into active electronics.
Step 1) Here are the parts you need to get started with the preamp. I forgot to show a stereo jack which is needed to turn on the battery when a jack is plugged into the guitar. I start by drilling a hole in the perf board in order to mount it to the push pull potentiometer. The first attempt failed because the perf board is brittle and the little holes cause the drill bit to get caught. The second attempt, I drilled as much as I thought I could and then continued to drill by turning the drill bit by hand. It is not the most beautiful hole but it works.
Step 2) Next, I decided it would be a good idea to trace the perf board to my schematic notebook so that I could reference the layout for the next time I build a Tillman JFet Preamp. It also helps to visualize the best layout before commiting to it. I decided on a layout and got it all soldered up. After deciding were I want the components I cut off the extra perf board to make it as compact as possible.
Step 3) Here I added the Input and Output preamp wires as well as wired the circuits ground to the push pull potentiometers housing. At this point it would be wise to test the preamp out. It is a bit of extra work to solder on the battery leads and the jacks necessary for testing but it can save you a headache later. I did not test the preamp at this point because I am lazy but luckily it worked the first time. Next I open up the Stratocaster and remove the potentiometer that I plan to replace. I check the available space to make sure the preamp and pot have enough room. I had to re-mount the preamp crooked in order for it to fit properly.
Step 4) Step four is just a matter of wiring up all the connections as shown in the diagram in the first picture of step four. This diagram shows how to wire the preamp so that it is activated when the push/pull is pulled up. When the push/pull potentiometer is down, the preamp is bypassed. To have the preamp turned on in the down position, simply reverse the connections on the DPDT switch sothe top left becomes bottom left, top right becomes bottom right and visa versa. The wiring for having the preamp always in the circuit is much easier. You can do this by wiring the wire that goes to the output jack to the input of the preamp and the output of the Tillman preamp back to the output jack. The reason I wanted to be able to bypass the preamp is just a precaution for incase the battery goes dead at a gig. It is also nice to be able to show people the difference in sound. I mount the battery in the back cavity of the strat for easier access and push its wires through the hole to the main cavity. Putting a bit of foam on the back plate will keep the battery in place.
Now I have to replace the old mono jack with a stereo jack. The stereo jack is needed to turn the battery on when a jack is inserted into the guitar. It is cleverly wired so that the batteries negative lead which connects to the guitars ground is only connected to ground when the plug is in. This happens because guitar cables are mono and the ring and sleeve end up both touching the cable ground which bridges the connection and turns the battery on. Clever! I install the stereo jack but I notice that the jack is nearly touching the shielding. This would cause the signal to go to ground and result in no sound. The solution is simply to cover the shielding in that area with some non conductive material. In this case I just glued some sandpaper around the problem area. Any non conductive material would do like duct tape but all I had was some sand paper.
Now I connect the batteries positive lead to the Tillman preamp then mount the preamp and potentiometer in place. I finish the wiring which is just a matter of connecting the guitars volume control output to the input of the preamp and wiring up the potentiometer part of the push/pull pot as it originally was with the old tone pot. All finished! Put the Strat back together, put on the strings, plug it in and enjoy!
Check if the problem is described here, Troubleshooting Guitar Wiring.
If you still have not found the problem, I suggest you join this forum, The DIY Stomp Boxes Forum. It is an incredibly helpful resource that I have recently started to use when I need help troubleshooting. As long as you are polite and give a detailed description of the problem you are sure to get some valueable help within a matter of minutes. Most people on this forum are far more knowledgable about active electronics than I am and will be able to help you better and faster than I could.