Since I just joined the forum, I thought I might offer this up. On various sites, I see many questions about the Echoplex EP-3 used by Van Halen, Jimmy Page, my hero Eddie Hazel and others. I wondered about it as well.
Sometime back, I was noodling about in my parts bin and came across a 9VAC 300ma adapter used to charge my old Nokia phone. I had some MPF102 JFETs and had never tried an EP-3, so I built this. The 9VAC adapter goes into a simple voltage doubler. Because of the low current draw of the MPF, the filter cap charges to 24VDC, which after the drop across the 6.8K resistor came to exactly 22VDC on my unit. A larger current-rated adapter may cause higher voltages on top of the stacked caps. The 300mA 9VAC adapter I used (a Nokia phone charger, but a 9VAC answering machine adapter would also probably work well) worked perfectly.
You see many criticisms on forums about 9V EP-3's not having the correct vintage voltages. Well, this one does. I have built it, it works and works WELL. I was skeptical about the mojo associated with this preamp, but it DID make a difference in how my JCM800 preamp felt and sounded. I like it very much! The MPF seems to work well in this otherwise note-for-note copy of the EP-3 circuit. I've got about 500 hours on mine so far, so the circuit seems stable.
One warning, it's NOT a boost as much as a tone-coloring buffer. It has a very mild gain. What makes it special is that it creates an accentuated high mid which seems to impart a clarity on the guitar signal and really comes through in distorted amps. These are my impressions. You may build it and hate it. So it goes!
Of course, it is tied to an adapter. Sorry about that, but I didn't have a charge pump in my junk box! I mounted mine in my rack. Forgive the crudeness of the drawing. I drew it up quickly for a fellow on an amp forum. If you build it, I hope you like it! Bill.
My Greatest Respect to Mike Battle, the Echoplex circuit's designer!