Gecko 1: An 18-Watt Lite III Build
A Davis Model 106 PA donated its power and output transformers for this project. The Davis was non-functional when I got it, but after scrounging around the web, I stumbled on some pages written by Paul Ruby and hosted on the www.ax84.com web site where he described how he had converted a Davis 106 to an 18-Watt Lite schematic. That was enough to get me going.
This was the first amplifier I worked on after a 20 year break from tinkering with tube electronics. Originally it started out as a modification of the 18-Watt Lite Minimalist schematic from the 18-Watt forum. I included a switchable output stage that could run with either push-pull EL84s or with push-pull 12AU7A triodes. It also had a switchable preamp in the mix that allowed me to go from a parallel to series triodes. Lead dress gave me serious problems. I had enough trouble coming back up to speed with amp building, that eventually I simplified the design to use the stock 18-Watt Lite III schematic. That's why you see an extra socket and a couple of unused switch holes in the chassis.
The large square hole in the chassis is for a big-old surplus power supply choke that I orginally installed there. I didn't like the added weight (just under eight pounds), so I opted to replace it with a 150Ω power resistor.
One of the problems I ran into with converting the Davis was the output transformer. The leads were very short; a few were just an inch or so long. To deal with this I made a garolite base for the OT with turrets arranged such to accept the leads from the bottom of the transformer. The turrets would then point downward into the chassis where I could solder their connections. It worked better than I expected.
In keeping with the minimalist approach of this amp, the controls are simple and unmarked. From left to right they are Gain, Tone, and Master-Volume. The master volume is a post-PI canceling type, similar to those used in the Matchless amps.
I made the cabinet from poplar wood. Its corners are butt-jointed and reinforced by oak doweling. It is covered with a Herculon weave that is finished with two coats of shellac. The gecko emblem was designed by my son's friend, Buddy. I scanned his artwork and sent it over to Rob Weigel Laser Engraving in Portland, OR. He etched it onto a sheet of black anodized aluminum for me. The corner protectors and the spring-loaded handle came from Parts Express.