I broke down and bought a pair of Quad II amplifiers from eBay. Like most of these old pieces of audio gear, the seller claimed no ability to test them and they were sold as is. The pictures looked good, and the only thing missing was one fuse-holder cap. I cleaned them up a bit and tested the tubes. The original GEC KT66’s and the EF86’s all measured strong. I through the switch on each amp and tested it on the bench. They seemed to be working well, so I gave them a listen.
They have that simple tube amplifier clarity, and warmth that keeps me coming back. So at this point I decided to do a passive parts swap. There isn’t an easy way to replace the power supply electrolytic caps, so I made a circuit board to hold a set of modern PCB caps. The circuit boards are shown below:
The amps really sound outstanding now, and are ready for a few more decades of use.