Eico AF4

I just recently had a Eico AF4 sent to me for a restore and some speaker posts. These were rated at  4 watt RMS per channel tube amps using a single ended EL84 power tube per channel. The power tubes are driven with a 7247 gain stage. Even though it has a tone pot, there is no active gain in bass or treble, but only a bass or treble cut. The rectifier is an EZ81 . Even though the front selector shows a phono input, there is no phono stage to drive a MM cartridge. The crystal input is an additional line input, thus it has 3 sets of stereo inputs.

They had these weird black 5 pin speaker connectors on the back and another to provide external power. The owner did not have the original male connectors that would go to the speaker wire, so I made up a plexiglass panel and mounted 2 pairs of banana plugs with the 8 ohm taps connected to them. He wanted the rca jacks and selector control left as is.

The restoration consisted of a new Hayseed Hamfest 3 section filter cap, all the other electrolytic caps in the amp were replaced. The coupling caps were Russian PIO K40Y. All the resistors in the signal path, cathode and plate were Carbon film. Since these were sold as kits, the original builder did not  do a real neat job of running cabling in the amp, and had to do a lot of work cleaning this up. Also it only had one ground point on a terminal strip, and the bottom of the strip touched the chassis. They used a lot of the tabs on the tube sockets in different spots in the amp for grounds. These over time will oxidize and there will be increased resistance, which will eventually lead to some hum. I installed a common ground tab and ran ground wiring from any of the tube tabs and other spots back to the main ground tab.

The unit tested as 4.5 WRMS at 1khz and I was very surprised how good it sounded. You really need a pair of efficient speakers if you want to use one of these low power amps.It sounds very much like an Eico HF81, with a nice warm midrange and nice bass and highs. Since these are rare units I thought I would document this restore.

Above are the stock speaker jacks that comes with AF4 amplifier.

Above are the new board made up with speaker binding posts.