Curing a sick CAT · Decades of Musings on HiFi

This Cat was not all that well when it arrived in the early noughties to be checked out.

What was wrong

The output tubes had died and had taken out many of the cathode resistors, so the JL-2 amplifier was not working well. In addition, the owner complained of the need to re-bias and of a persistent hum.

Photograph: the amplifier in a listening room on the floor in front of equipment racks, with a row of output tubes visible — The amplifier in the listening room.

What was done

New sets of tubes were procured (Russian 6550), tested and burned in.

The biasing was automated using the automatic bias module I designed for diyhifisupply.com (long unavailable). The amplifier no longer needs to be re-biased by hand.

The hum was cured with a safe earth lifter using high-current diodes.

Photograph: the underside of the amplifier chassis, showing point-to-point wiring, circuit boards and a bank of large blue capacitors — Under the chassis: the original point-to-point wiring.

Photograph: close-up of the underside with the automatic bias modules, black boards with red WIMA capacitors and blue indicator lights, mounted among the original wiring — The automatic bias modules, black boards with red WIMA capacitors, fitted among the original wiring.

Photograph: another close-up of an automatic bias module and the surrounding wiring — A second view of the bias module installation.

Photograph: close-up of resistor strips and cable looms next to a bias module — Resistors and wiring next to a bias module.

Photograph: close-up of a bias module with its terminal blocks and the wiring running to it — A bias module with its terminal blocks.

Photograph: a bank of large blue capacitors mounted on the chassis — The bank of large blue capacitors.

Result

With a good cleanup and the restoration, the amplifier returned to its excellent sound quality and served for many more years without incidents.