Harman Kardon A50K / A500 Integrated Tube Amplifier

Harman Kardon A50K tube integrated amplifier, the kit version of the A500 amplifier, serial number 6001200. This is an HK A50K integrated stereo amplifier from the golden age of tube amps. I had an HK A300 in college many years ago and was always impressed with its sound. So–as much for a hobby as anything, I researched these ‘Award Series’ amps with the intent of updating them to make them safe and reliable. I have been very focused in my efforts—I am not a generalist, but have concentrated on the tube type HK amps and receivers—and learned all that I can about these specific units. The ‘restoration’ of these (and any other 40 year old tube radio, tuner amp, etc.) per the experts includes replacing the electrolytic and signal coupling capacitors. These specific capacitors do not age well, and it’s generally agreed that ‘recapping’ is an essential requirement if you want to keep a tube unit reliable and safe.

Proper restoration of the A50K’s is pretty extensive. There are a total of 24 electrolytic and coupling capacitors to be replaced. In addition, the A500 has adjustable output tube bias balance circuits consisting of fixed resistors and potentiometers. Before I undertook the A50K restoration, I reviewed my plans with the ‘recognized HK expert’. He indicated that these pots and resistors should be replaced as part of the restoration—they are too critical to take chance on possible future failure. The last three pictures show the ‘before’ and ‘after’ of the coupling and electrolytic capacitors and potentiometer replacement. The bottom cover was removed for these pictures. After recapping, I have verified all the B+ voltages to be within original specifications.

In addition to replacing the electrolytic and coupling capacitors, I have tested all the tubes on my B&K 707 tube tester—and replaced any that were marginal. The four 7355 output tubes are matched RCA’s, all dating from 1961.

Appearance condition: The condition is good, with some minor issues. The faceplate is in pretty good condition, a few scratches, and one noticeable mark on the lower RH corner–see pics. The chassis is very nice, the faceplate lettering is decent, light in a few places, the chassis lettering excellent, the chassis itself excellent–clean, nice and shiny. The knobs are in very good condition. All-in-all a very nice appearance for the age of the amp.

As mentioned, the A50K has a bias balance feature for the output tubes. When properly set, each output tube carries the same amount of load. Some tube amps have built in meters to set the bias balance—not present on the A500’s –so the procedure has been either to use a intermodulation distortion analyzer (not at all common) or to measure inside the unit with the bottom cover off—I don’t like doing that with that much high voltage nearby. So, I ran the points to test jacks on the chassis to allow setting the bias balance with the unit buttoned up. The potential on the test points is safe–less than +/-10 volts. This should be done whenever the output tubes are changed, or every 100 hours or so of operation.