Harman Kardon A500 integrated tube amplifier, refurbed to original, 25WPC, nice

  

This is a very nice Harman Kardon A500 tube integrated amplifier, serial # 3820398. See feedback on my amps at the end of this listing. Newest feedback at the bottom. This is an HK A500 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.

The last picture is me with my A300 in 1965. 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 never put power to these amps without first replacing the power supply electrolytic and paper/oil coupling capacitors. The restoration of these and any other 50 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. Some people believe that'reforming' capacitors (bring them up to voltage with a variac over a few days) will provide safe operation, but I would rather use new capacitors even though that is a much more expensive and labor intensive route. Interesting enough, most other components tubes, transformers, wiring, most resistors, coils, etc. Do not generally fail with age.

Switches and potentiometers volume, tone, balance, etc. May become noisy, but usually can be restored with proper use of quality cleaning (DeOxit) solutions. I believe it should be the intent of restoration to bring the amp back to the quality and sound of the original unit as originally designed by the HK engineers and I have selected the components to match the original design with one exception.

There have been improvements in rectifying diodes since these units were first built, and that's one area where I provide an upgrade. New diodes, with fast turnoff /soft recovery are now available and because of their characteristics, provide a cleaner rectifying operation than the original diodes--fear not, the "tube sound" is not diminished in any way with these diodes--only a cleaner power system. A previous owner had replaced the coupling capacitors using Electronic Concepts Inc. (ECI) capacitors, a good choice of a film cap. I replaced all the power supply caps, resistors as necessary, installed new diodes, added bias balance control, new bias balance pots and replaced the pilot lamp with an LED lamp. Proper restoration of the A500 is extensive. There are a total of 24 electrolytic and coupling capacitor sections to be replaced. In addition, the A500 has adjustable output tube bias balance circuits consisting of fixed resistors and potentiometers.

Before I undertook the A500 restorations, I reviewed my plans with the "recognized HK expert". He indicated that these pots and resistors should be replaced as part of the restoration as they are too critical to take chance on possible future failure. The bottom cover was removed for pictures.

After recapping, I have verified all the B+ voltages to be within original specifications. In addition to replacing the electrolytic and coupling capacitors as necessary, I have tested all the tubes on my B&K 707 tube tester and replaced any that were marginal. The four output tubes are a matched GE 7355 set. Appearance condition : The condition is nice with some minor exceptions.

The chassis top is generally clean, there is some staining, the faceplate is decent, at some time a previous owner appears to have sprayed the faceplate with a cert light coat of tinted translucent paint which gives the amp a more of a gold hue--it's hard to capture the color, I think pics. 4 and 5 capture it fairly closely, not unattractive, but closer to the gold hue of Fishers than the champagne of HK's. I don't think you would notice the hue change unless you had a companion HK tuner placed adjacent to the A500.

The inside of the chassis, which is important electrically, has no issues. As mentioned above, the A500 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 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. Work performed on this A500 consisted of.

Replaced the original coupling and electrolytic capacitors--not because they have failed, but to increase reliability and bring the unit back into specifications. Capacitor failures can ruin your day (and your equipment). Replaced the power supply rectifying diodes with modern diodes that perform the identical function, but with less noise. Tested all the tubes on my B&K 707 tube tester and replaced all tubes that did not test strong. Cleaned all switches, controls and tube sockets with Deoxit cleaner--D5 for tube sockets, switches and contacts, Fader F5 for the potentiometers. Verified all controls, B+ voltages and operations. Replaced the bias control pots and resistors. Added bias balance set test points and provided a meter to set the bias balance.

Replaced dial lamp with violet LED, longer life and a little less load on the filament transformer--color is correct HK violet. Ran amp for a minimum of 16 hours after restoration--with no issues. If you are reading this, you probably are familiar with the "tube sound". It's just mellow, and clear and wonderful (my opinion). The HK tube amps were (and are) highly respected units from the'golden age' of tube systems.

Probably says something for HK in that they are one of the very few hi-fi manufacturers from the 60's that are still in business. The main specs of the A500 are.

IHFM music output power: 25 watts per channel. These tube amps really like high efficiency speakers, many folks use Klipsch speakers with efficiencies in the 91-94 db range, I personally like the Electrovoice TRX12B's. How much power you need also depends on your room size and listening style. 5 db 12-35,000 cps at normal listening levels.

Output impedance: 4, 8 and 16 ohm. Stereo inputs: low level 2.5 mv. For magnetic phono cartridges and tape heads (from tape recorders).

Stereo inputs: high level 200 mv. This is suitable input for tuners, CD decks, and most mp3 players, tablets and smart phones. Third channel output with volume control (called ambience): to feed a center channel amp, reverberation unit remember those?

Stereo tape out--to feed a tape recorder. Headphone jack--does not disconnect power to the speakers when in use. Separate Bass and Treble controls for each channel--more knobs to turn. This is not your son's amplifier. This is a tube unit, and as such, has voltages inside the chassis that can be lethal, 350+ volts.

Tubes and transformers get hot and while the unit is rated for a 100% duty cycle, I suggest turning it off when not in use. My goal was to make this amplifier as safe and reliable as I know how to without losing the wonderful "tube sound". I believe I have accomplished this. So, if you are into tubes, or just starting, here is an amplifier that is ready to go right out of the box. If your country is not listed, contact me, maybe we can work something out.

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This item is in the category "Consumer Electronics\Vintage Electronics\Vintage Audio & Video\Vintage Amplifiers & Tube Amps". The seller is "camaroguy62" and is located in Fenton, Michigan. This item can be shipped to United States, Canada, United Kingdom, Denmark, Romania, Slovakia, Bulgaria, Czech republic, Finland, Hungary, Latvia, Lithuania, Malta, Estonia, Australia, Greece, Portugal, Cyprus, Slovenia, Japan, China, Sweden, South Korea, Indonesia, Taiwan, South africa, Thailand, Belgium, France, Hong Kong, Ireland, Netherlands, Poland, Spain, Italy, Germany, Austria, Bahamas, Israel, Mexico, New Zealand, Singapore, Switzerland, Norway, Saudi arabia, Ukraine, United arab emirates, Qatar, Kuwait, Bahrain, Croatia, Malaysia, Brazil, Chile, Colombia, Costa rica, Panama, Trinidad and tobago, Guatemala, Honduras, Jamaica, Barbados, Bangladesh, Bermuda, Bolivia, Ecuador, Egypt, Guadeloupe, Iceland, Jordan, Sri lanka, Luxembourg, Macao, Martinique, Maldives, Peru, Pakistan, Reunion, Viet nam, Uruguay.

• Model: A500
• Country/Region of Manufacture: United States
• Type: Integrated Amplifier
• Brand: Harman Kardon
• Country of Manufacture: United States