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

  
This is a very nice Harman Kardon A500 tube integrated amplifier, serial #5768271. See feedback on my amps at the end of this listing. For those of you that have followed my listings, you may have noticed that the prices are creeping up. I see the main reason for this to be the resurgence in appreciation of vinyl and tube amps, and consequently the cost of'raw material' for restorations has increased rather dramatically. 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. 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 60 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 the 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. 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.

This particular amp was partially restored by a very skilled tech before I bought it. Essentially everything he did was essentially the same as I do. After I received this amp, I did some additional work, but it was a nice candidate when I bought it. In addition to replacing the electrolytic and coupling capacitors, (using premium Sprague/Vishay 716 caps for the non phono pre-amp coupIing and Russian K42Y Paper-in-oil caps for the power tubes), have tested all the tubes on my B&K 707 tube tester and replaced any that were marginal. The four output tubes are a performance matched GE 7355 set.

Appearance condition : The condition is very nice with some minor exceptions. The chassis top is decently clean, has three small stains near the vertical front shield. Some time ago a previous owner attempted to clean a portion of the faceplate, the area below the RH knobs has a slightly cloudy appearance. This is either noticeable or disappears depending on the lighting. All the lettering on the faceplate and chassis is legible and clear.

The knobs are all good, they have been refinished, and the inside of the chassis, which is important electrically, has no issues. It's difficult to show the true color of the amp, it's essentially an off-silver color. I think the outdoor pics capture the color best. Also, the on-off button photographs blue, the actual color is the correct HK violet.

The plastic pilot lamp is really violet, not blue as it looks like in the picture. ALSO, AFTER THE PICTURES WERE TAKEN, I REPLACED THE CHEWED UP AMBIENCE CONTROL KNOB WITH A NICE CORRECT MATCHING ALUMINUM KNOB.

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.

As the line voltage these days tends to be somewhat higher than in the 1960's I've added a thermistor inside the amp to slightly reduce the internal high DC voltages. Bring it closer to what it would see when new.

Work performed on this A500 consisted of. Replaced all of the 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. Replaced the pilot lamp with a LED. Added bias balance set test points and provided a meter to set the bias balance. Added a voltage reducing thermistor to slightly lower the internal voltages. 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 90-95 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, ECHO Dot 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 for your consideration. If your country is not listed, contact me, maybe we can work something out. I've not had that issue, but want to state my position.

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The item "Harman Kardon A500 integrated tube amplifier, refurbed to original, 25WPC" is in sale since Thursday, December 31, 2020. 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 worldwide.

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