Harman Kardon Citation II Stereo Tube Power Amplifier just reconditioned

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This listing is for a Harman Kardon Citation II tube power amp that has been reconditioned by an electronics engineer with a PHD from FSU, and an astronaut. Prefer not to mention people's names but he is rather well known. He owned 10 of them and has consigned them to us to sell for him. Prior to consigning them he went through each amp and made sure it plays correctly and is biased correctly. This is the third of the ten we have.
A total of 2 amps are available in this ad, each item is one amp, for 00 plus 0 shipping each.
A rare
opportunity.
Here is what he did to them.
"All have modern filter capacitors of higher value and better quality than the original 50's/60's caps used by Harmon Kardon. All bias power supplies were converted from 'half-wave rectification using a single selenium rectifier to full-wave rectification using silicon bridge rectifiers and larger capacitance in the supplies' filter caps. All have negative temperature coefficient thermistors that lower the current surge at power-on to provide "soft" turn-on.
The bias meters' series resistors on all twelve of the amps have been changed so that the meter's bias mark represents only 67 mA. of output tube plate current rather than the 100 mA. that Harmon Kardon used. At 450 volts on the KT-88 or 6550 plates, 0.1 amp means 45 watts tube plate power dissipation. This was not a good idea since 6550s were rated at 30 or 35 watts and KT-88s at 40 watts. So, Harmon Kardon operated the Citation II output tubes well in excess of their manufacturers' plate power ratings, significantly lowering their operating life time. At 67 mA., my amps' output tubes dissipate only 30 watts, which does not exceed the rating of a 6550 and is comfortably below the 40 watt rating of a KT-99 and even more so for the 45-watt rated KT-90 power tube.
Every channel of every amplifier delivers at least 62 watts into 16-ohm resistive loads at 0.5% THD if the line voltage is held at 120VAC, which I did with a 2kW Variac. Load resistance of 16 ohms was used because that is the output tap used in the four-amp configuration to produce over 500 watts/channel (Imagine two such configurations, each capable of 500 watts @ 0.5% THD driving a pair of B&W 802Ds, which is what they do in our A/V room). Most single amp channels produced over 70 watts under the same conditions. It is my belief that every channel would have made or beaten the 70-watt figure if I had new vacuum tubes, but most of my tubes have at least four or five years of use. This is somewhat hazardous in that the output tubes can get gassy as they age and subsequently begin to "run away." The plate current in this circumstance can get high enough to damage the output transformer, an event that I have personally witnessed twice. A couple of months ago, I paid over 0.00 to an eBay vendor for a pair of Citation II output transformers and had previously had one damaged transformer rewound 15 years ago at a cost greater than that.
All but a couple of my amps have power and output transformers repainted in semi-gloss black because I did not like the factory color of brown. I apparently am not the only one since the two spare output transformers recently purchased as mentioned above were also semigloss black
Since I rebuilt all of my Citation II's with modern power supply filter capacitors of larger capacitance, at least 70 watts/channel @0.5% THD was realized on these amps.  That handily beat Harmon Kardon's rating of 60 WPC @ 0.5%, a strong selling point.
My personal recommendation is to leave the output tube bias at 30 watts, since that it within even the 6550's maximum plate rating.  It was never a good idea to run either the 655o's or the KT-88's at 45 watts since that power dissipation level exceeds those tubes maximum rating of 30 and 35 watts, respectively.
A further argument to leave my mod in is that Jim McShane, who for years was well known for offering advice and circuit mod kits for the Citation II, also recommended reducing output tube bias to a level within the output tubes ratings.  Jim, however, did it in a less satisfactory way.  He effected the reduction by increasing the KT-90 cathode resistors from 15 ohms to a higher value.  Since the voltage across this resistor sets the bias meter level, it does work, but it also changes the cathode degeneration, that is, the element of negative feedback for the KT-90.  My method only affects the meter reading, it does not alter the actual circuit parameters."
Below was written by Bob Carver
The Citation II was Stu Hegeman’s penultimate masterpiece,
inspired
by his unwavering desire to build a power amplifier equal to his
stunningly great Citation I preamplifier. I consider the Citation II
power amplifier to be one of the most original designs in audio history,
introducing technology that had never before been seen in an audio
product. The Citation II’s high-gain, wide-bandwidth video-pentode
design yielded performance that was difficult to believe.
Stu Hegeman’s circuits produced a sound that had a life and a breadth
and image depth that were stunning. They were the ultimate in
soundstaging and sense of immersion. The Citation II’s circuit was
most elaborate, the thinking behind it exceptionally advanced, the sound
spectacular! It consisted of a pentode input stage, followed by a pair
of 12BY7 video pentodes as a differential phase splitter (the video
pentode supplied a bandwidth of well over a megahertz in the input
stage). The video pentodes drove the grids of the output tubes to full
power. This had never been done before, and it resulted in a wide power
response that was second to none. The output stage utilized a pair of
KT88s for each channel, each valve having its own bias control as well
as an AC balance control. (A small meter at the chassis rear measured
bias and AC balance.)
Three feedback loops provided 32dB of overall
feedback with unconditional stability. The output transformers were
superb, built by the Freed company in New York. Huge, well-potted units,
they boasted extremely wide response thanks to the ultra-low leakage
inductance. Only the highest-grade core materials were used, which
lowered the effect of core distortion to a region well below the limit
of human hearing. With feedback, the Citation II transformers were
capable of high-frequency response up to 270kHz, a response that most
designers of the time thought could not be achieved.
This 70 pound amp is quite a feat of engineering. It's
impressive weight and construction is equally matched by the impressive
looking line of tubes, and the conservative 60 WPC power rating. But the
really impressive work is "under the hood". Hegeman felt that a good
amplifier should not be bandwidth limited to the audible range, but
should extend farther in both extremes, so the amplifier can handle the
audible range without any of the artifacts of limited bandwidth
amplifiers; such as phase shift at the frequency extremes.
This amplifier consists of a pentode input
stage, followed by a pair of pentodes as a differential phase splitter.
The pentodes Hegeman chose were 12BY7 video pentodes. This choice of
tube gives the input stage a bandwidth of well over a megahertz. The
output tubes are a pair of KT88's per channel. There is a individual
bias controls for each output tube as well as an AC balance control.
There is a meter on the back of the amp to measure bias and AC balance.
AC balance requires a set of 25 Watt 16 ohm dummy loads be substituted
for the speakers. A test signal output RCA jack is supplied on the back
of the amplifier. The test signal is fed into each channel and the AC
balance pot is adjusted until the meter reads a null.
The output tubes drive a potted transformer
which is arguably the best ever built. It was wound by Freed in New
York. It has a full power bandwidth of up to about 230 HKz. These
transformers look like iron, but sound like gold. They are also
scrounged by Bruce Moore for use in his MFA amplifiers. I'm sorry to say
that many a Citation II has been gutted for it's iron.
There are three nested feedback loops. The first
is from the plates of the phase splitter tubes, to their grids. The
second is from the plates of the output tubes to the grids of the phase
splitter tubes. The third is from the output transformer secondary to
the cathode of the input tube. The total amount of feedback is about 32
dB. This about 10 dB more feedback than a traditional Mullard or
Williamson type tube amp. The multiple loops, and fantastic output iron
make this possible. The time constants of each loop have been carefully
calculated. 10 KHZ square waves (at about 2/3 power) look amazingly
good, with just a tiny hint of ringing (one period). This amplifier is
amazingly stable for any tube amplifier, let alone a tube amp with that
much feedback.
When the passive components in this amp are
updated, this amp sounds just amazing. It produces music with an
authority and an ease that is unequaled in the audio world. It tends to
have a dry and "correct" sound rather than a "tube amp" sound. If you
want the warmth and carmel sound of vintage tubes, this may not be the
amplifier for you. If you want your music served up honestly with a bare
minimum of distortion (and no solid state distortion), this may be the
amp for you.
Note:
This amp will Eat Quad ESL's, it sounds good, but can fry tweeters instantly
Offered for 00 plus 0 domestic shipping. Guaranteed to play correctly and be as described. Always over-packed and insured.
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Offered for 00.00 with 0 domestic shipping.
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