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In Reply to: RE: Harmon Kardon Citation II article in TAS Legacy segment by Bob Carver.. March 2017 posted by Julien43 on March 17, 2017 at 11:40:36
Here's the whole text:
It was a hot mid-summer's night and I was in Chicago at the consumer electronics show.
I was in my booth pitching my wares, a new amp and preamp, and it was hot inside. Not the amplifier, but the convention floor, and I was getting thirsty. I wanted something cold to drink, so I decided to sneak away from my booth for a moment to get a coke. As I was sneaking, a colleague happened to walk by and said, " Hey Bob, have you seen Stu Hegeman's new preamp?"
I found myself getting excited, as Stewart Hegeman, the master designer of so many classic vacuum tube amplifiers and my very own hero, was here! Stu Hegeman was a true genius, having designed the Citation I, II, probably the III and countless other amplifiers for Sidney Harmon and for Lafayette.
We all were just entering the solid state age, and Stu had started his own company to introduce his latest creation, the solid state Hapi I preamp. I just had to see it! So instead of getting my coke, I made a bee-line to Hegeman's booth, and there I met the master and my hero for the first time. I introduced myself and HE said "I know who you are, you're the kid who designed the Phase Linear." Kid? Anyway, I was standing there mesmerized and in awe of my hero and could hardly concentrate on what he was saying, much less COMPREHEND what he was saying.
And so we began talking about amps and preamps, ultimately leading to a quiet corner in a close-by restaurant. Very close, as it was part of the convention itself. I finally got my coke. I could not believe I was in the presence of THE Stewart Hegeman, and he was talking to ME! We talked and talked about big solid state amps, tube preamps, solid state preamps, loudspeakers, ionic tweeters, recording lathes and finally tube power amps. And what a scientific talk it was! As two hours went by, he admitted to having been caught up in the ultra-linear fad of his day, that it was the biggest blunder of his career, and he did it only once and would NEVER design an amp with an ultra-linear output stage ever again.
And he didn't. The Citation V was pure pentode, as was the subsequent Lafayette 550 and everything else he designed from then on. I asked how come. He explained that when the plate pulls the top of the output transformer winding towards ground, the ultra-linear tap pushes the screen grid so low that it renders the tube unable to drive difficult loads. In addition, he pointed out that the normal idle potential on the screen grid regularly exceeded a safe voltage, often causing output tubes to blow up. The tube manufacturers hated it, but had to go along or lose market share, and so changed the specification for screen voltage in order to allow ultra-linear output stages. I wonder if they changed the tube design, or just changed the screen voltage specification.
Back to Stu. Anyway, when I asked him about his favorite output tube, he said the mighty 6550 was the one to use, and when I queried about KT88's, he held up his little pinkie finger as if to hold a tea-cup and said in a mock British accent. " Brits you know, if you want watered down tea."
He loved to use wide-band video pentodes in his amp designs, and did so when he could. (Read that as cost-no-object.) They were expensive then. "Why would anyone want to use a triode front-end when they could use a pentode?" he mused. Stu was single handily responsible for one of the worlds great amplifier topologies, the wide-band video pentode design. Never been done before, and it added a new category of stunning topologies to our universe. We talked with our arms and hands as much as our mouths, and we both usually had pencil and paper in front of us to help. Not only that, but my patent attorney had already trained me to continuously write down stuff in my "inventor's notebook", so when Stu Hegeman drew a diagram on a piece of paper, I saved it, trimmed it with a pair of scissors and taped it into my inventor's notebook that night in my hotel room. And I still have it to this day. ( My attorneys make me save everything in those notebooks forever.)
Iâ€™ve been collecting vintage amplifier parts for a lifetime, always with the notion of building a small museum someday. But lately a better idea has come over me, and thatâ€™s to use them to build an amplifier. And here it is.
Successful cross pollination:
With a simple piece of inventor's notebook paper, Stu designed the wide-band video pentode front-end along with his 550 phase inverter, all the while leaving the rest of the circuit and the details of the design for me to solve over a vast gulf of intervening years. I designed the output stage with his favorite output tube, the mighty General Electric 6550, the main bias and power supplies, and as it turned out, sonic performance that cannot easily be believed.
Very hot output tubes:
Stu talked about the need to run the output tubes extremely hot to get the distortion low enough to satisfy his boss, Sidney Harmon. (The Citation division of Harmon Kardon later relented on this point when too many output tubes began failing.) Consequently I had to invent a DC restorer circuit using a 6AL5 / 5726 tube; it eliminated the troublesome need to idle each output tube at 50 watts ( ! ), yielding instead an idle power of about 12 watts per tube. And it lowers distortion way down while simultaneously eliminating all signal related DC operating point shifts, a real sore point for vacuum tube amplifier designers. It works by keeping the DC component on each output tube grid the exact correct value through the entire audio signal swing, allowing perfect performance up to and even beyond clipping.
If you're still awake, read on.
All vintage parts:
This amplifier uses all vintage parts except for paint, the chassis and some of the small circuit parts. Even the transformer covers, tube sockets, and transformers themselves are vintage. The front end tube is an Amperex wide-band video pentode, the tube following is a General Electric 12BH7 driver, and the DC restorer is a JAN 6AL5 / 5726. Output tubes are new vintage General Electric 6550's, double tested, then burned in for three days and tested again. The best of the best. All are NOS. The transformers are MASSIVE vintage iron; each mono-block amp weighs in at 42 pounds, 84 lbs in all. Power is an easy 180 watts rms with a power bandwidth from 23 Hz to 55kHz, frequency response 2Hz to 95 kHz, and distortion less than 0.15 % at 220 watts out. Even more onto six ohms.
It's been so over designed that I expect it will last at least 50 years without need for service. In fifeteen years the wide-band pentode should be replaced, whether it needs replacing or not, and the 12BH7 driver tube should be replaced after twenty years, again whether it needs it or not. Here is the best part: even the mighty 6550's should last 50 years unless they have an unforeseen catastrophic failure. No need to replace them unless they won't bias up.
The sound: This amplifier stands with a small handful of the world's great vacuum tube amplifiers.
And beyond: It possess huge energy storage that Stu could only dream of, has the Hegeman phase inverter configurerd by me to have built-in auto-balance, a DC restorer and twelve output tubes in all. Nothing can touch it.
The features: Four output terminals; one, two, four and eight ohms. A bias control. A switch that changes feedback from classical ( vintage ) to contemporary. A jack for the bias meter. It has built-in auto balance, a volume control and a power switch with in-rush current limiting. And finally, a chrome roll bar for the front end tubes.
This amp was lovingly built by Joey "Tubular Joe" Bonin, the best tube amplifier builder and craftsman of all time right here in the rain forest of the Northwest.
Hi, Tubular Joe here. Just wanted to let you to know a few aspects and features of these amplifiers. They are hand-wired, point-to-point with star grounds. Even the tagboards are Stewart Hegeman style, inspired by his teachings on the big Citations amplifiers. They are military-style epoxy with bright silver plated swaged turret terminals. Using them keeps internal wires short, neat and hum-free. The transformer covers are lined with 3/16" inch thick sound and vibration-deadening panels. Hope you like them. Enjoy!
Bob here. If one ever breaks, as long as I'm alive I'll fix it free if you get it to me. Except for the tubes.
One last thing. Stu Hegeman and I will autograph the the amp to you with your name in gold. Example: "To our friend John - enjoy the music! Bob Carver, Stu Hegeman". Except your name of course, not Johnâ€™s. If you wish. Since Hegeman is no longer with us, his name will be a facsimile in gold, whereas mine will be by hand. Itâ€™ll be hard to tell. Or, I can write â€œ In tribute to Stu Hegemanâ€¦..â€ You choose. I prefer the facsimile myself.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Interesting! A PP pair of 6550's in pure pentode would give a lot of power. Enough for most speakers, although I get why you electrostatic owners would want a quad.
I am sure I own a lot of wide-band video pentodes, since I was given a tube collection from a TV repair man, but since he talks about 12BH7 driver, I am not sure where this fits. Did he like the as a driver or an output or both? If I am remembering correctly one tube I Identified as a video amp tube was very large with a top cap, and more suitable as an output tube. Of course there are multistage video amplifiers.
He liked the pentode voltage amp stage because he didn't have to fight Miller capacitance as you do with a triode.That can devastate higher frequencies.The Deuce drives ESLs with ease.A lot of McShaned Deuces are paired with Martin Logans,Quads,and Magnepans.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
I looked up Miller capacitance. That makes sense. I used to sell Acoustat and Magnapan. I know how power hungry they can be.
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