I'm curious what "the Carver mod" was all about.
BillWojo
This is direct from Bob C's original listing:
HOT-RODDED CITATION II POWER AMPLIFIER WITH 115 WATTS PER CHANNEL
Hot-Rod
Usually the expression “hot-rodded” sounds somewhat flip. However in this case, it evidences a remarkable new output stage combining Stu Hegeman's classic design and Mike Matthews’ new Tung-Sol KT120 output tube. This amplifier will come to you with Tung-Sol KT120's and 115 watts per channel, OR with Silver Eagle KT88's and 60 watts per channel. At the close of the auction, the choice will be yours.
Remarkable New Tube
Several months ago the new Tung-Sol KT120 vacuum tubes became available, and I just had to try them! I found them to be remarkable......no, great tubes with distortion, bandwidth, and fidelity as good as anything I have ever experienced. In order to obtain the best from these remarkable new tubes it was necessary to modify the turns ratio of the output transformer by a factor of 1.414. This lowered the impedance and optimally matched the new tubes, and I didn’t even have to alter the transformer design or change the wires inside; all I had to do was connect the external wires up in a new way such that the effective driving point impedance was optimized for these tubes.
Success
I was rewarded with a new and powerful amplifier that sounded as sweet and as expansive as it did prior to the hot-rodding, yet it easily outperformed the original amplifier.
A Hot Mid-Summer’s Night
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 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. 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 preamps and amplifiers, 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 the 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 why. 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 can’t help but wonder if they REALLY changed the tube design, or simply changed the screen voltage specification.
The Mighty 6550
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.
Back to Pure Pentode
I’ve updated and modified this amplifier to represent an expression of Stu Hegeman’s latest thinking regarding power amplifier designs; it’s a way of thinking about amplifier designs that he was unable to implement for a variety of technical and political reasons. These modifications include a return to a pure pentode output stage, allowing substantially more output drive current than possible with, in his own words, the devil-begotten ultra-linear output stage. Strong words, but they were Stu’s. I don’t think Sidney Harman would have been pleased if he heard Stu saying that.
Greater Energy Storage
When Stu designed this amplifier, huge amounts of energy storage were not practical. He did the best he could with the available energy storage electrolytic capacitors of his day. In the vast intervening gulf of 50 years, the technology associated with capacitor energy storage has allowed an increase of approximately ten fold. This amplifier has had its energy storage increased substantially by the addition of HUGE capacitors that Stu could only dream about. This eliminates every last vestige of “DC bounce” on musical transients. DC bounce and low-frequency stability has always been the result of compromises made by Stu in order to design an amplifier for the real world, that could be BUILT in the real world. Not any more.
Very Hot Output Tubes and The DC Restorer
Stu talked about the need to run the output tubes extremely hot to get the distortion low enough to satisfy his boss, Sidney Harman. (The Citation division of Harman Kardon later relented on this point when too many output tubes began failing.) Consequently I had to invent a DC restorer circuit; it eliminated the troublesome need to idle each output tube at 50 watts ( ! ). This circuit allows the output stage to idle at approximately 12 watts per tube and retains the low distortion that Sidney wanted for his amplifiers. 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.
Super Long-Lasting Output Tubes
The DC restorer allows simultaneous low distortion and low idle power, allowing extreme longevity for the output tubes.
The final distortion level in this amplifier is so low that I am embarrassed to write it down here.
This amplifier will arrive with vintage GE 6550’s or KT88's, your choice. There is also a one year warranty, standard. If it ever breaks and you get it to me in the first year, I’ll fix it free of charge, and additionally I will give you a great deal if it EVER needs fixing.
Features
Four output terminals; common ground, four, eight and sixteen ohms. A bias meter, bias control and a balance control.
One more thing: I accept ANY form of rational payment.
The man with the big muscles is me, holding another amp inspired by Stu Hegeman's Citation-II
Bob Carver
MA
