A BRAND NEW SILVER SEVEN VACUUM TUBE AMPLIFIER DELIVERING NINE HUNDRED WATTS PER CHANNEL AND CLEARLY THE MOST ADVANCED OVER-THE-TOP AMPLIFIER I HAVE EVER BUILT.    BOB CARVER

 

 

Tubular Joe and I finished a brand new amplifier and here it is.

 

HOW IT ALL STARTED 

 

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.

 

 I found him in his booth; 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. This conversation led to me confessing that I had always wanted to build a big tube amp that was painted the same color as my first car, a '49 metallic burgundy Mercury automobile.  So I started on it. And here it is!

 

A BREND NEW POWER AMPLIFIER, THE SILVER SEVEN NINE HUNDRED

 

Several months ago the new Tung-Sol KT120  and new KT 100 mk II vacuum tubes became available, and I just had to try them! I found them to be utterly remarkable, with 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 design a new output stage , and what better one to start with than my own Silver Seven vacuum tube amplifier?

Success !

I was rewarded with a new and powerful amplifier that sounded as sweet and as expansive as my original Silver Seven, perhaps even better, yet easily outperformed it by delivering substantially more power and with less distortion (not that it needed it) than the original.

 

The Mighty 6550 vs the KT120 and the KT100 mk II

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 the "kinkless" tetrodes, (the KT tubes), 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.

This design uses Stu's front end approach using a 12BY7. For this new Silver Seven I've used the more powerful, lower distortion version, the 12GN7 and I’ve updated and modified this amplifier to represent an expression of my  latest thinking regarding power amplifier design.  These modifications include a return to a pure pentode output stage, allowing substantially more output drive current than possible with, in (Stu Hegeman's) own words, the devil-begotten ultra-linear output stage. Strong words, but as we shall see, Stu was right. This amp has a DC filament supply, unheard of in power amps, but a necessary feature when using high-bandwidth pentodes in the front-end. This is the lowest noise power amp ever, with an A-weighted signal to noise ratio of minus 120 dB! Power output is hard to believe, and the sound is even more difficult to believe.

                                            

THE DESIGN OF THE NEW SILVER SEVEN

I started the design of this new amplifier by changing the color in order to protect the innocent. I changed the color from black to a beautiful strawberry burgundy red with chrome and silver highlights, just like my car.  This new Silver Seven has every known de-lux circuit embodiment known to man, woman, or minor gods. As mentioned earlier, I used a 12GN7 vacuum tube for the front-end. I followed that high speed tube with several class-A drivers for the output tube grids. A cascode voltage amp is used for the gain stage that runs the dual regulator tubes, a pair of power output tubes. In the old days, huge amounts of energy storage were not practical. But today, in the vast intervening gulf of years, the technology associated with capacitor energy storage has allowed an increase of approximately ten fold in power supply capacity. This amplifier has had its energy storage increased substantially by the addition of HUGE capacitors that Stu could only dream about.  This, combined with the additional  DC restorer circuit eliminates every last vestige of “DC bounce” on musical transients. DC bounce and low-frequency stability have always been the result of compromises made by all amplifier designers in order to design an amplifier for the real world, that could be BUILT in the real world. Not any more, thanks to the DC restorer, HUGE capacitors, and my new output transformer which I wound my very own self. Not only that, the DC restorer 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.

 The DC Restorer and 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, and the output tubes should last 50 years unless they have a catastrophic failure or won't bias up. This amp has a lifetime guarantee. It's my life and as long as I'm alive I will fix it free. I'll replace the output tubes free if they should become defective, even if you drop the amp on them.  It's the least I can do.  I know they will not become weak in our lifetimes.

 

Features

Four output terminals; common ground, one, two, four, eight and sixteen ohms. A bias meter and a bias control. It has a volume control and a built-in tube tester for the output tubes.  It comes in four chassis; two power supplies and two amplifiers. Built by me and Tubular Joe. Shipping weight is about 325 lbs for all four chassis plus the tubes.

One more thing: I accept ANY form of rational payment, and the pictures are of me, Bob Carver.
Bob Carver