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Modjeski Reference Tubes

By Roger A. Modjeski

You wouldn't choose instant coffee to make cappuccino, would you? Nor would you choose diet margarine to make a pound cake. At least I hope you wouldn't if you had invited me to dinner. But that has been the predicament of the owners of fine tube audio equipment when they needed replacement tubes.

Most of the tubes available, even from the best manufacturers, are incapable of delivering the sumptuous qualities and musical detail that distinguish tube equipment at its best.

Through personally testing tubes, I found that measuring the noise voltage alone was insufficient for determining the acceptability of tubes. Taking two tubes that measured the same noise voltage I could still distinguish a difference, I defined this quality of noise as RAM factor, and designed a circuit to measure it. For the first time it was possible to objectively isolate and measure one of the key variables that accounted for sonic differences and variation heretofore unexplained.

Even after the discovery of the RAM factor I still had a sizable logistics problem. Testing thousands of tubes to my exacting standards, including repeated measurements at different times, at different temperatures, in addition to "burn in" time, was taking many days to produce the small percentage of tubes which qualified for listening to music.

Solving that problem took another year. I had to develop my own computer-controlled testing system, This system is run by a specially modified Apple II+ computer which tests 128 tubes simultaneously. The computer takes 200 different readings from each tube, averages the readings and records them.
After the tubes are tested, the computer prints a label for each one with all of the test results for that particular tube. Furthermore, the grading of the tubes is done by sections, unless the tube is destined for a head amp, where it must rate a perfect "A" for parallel operation. Typically, this "A" grade is the best tube in 10 tested. The "SA" grade is the best one out of 100 tested.

The double letter indicates the separate ratings of sections 1 and 2 in the tube. When we grade a tube, we grade it by the lower rating of the two sections. Our letter grade designations represent the guaranteed minimum performance. Even our tubes with ratings such as "BB" and "CC" are good choices for use in high level stages and cathode followers and are distinctly better than random untested tubes.

RAM TUBE WORKS also provides computer-tested and matched tubes for power amplifiers. Front end tubes are normally dual triodes and are similar to pre-amp tubes. Driver tubes are graded for tight gain matching and operating bias so that they enhance performance in push-pull balanced circuits. Output tubes are computer matched for bias current and transconductance (Gain). This is essential for amplifiers (Dynas and others) with no AC (Gain) or DC (Bias) balance controls. You will be amazed by the sound of your amplifier equipped with computer-balanced, matched tubes.

Each tube is individually serial-numbered on the box and on the glass. Tubes are warranteed for 90 days to meet specifications. Any warranty claims MUST include the original box upon return.