Some tube facts

[Rameish 0]

Some of you hunt for bargain basement (in terms of price) but branded tubes. Here's some info from one of the most respectable of tube gurus. He is of course talking about RAM tubes culling process. Of course there's a bit of marketing involved but basically it's true. Have fun tube guys. The following words are Roger's obviously and I don't claim to understand all of it LOL.

 

Modjeski Reference Tubes

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.

 

 

Part II

 

Modjeski Reference Tubes

THE VIRTUES OF POWER TUBE MATCHING

By Roger A. Modjeski

 

My experience gained from testing thousands of EL-34's, 6550's, KT-88's, and other tubes has produced a wide bell curve of the two most important parameters, Bias and Transconductance. The center of this curve is the "bogey" value that the manufacturer is trying to hit, and the ends show the cutoff points of what he is willing to allow out of the factory. The tubes at the ends are still perfectly functional, but may not work well in all amplifiers, and will certainly not work well if mated with another from the opposite end of the curve.

 

Some amplifiers do not have enough range on the bias pots to handle the range of tubes so that selection-must be made from a particular part of the curve to ever achieve bias.

 

In the "good old days," I'm told, the spread of values was much less, allowing reasonable performance with random selection. Today, random selection is strongly not recommended-I'll tell you why.

 

Looking at bias voltage in a typical batch of EL-34's from the best producer, it will typically range from -32 to -42 volts for 50mA of plate current. At a typical transconductance for 7000 umhos (7 mA/volt) the 10 volt difference will cause a 70 mA bias difference. Obviously, one tube will be on and the other cut off if they are biased with the same adjustment pot. I match the bias voltage to. 0.5 volts or better, and thus have only a 3.5 mA (0.3 x 7 mA/v) difference in plate currents, which is only 4% maximum at 50 mA.

 

 

Now, those of you with individual bias adjustments are probably thinking that you've got it all handled because you can adjust for these different grid voltages. Well, you can-but in doing so, you will create a greater problem.

 

Say you put a -32 volt tube in one socket of a push-pull pair and a -42v in the other and bias them so that their currents are equal. Unless you can also adjust the a.c. balance of the driver to provide 32v of drive to one and 42v to the other, you will have very unsymmetrical clipping and reduced power output.

 

In most amplifiers with a.c. balance, the range of adjustment is nowhere near this, as that adjustment is meant to balance the driver and not current for widely unmatched output tubes.

 

In our modern, larger amplifiers, multiple pairs are used in push-pull parallel to achieve higher powers. It takes four of the popular output tubes to safely attain 100 watts, and here matching is even more critical.

To fully appreciate the problem, you must know a bit about the operation of a tube output stage. In push-pull parallel, we have four (or more) tubes connected so that two (in parallel) handle the positive half of the output signal, and the other two handle the other half. Even if you do find two tubes which are 5v off, there will be 35 mA different in bias current, which is still intolerable.

 

Ideally, they are all biased at the same negative grid voltage and have the same current through them. As we apply positive signal, the grid voltage goes up on the first pair of tubes, raising their current in a shared manner. For this to happen, they must have the same transconductance (Gm), which is the measure of how many mA/volt the plate current changes. Imagine if one has 7000 umhos (7mA/v) and its partner has 5000 umhos (5mA/v).

 

By the time they hit 0v (coming up from the -35v bias), one is conducting 175mA and the other is conducting 245 mA, which is neither fair, nor a good idea. Now imagine if these tubes didn't even start at the same bias, but were adjusted by individual pots to -32v and -42v as per the manufacturer's method of achieving equal bias currents. What then happens is that the -32v tube clamps the drive when it hits 32v (0 on the grid), and the -42v tube's grid never gets higher than -10v. Since most amplifiers require bringing the grid up to zero for full power, with unmatched tubes, you will never attain maximum performance.

 

Here's an analogy for the mechanically-minded. Imagine a VW-type engine where the cylinders are opposed and made separately, and the manufacturer puts together engines where the pistons, rods, and cylinders are not very uniform. In this engine there is only one arm on the crank (like the driver in our amplifier), and all the rods are connected to it so that they all have the same length of throw. One "pair" of pistons should top out together when the other pair hits bottom. If any of them has a short rod, it will achieve less compression and therefore less power. If he matched the rods and selected cylinders of the same bore, he would then get equal power per constraint of that one crank, as we have with our single (symmetrical) drive voltage. With unequal push rods, we're going to end up with very different compressions at the end of the stroke, and thus very different output power from each cylinder.

 

Fortunately, this nasty problem has a solution-simply match the tubes for Bias, and Gm, and use one bias adjustment. Then the amplifier will be working at its best, and the consumer will have the simplest possible set-up.

 

At RAM Tube Works, we match for Bias within 4% and Gm within 8%. We provide tubes in any size sets at no extra charge. We give you the full data for each tube so you may see the matching for yourself. We also reject tubes which have high grid leakage (gas), low power output, unreasonable bias or Gm, and other flaws.

Edited September 25, 2003 by Rameish

[Mackie 0]

Thanks Rameish, this is one for my notebook.

[Barradio 0]

WHOA can it put in the Sticky thread for references.

[Rameish 0]

Now you know why I say tube are a PITA LOL. I had some RAM EL34 tube way back when, and I must say they all aged evenly. So evenly that biasing was a breeze. Of course if you're using an amp with cathode bias (aka auto bias) then there's not much you can do to alter the sound other than tuberolling.

 

Take a 12AX7 tube for example (also true for other types). Both sections within the one single tube needs to be matched. So to get a matched pair you would need a second tube with similar readings and matched quad.... But it doesn't end there. The tube needs to have similar readings generally after burn-in at max plate volt for at least 24 hrs. These were done to all Mullard tubes at the factory in the past - hence you will normally see a sorta burn mark on the top (where the mercury coating usually is).

 

Now we then come to the BIG question. If all this matching requires hundreads to thousands of tube of one particular manufacture date, then, the super rare tubes like telefunkens,BB (Holland), can't be matched can they? Or only a basic matching is done.

 

If you have an amp like the Cayin which only uses one 12AU7 and one 12AX7, that becomes less of a headache. However most tube amps require at least matched pairs or matched quads.

 

As for the 12AX7 - Svetlana makes a nice mullard copy (non-military box plate type). The Box-Plate type aka CV4004 is warmer than the non-box plate type.

 

Having said all that, I must add that every once in a while I feel the urge to go toobs again. But then I remember my madman days where I started to hoard NOS tubes and was very picky about everything being matched to the most stringent standards. Ten to double check, I sent them in to Zenn Audio and paid an additioanl fee to double check that the matching was not bullsh*t. I caught some vendors that way. Zenn has one of the best tube testers around. I think he charges a setup fee of S$20 for each tube and a test fee of S$5 per tube. SO it's only worthwhile if you have a stash.

 

 

[N@Z 0]

Posted the link to this thread in the SGheadphone articles, reviews, articles, etc by our very own! sticky thread.