In this month’s Roger’s Corner, we are continuing the discussion on tubes and demystifying some concepts about tube matching. Some 30 years ago I purchased my first tube component and trusted my dealer to educate me on tubes but later learned he was not that knowledgeable on the subject. Over the years, as I purchased more tube components, I found myself making the mistake of trying to seek advice on tubes from the “experts” on the audio forums. So down the rabbit hole I went spending on this tube and that, new production, NOS, and on and on as many audiophiles still do today in the quest to find the right “sound.” If you read last month’s Roger’s Corner, you might now know a bit more about tubes and sound.
After buying a Music Reference RM-9 amplifier, without thinking I retubed it with a “matched” octet of NOS Siemens EL-34s. Not inexpensive to say the least. I was living in Santa Barbara at the time and found out that Roger Modjeski lived up the hill about 3 miles from me. I took him up on an offer to test the amplifier and upon seeing the Siemens tubes asked where I purchased them and at what cost. After telling him Roger wryly mentioned the vendor was a RAM Tubes reseller and the price was marked up 30%. Upon retesting the tubes, Roger found they were matched, but not as an octet, nor even quads, just pairs and not closely matched at that. Indeed, I felt dumb at that point, but this was the moment I started my tube education with Roger. Many years later while working for him I mastered the RAM Tubes testers and Bell curve.
The number of tube vendors has proliferated over the years. The continued audiophile fixation on NOS tubes has continued to drive up prices as stock dwindles. I could recall a time when no one would touch an RCA 6SN7, but when the “premium” brands started drying up they became one of the hottest NOS 6SN7s on the market. While on the topic, let me tell you something about NOS tubes. Vendors grading them as low or extremely low noise (one huckleberry comes to mind) are propagating a myth. The truly quiet and low microphonic NOS and military spec tubes were cherry picked long ago. Today’s NOS tubes do not compare yet audiophiles feel compelled to put them in circuits such as preamplifier gain stages, as well as sensitive phono sections where low noise and microphonic tubes are required.
Rising tube costs are also the result of the dwindling number of tube manufacturers. The St. Petersburg factory produced very good tubes under the Svetlana and Sovtek brand. After its destruction production reappeared later under different ownership where pricing was higher and quality inconsistent. Roger knew because he bought tubes from both factories. The demise of the Shuguang factory is another example, and while others like Linlai have emerged the focus now appears to be on manufacturing “high fidelity” tubes with high pricing to match. Then there are the boutique tube manufacturers with other worldly pricing and tubes with specifications not easily verified. When spending good money on a component, you should not expect to have to pay a lot to retube it, or if so, you should expect equivalent quality in return.
Mu, GM, & Rp & How Tubes Are Matched
By Roger A. Modjeski
Voltage gain (Mu), transconductance (Gm) and plate resistance (Rp) are the three electrical characteristics of a vacuum tube. There is confusion among audiophiles about how these characteristics affect tubes and their performance in components. In this discussion we will look at these characteristics, how each is measured, and how each interacts within a circuit. We will also discuss tube testing, tube matching, and which characteristics are most important when matching tubes for a given circuit. Also, how these measurements are applied differs for triodes and pentodes. Both have Gm and Mu, but Mu is the important characteristic in triodes while Gm is most important in pentodes. As such, these parameters are the dominant characteristics affecting performance when each is used in a typical circuit.
Triodes
With triodes, as in Ohm’s Law, two parameters relate to make a third. The parameters are locked into a relationship so that knowing any two will yield the third. In this case Mu = Gm x Rp where Gm is units of amps per volt (mho) and Rp is units of volts per amp (ohms). When multiplied their units cancel, making Mu unitless as it should be. A Mu of 30 means that what goes in comes out 30 times bigger. Triodes are widely used for voltage amplification in preamplifiers which is why Mu is the more important measurement affecting what you hear. If you have a tube where Mu = 30 in one channel and 33 in the other, you will have a 1 dB channel imbalance. So, if two triodes are matched for Gm, there is no assurance these tubes will have equal voltage gain. Since Mu is Gm x Rp, if Gm is matched to Mu, Rp must also be matched.
However, Rp is rarely measured and while the famous Hickock tube testers are sought for their ability to measure Gm, this was more of a marketing ploy and less useful for real-world testing. If the tube vendor measuring Gm would give you the Rp then you could calculate the Mu by simple multiplication. The reason you do not get the Rp data is that it is hard to measure, and no commercial tube tester ever measured it. Audiophiles want everything to be matched, though they rarely know what it should be matched for, and are buying tubes just because the vendor says they are matched. However, they may or may not be matched for something unimportant while something important is not matched. Gm and Mu are a perfect example.
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