Why an amplifier's sound changes when you change tubes


BY ROGER MODJESKI

Over the years, it has been noted by many astute listeners that changing tubes often produces a marked change in the sound of their equipment. While these changes are often attributed to the tubes alone, it is almost always a case of tube-and-unit interaction. Therefore, it is incorrect to say that a certain tube sounds a certain way. In this article we shall look at the ways in which tubes affect the equipment they go into.

First, let us look at some of the problems involved in making tubes. Given a batch of tubes from a maker (factory) there will be a spread of parameters. Each maker's noise, microphonics, gain and operating point will fall into a bell curve due to the nature of the machines that make the tubes, the materials for the batch, and control over processes. A bell curve of gain is a good example. (See above)

In an ideal world, all the 6DJ8 makers would adjust their grid lathes to get the same target value of 30. They actually try, but they have a specification of l or 2 or 3 depending on how important this parameter is to them. It is very hard to wind that little 6DJ8 frame grid to get Mu of 30 1. I encourage you to break open an old 6DJ8, pull off the top mica, cut the lower supports and carefully pull out the two posts (about 1/8" apart) on either side of the hollow cathode sleeve in the center. Look at the fineness of the grid wire wound on the posts and the spacing of those hundreds of turns. The voltage gain (Mu) of 30, which we want to control to 3% is the ratio of the diameter of that invisibly fine wire to the spacing between turns. It is amazing they can do it at all! Tear apart a 12AX7 any you will see how much easier it is to wind that grid.

Controlling gain by Mu alone is not sufficient, as MU is the product of transconductance (gm) and plate resistance (Rp). Although most triode circuits are designed to let Mu predominate, sometimes gm is more in control of circuit operation. This is typically the case in cascode circuits. Here we have the opportunity for a larger variation because two tubes with the same Mu may not have the same gm as Rp differs.

Now add in the variation in operating point (bias) and it is easy to see why different maker's tubes sound different-even to the point of not functioning in some overly sensitive circuits. An extreme case of this is the 12AX7 in the Moscode Minuette. George Kay and I found that while the Tugsram 12AX7 worked well in the front end of his unit, the smooth plate Telefunken did not. The latter tube caused the gain to fall 10-20 dB and the distortion to increase 10-100 times. Any user discovering this in his home would certainly hear startling differences between the Tungsram and Telefunken 12AX7. But the main thing he would be hearing is the difference between a functioning preamp and a malfunctioning one, though it still played.

A less dramatic situation is found in feedback-type RIAA phono circuits; these include ARC, Modulus, and others. The problem stems from the fact that there is not enough loop gain for the feedback to provide consistent equalization at low frequencies. This means that the shape of the low end, and low-to-high tonal balance will vary with the Mu of the tubes. I remember magazines of the late '70's encouraging manufacturers to get this RIAA EQ accurate to some 10 millibel. It is impossible to get .l dB when tube variations cause 2 dB errors. I believe this RIAA EQ shift to be the major effect one hears when changing phono tubes in active RIAA circuits. Fortunately, passive EQ circuits are free from this effect, and therefore RIAA consistent over tube life.

Moving on to noise, there are four areas to look into. AC hum is rarely a problem as most designers use DC regulated filament supplies. But for those with some ripple, the filament construction becomes a factor. The 6DJ8 and 12AX7 are made with folded or coiled filaments depending on the maker-it is not part of the specification. A coiled filament will cancel hum whereas a folded one will not.

Noise, however, is the big battle. All tubes have thermal noise and in a perfectly made one there is only the thermal noise, which cannot be improved upon. By the way, a 6DJ8 has a signal-to-noise ratio (s/n) that is 8 dB better than a 12AX7-no wonder it is the tube of choice for phono inputs. In addition to this thermal noise, there is excess noise which can be large-this is the noise we grade for. A RAM "A" grade has only 3dB excess noise.

After listening to and measuring noise over the years, I determined that two tubes could measure the same total noise but one would be more annoying than another. I designed a device to quantify the annoyance factor in the noise; that became the RAM factor.

The last area of noise performance is susceptibility to microphonics-we all know the sound emitted by tapping on a tube or the chassis. The sound is caused by the relative motion of the grid and cathode. Many tubes use all sorts of fancy mica insulators to hold things steady, but large variations occur from tube to tube and between makers. Low microphonics is important to imaging and detail in that a microphonic tube will receive sound from the speakers and put it back in the chain in a very non-musical way.

I am often asked if X tube is bright or Y tube is bloated, etc. This paper is the answer to that question. I cannot ascribe a particular sonic quality to a particular tube because tubes in themselves do not have a sound. This information was gathered over my last 25 years of working with tubes and tube circuits. It is a combination of many books, experiments and visits to tube makers.

Roger A. Modjeski

Music Reference / RAM Tubeworks



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