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The Inner Workings of Vacuum Tube Buffers

This article takes the lid off vacuum tube buffer circuitry to reveal its hidden, inner workings, and explores why some musicians prefer the tone of tube buffers to their solid-state, transistorised op-amp (operational amplifier) counterparts.

Just what is a ‘Buffer’?

Cathode follower circuit — Figure 1. Vacuum tube buffer circuit, a.k.a. the 'Cathode Follower'.

Buffer circuitry is found in all kinds of electronic audio hardware from budget transistorised guitar stompboxes to high-end vintage tube studio compressors. Many Effectrode effect pedals are built on a simple tube buffer shown in fig. 1. This circuit originally appeared in the RCA Receiving Tube Manual back in the 1940s—the same text Leo Fender referred to when designing his first guitar amplifiers. The men in white coats refer to this circuit as a cathode follower stage but it’s also commonly known as a ‘buffer’. This is because the circuit has high input impedance and low output impedance meaning it can effectively isolate or buffer one part of an audio circuit from another to prevent loading. A practical application of this is the use of a buffer to isolate a high impedance signal source, such as guitar pickup from a long length of cable to prevent high frequency loss.

Why Tubes?

Next to an op-amp a tube is an expensive, fragile, bulky and power hungry device that takes a while to warm-up and gets pretty hot during operation. Yet despite these technical shortcomings, there are a few crackpot, fringe audio designers who are prepared to go to great lengths to understand and work with this archaic and virtually obsolete technology. Their reasons? Better tone of course—what else! The description cited earlier is just the bog-standard textbook definition of what a buffer is, but as with most things, there’s devil in the details: a tube buffer circuit does a lot more to an electrical signal than simply just buffering it. Their subtle inner workings also affect the tone of musical instruments in quite profound ways that can work to the advantage of guitarists, musicians and recording engineers [see ‘The Cool Sound of Tubes’ article written by Eric Barbour]. Tube buffers can fatten-up weak, thin solo guitar parts, add sparkle or life to chord work, impart additional depth and richness to ‘flat’, one-dimensional sounds, add ‘shine’ to a guitar track or help it sit better in the mix and even bring a lack-lustre, solid-state transistor guitar amp to life. Let’s take a closer look at the construction, materials and physics of this tube buffer and see what it reveals about this vintage circuit’s nature as a tone enhancer.

Tubes and Opamps Look Different

Although vacuum tubes and op-amps both perform the task of electronic signal amplification, they’re physically very different devices: So different in fact, that tubes look like they came from Mars, or some other distant, alien world. A vacuum tube is constructed from metal electrodes and wires made up of several different metals, including nickel, tungsten, molybdenum, copper, strontium and barium oxides supported within a glass tube using insulating mica washers. Although much of their assembly was automated using machines some of it could only be completed by hand and this coupled with some of the rare and exotic materials utilised in their construction makes them considerably more expensive to manufacture than an op-amp.

tube_grid_and_cathode_320px — Figure 2. Metal sculpture – close-up of grid and cathode in a 6J5 triode tube. Photograph taken by Thomas Mayer.

A tube is also a relatively large device and requires larger quantities of raw materials, further elevating fabrication costs. For comparison an op-amp contains a tiny integrated circuit (IC) inside a plastic housing. The IC itself is a tiny silicon wafer (or die) and the circuit is created using hi-tech etching and deposition processes to create transistors, capacitors, resistors and connections on its surface. Fig 3. below shows a 741 op-amp with the outer casing removed. The photo is greatly magnified and the die containing all the circuitry is only 1.3 mm square. The transistors are made of silicon (a semiconductor) containing impurities of boron or phosphorus, the capacitors from insulation materials and the internal connections from aluminium.

Tubes and Opamps Operate on Different Principles

741_opamp_die2_320px — Figure 3. Enter the labyrinth – the silicon guts of a 741 operational amplifier. Photograph taken by Gary Lecomte.

The fundamental physics of how tubes and transistors operate is also completely different. In a vacuum tube electrons are emitted from a heated ‘cathode’ (the electrode that glows orange inside the tube) and are attracted to another electrode, the ‘anode’ which is held at a high voltage of several hundred volts. Signal amplification is achieved by control of the electron stream (a current) by varying a voltage on an electrode called the ‘grid’. The electrons travel at high velocity in a vacuum, whereas in a transistor they make their way more slowly through a crystalline silicon lattice. Now, please let me apologise in advance for the next paragraph: things get a little technical because the principle of operation of a transistor is not as straightforward as the simple old triode tube.

Here we go… In an op-amp the tiny transistors are made of a sandwich of silicon with boron and phosphorus impurities deliberately introduced by a ‘doping’ process. Doping silicon with phosphorus introduces extra electrons into the crystal lattice, whereas doping with boron causes a lack of electrons. The outer phosphorus-doped layers of the silicon lattice are called the ‘collector’ (analogous to the anode in a tube) and ‘emitter’ (analogous to the cathode) and the thin inner boron-doped layer is the ‘base’. The base controls the flow of electrons from the emitter to the collector which is connected to the power supply rail which is held at a voltage of several volts. Small currents on the base affect the conductive properties at the boundary between boron-doped and phosphorus-doped silicon. The physics of transistors is much more abstract than tubes, however that’s essentially how they operate.

Tube Circuit Minimalism

The circuit topologies of tube and op-amp buffer circuits also differ. Again, this is partly down to the differences their physics, but also because of many years of development in the art of electronics and circuit design in the quest for the ideal amplifier. Tube circuits, being more primitive and less sophisticated, invariably utilise fewer components and are more straight forward in their operation. The component count in the RCA buffer circuit is minimal with just a handful of resistors, capacitors (passive components) and the vacuum tube (the active component).

741_op-amp_schematic_320px — Figure 4. Schematic of a '741' op-amp buffer. Simple on the outside… busy on the inside.

Contrast with the op-amp buffer circuit in fig 4. Although its external physical appearance appears simple, it actually contains dozens of tiny transistors, capacitors and resistors. An op-amp is intrinsically more complex than a tube, in fact it contains more components than a Fender ‘Tweed’ guitar amp! Fig. 3 shows an excellent close-up view of the guts of a 741 op-amp—a labyrinth of current sources, current mirrors, differential and push-pull amplifiers, temperature compensation and current limiting circuitry etched onto a tiny silicon wafer. All this circuitry is in the pursuit of technical perfection, the dream of an amplifier with wide, flat frequency response, vanishingly low distortion and low output impedance, and the op-amp, to all intents and purposes realises this. Near perfect specifications realised through circuitry that harnesses ‘negative feedback’, an invention first conceived by Harold Black whilst working at Bell Labs during the 1920s and early 30s—more on negative feedback later.

Before closing this section I have to say, as an audio design engineer, I get a real buzz out of working with the idiosyncrasies of vacuum tubes; there’s something about their simplicity and their limitations that appeals to me. For example, a 12AX7 contains only two discrete amplifier sections, which means that it’s only practical to utilise a few tube stages in a given design, otherwise the circuitry becomes unwieldy and expensive. Op-amps are much smaller and much, much cheaper meaning a designer can throw as many of these at their circuit design as they like with far less concern for costs or complexity. You have to be a minimalist to work with tubes, and designing circuits with them is as much an art as it is a science. Their simplicity and limitations focus the engineer’s mind on the challenge of attempting do as much as they can with very little. And, simple systems have simple shortcomings, and the shortcomings of tubes can sound mightily impressive.

Tubes are Electrifying!

High Voltage warning sign — Vacuum tubes operate at high voltages, sometimes as much as 500 volts!

Op-amps operate at much lower voltages than tubes. In solid-state gear, such as an effects pedal, the op-amp power supply rail is typically derived from a 9 volt battery. Some pedals operate at slightly higher voltages of 12VDC or 18VDC and professional studio equipment usually has positive and negative supply rails of +/-15VDC, allowing almost 30V total rail-to-rail signal swing. This gives greater headroom before onset of signal ‘clipping’, however is still low when compared with vacuum tube circuitry operating from a 300VDC or 400VDC rail.

300V is a huge potential difference and gives the tube cathode follower a wide linear region of operation and excellent dynamic range. Not only that, if it is driven into the non-linear region then ‘soft’ clipping occurs, generating a spectrum of harmonic components that sounds subjectively more pleasant than the abrupt ‘hard’ clipping in op-amps.

Now, although tube buffers operate at high voltage, the currents flowing within them are lower than their solid-state counterparts. Looking more closely at the op-amp buffer circuit in fig 4. it can be seen there are 25Ω and 50Ω resistors in the push-pull output stage. This is a much lower output resistance than that of a tube buffer—where the cathode resistor is measured in tens of kilo ohms—indicating that higher currents are flowing. Current density—that is the number of electrons flowing—is several orders of magnitudes higher in a solid-state buffer. Further, at higher frequencies proportionally more of these currents flow on the surface (the “skin effect“) of the conductor due to its inductance. This means electron density is not uniform, being greater on the surface than in the centre of the conductor.

On a philosophical note, it’s interesting to ponder how these physical phenomena might ultimately affect the tone of audio equipment. Most experienced R.F. engineers will categorically tell you that such things have no significance at audio frequencies, but I’m not sure they’re entirely right about that. Firstly, because the increased heavy traffic in a solid-state device must lead to more collisions between electrons and the metal, or silicon, atoms in the crystal lattice of the conductor. And secondly, the much lower operating voltages must result in significantly lower electron mobility than in a tube circuit. This is physics on scales smaller than an atom—a bizarre quantum world at the limits of our understanding—but this physics may explain why so many believe vacuum tube and solid-state amplification, and effectification (including buffers) do not sound the same.

Nicer Negative Feedback

It’s worth highlighting how negative feedback is used in tube and op-amp buffer circuits. Yes, that’s different too. In the op-amp buffer feedback is externally applied from the output back to its inverting input with the well understood advantages of lowering output impedance and lowering harmonic distortion. This negative feedback control loop is non-localised, that is, it’s applied across the transistor stages within the guts of the op-amp. The effect of such non-localised (or global) feedback on tone is the subject of heated debate amongst audio engineers and I must confess I’m not an enthusiast for it. I prefer the feedback arrangement in the cathode follower, which is beautiful in its simplicity. Here the negative feedback is intrinsic to the tube and much less contrived—the principle of operation is elegance itself. If the cathode is made more positive, then the grid becomes relatively more negative and less current flows, decreasing the output voltage. Conversely, if the cathode is made less positive, the grid becomes relatively more positive and more current flows causing an increase the output voltage. The feedback control in the tube buffer is down to the fundamental physics of vacuum tubes, whereas in the op-amp buffer the feedback mechanism is imposed by design.

mechanical_negative_feedback — Figure 5. This mechanical negative feedback system gives a nice visual representation of how feedback works.

In Conclusion

It’s been the aim of this article to highlight the physical and operational differences between tube and op-amp buffer circuits. It’s common engineering practice to evaluate audio circuitry solely using easily measurable parameters such as frequency response, harmonic distortion, etc. On paper op-amp parameters can look very impressive; much better than tube specs. But what is ‘better’? Behind the specs there’s real physics happening. In tubes high voltages move hot electrons through empty space, whereas op-amps are a maze of cool crystalline circuits working at low voltage. Graphs and specifications don’t tell us that. Further, they can blind us to what’s really important: Does it sound good? For an engineer technical specs are useful tools in circuit design, but they’re only guidelines, roads on a map to a destination. And that destination? Well, it’s great tone; for a musician it’s always comes down to tone, and the ‘feel’ of their gear and, does it inspire to create. That’s what really counts, and this is where tube buffers score, bigtime.

Special thanks to Thomas Mayer for kindly granting permission to use his beautifully detailed close-up picture of the 6J5 tube—I’ve not seen anyone else achieve such detailed and well illuminated close-up photographs of tubes before. If you’re interested in seeing more examples of his superb work then you might want to check out his blog VinylSavor. Additionally, I would like to thank Gary Lecomte of Chemlec Consulting and Design for his superb close-up of the inside of a 741 opamp. You can find out how he managed to take this amazing picture on his website.

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The Inner Workings of Vacuum Tube Buffers