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Vacuum Tubes and Transistors Compared

Transistors vs. Tubes – Brief Feature Comparison, adapted from IEEE & Eric Barbour’s 1998 “Cool Sound of Tubes” article.

Vacuum Tubes: Advantages

Superior sound quality.
Highly linear without negative feedback, especially small-signal types.
Smooth clipping is widely considered more musical than transistors.
Tolerant of large overloads and voltage spikes.
Characteristics highly independent of temperature, greatly simplifying biasing.
Wider dynamic range than transistors circuits, due to higher operating voltages and overload tolerance.
Device capacitances vary only slightly with signal voltages (Miller effect).
Capacitive coupling can be done with small, high-quality film capacitors, due to inherently high-impedances of tube ciruits.
Circuit designs tend to be simpler than transistorized equivalents, which are greatly complicated by the need to linearize intrinsically non-linear transistors.
Operation is usually in Class A or Class AB, minimizing crossover notch distortion.
Output transformer in power amp protects speaker from DC voltage due to malfunction and protects tubes from shorts and blunts back-emf spikes from speaker.
Tubes can be relatively easily replaced by user.

Vacuum Tubes: Disadvantages

Bulky, hence less suitable for portable products.
Higher operating voltages generally required.
High power consumption; needs heater supply that generates waste heat and yields lower efficiency, notably for small-signal circuits.
Glass tubes are fragile, compared to metal transistors.
Sometimes more prone to microphonics than transistors, depending upon circuit and device.
Cathode electron-emitting materials are used up in operation.
High-impedance devices that need impedance matching transformer for low-impedance loads, like speakers; however, the magnetic cushion provided by an output transformer prevents the output tubes from blowing up.
Sometimes higher cost than equivalently powered transistors.

Transistors: Advantages

Usually lower cost and smaller than tubes, especially in small-signal circuits.
Can be combined in the millions on one cheap die to make an integrated circuit, whereas tubes are limited to at most three functional units per glass bulb.
Lower power consumption, less waste heat, and high efficiency than equivalent tubes, especially in small-signal circuits.
Can operate on lower-voltage supplies for greater safety, lower costs, tighter clearances.
Matching transformers not required for low-impedance loads.
Usually more physical ruggedness than tubes (depends upon construction).

Transistors: Disadvantages

Tendency toward higher distortion than equivalent tubed circuits.
Complex circuits and considerable negative feedback required for low distortion.
Sharp clipping, in a manner widely considered non-musical, due to considerable negative feedback commonly used. Does not gracefully roll-off or gently compress; instead, cuts off sharply, suddenly and abruptly with extremely hard edge.
Device capacitances tend to vary wildly with applied voltages (Miller effect).
Large unit-to-unit manufacturing tolerances and unreliable variations in key parameters, such as gain and threshold voltage.
Stored-charge effects add signal delay, which complicates high-frequency and feedback design.
Device parameters vary considerably with temperature, complicating biasing and increasing likelihood of thermal runaway, hotspots and unreproducible behavior.
Cooling is less efficient than with tubes, because lower operating temperature is required for reliability. Tubes prefer hot; transistors do not. Massive, expensive and unwieldy heat sinks are always required for power transistors, yet they are not always effective (power output transistors still blow up; whereas, tubes fade down gracefully over time with warning and usually without catatrophic results).
Power MOSFETs have high input capacitances that vary with voltage, complicating driver circuitry.
Class B totem-pole circuits are common, which cause severe crossover distortion, or else necessitate huge amounts of negative feedback to correct. This “measures well” for steady-state signals, but it completely “sucks the life out of” dynamic and transient signals such as music.
Less tolerant of overloads and voltage spikes than tubes. Except for their robust and forgiving heater filaments, it is very difficult, bordering on impossible, to blow out a tube with overvoltage; whereas, most transistors can be destroyed with as little as six volts, and every transistor can be destroyed by some voltage. Tubes are much harder to “zap.”
Nearly all transistor power amps have directly-coupled outputs that can damage speakers, even with active protection.
Capacitive coupling usually requires high-value electrolytic capacitors, which give audibly and measurably inferior performance at audio frequency extremes.
Greater tendency to pick up radio frequency interference and self-oscillate to the point of self-destruction, due to rectification by low-voltage diode junctions or slew-rate effects.
Maintenance more difficult; devices are not easily replaced by user.
Biasing more difficult, as temperature effects and device variations complicate circuitry and degrade performance.
Older transistors and ICs often become unavailable after only 20 years, and sometimes much less, making replacement difficult or impossible. Tubes have a staying power, proven over many decades.
Hardly scientific or objective, but whereas transistors operate on an invisibly microscopic, quantum scale, tubes exist and operate on an intuitive, human scale. You can see the heaters lit up, you can sometimes see a glowing plasma, and you can feel and hear the warmth. Everything about tubes exists in a more human realm than hard, cold transistors. Measure away, but it’s the sound that matters.

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Vacuum Tubes and Transistors Compared