In Part One of this series (Issue 183), I discussed the history of the of the vacuum tube. Since it all started with the development of the diode, we might as well start our deeper exploration of vacuum tube audio with this tube type.
The diode has a simple job to do. It only allows an electric current to be conducted in one direction. It is therefore useful for turning an alternating current (AC) into direct current (DC), and this type of device is called a rectifier. However, this same feat can be easily accomplished with a solid-state rectifier. Vacuum tube rectifiers require an extra power source, and have a much higher voltage drop than solid-state diodes. They are therefore much less energy efficient.
Early tube rectifiers were not all vacuum tubes. There were low-voltage rectifiers filled with argon called Tungar (a General Electric trademark) bulbs, that were used in battery chargers. Before the era when most homes were connected to the electric grid, many households relied on batteries to run their radio sets. There were commercial businesses that recharged batteries for private homes. These Tungar bulbs were very inefficient, with voltage drops that were often higher than the voltage they supplied. There were also rectifiers that were filled with mercury vapor, mainly used for high voltage, high current applications, which no doubt must have caused at least a few cases of poisoning. These mercury vapor rectifiers had the tendency to back-arc, destroying the equipment they powered.
After AC mains power became common in the 1920s, high-voltage rectifiers were needed to power radio sets. The giant defense contractor Raytheon started life as a manufacturer of expensive rectifiers for radios. The most common rectifier of the era was the UX-280, which remained in production for more than 50 years. There was no standard mounting base for rectifiers in those days, but many devices had a 4-pin base and were interchangeable.
The vacuum tube rectifiers that remain in use today all have the octal (8-pin) base, and were first developed by RCA, which held the license for this design. The early types were based on the UX-280, with a 5-volt filament. The first to appear on an octal base was the 5Z4, with subsequent development of the 5U4, 5U4G and 5U4GB, which is still in common use today in audio equipment and some guitar amps. The 5R4GY was developed to handle a higher voltage, and a ruggedized version, the 5R4WGB (nicknamed the “potato masher’) can still be easily found since it was the mainstay in Vietnam War-era military equipment.
5U4G rectifier tube. Courtesy of Adrian Wu.
5R4WGB rectifier tube. Courtesy of Adrian Wu.
Another 5U4 variant is the 274B made by Western Electric. Even though they have essentially the same electrical characteristics, the 274B sells at many times the price (around US $1,500 for new old stock) of the 5U4 nowadays.
Up to this point, the tube rectifiers mentioned above all have directly-heated cathodes. By the early 1930s, almost all rectifiers were full-wave, meaning each contained two anodes to rectify both phases of the AC cycle, and were typically used with a center-tapped transformer. Indirectly-heated rectifiers also started to appear at this point. These have the advantage of slow warm up, which means a delay in supplying the high tension (HT) voltage to allow the other tubes to warm up first. If the HT is applied before the cathode is warm enough to supply electrons, the cathode could be damaged by a process called cathode stripping.
With the American manufacturers standardizing rectifiers to using the octal base and 5V heaters, European manufacturers followed and basically just produced their own versions of American designs. The most common rectifiers found in vintage British tube hi-fi gear include the GZ30 (5Z4 equivalent), GZ31 (5U4), GZ32 (5V4), GZ33, and GZ37. The popular GZ34 (5AR4) was developed to fit into low-profile equipment, thanks to its short envelope. It acquired its name because it was intended to work with the EL34 power tube, frequently found in guitar amps and DIY amplifier kits of the era. It was first introduced by Philips with a metal base, and later made by Mullard on a Bakelite base. Marconi-Osram (M-OV) also produced their own version of American tubes such as the U50 (5Z4), U52 (like the 5U4, but indirectly heated) and U77 (GZ34).
A CV378 tube, equivalent to a GZ37. Courtesy of Adrian Wu.
There were many reasons why vacuum-tube rectifiers were rapidly replaced by silicon diodes. By the 1960s, most vacuum tubes had 6.3V (or 12.6V) heaters, but rectifiers still used 5V heaters, for no better reason than a lack of desire to introduce new products that were not backward-compatible. That meant equipment needed to have both a 5V and a 6.3V heater supply, adding to the cost. Silicon diodes were much cheaper, did not require a power supply, and had a much lower voltage drop. They could also tolerate much higher filter capacitance.
Having been made obsolete more than 50 years ago, these tubes have been given a new lease on life in recent years when a number of manufacturers of tube audio equipment started designing new products that use tube rectification. I am even aware of transistor amplifiers (from Lamm Industries to name one example) that make use of tube rectification! What accounts for this development?
There is no doubt a bit of nostalgia and hype surrounding the use of tube rectifiers. They do have some advantages over silicon diodes. Early silicon diodes had slow recovery; when a diode ceases to conduct forward current, it actually conducts reverse current for a short time. This reverse current ceases abruptly, and the resulting current spike can inject noise into the power supply. Depending on the design of the power supply, these spikes can significantly affect the sound quality of the amplifier. However, this effect can be eliminated by the use of Schottky diodes, which have negligible reverse current, or by using soft recovery diodes. Another advantage of tube rectifiers is their slow start up, which, as noted, protects the other tubes. Tube equipment that employs solid-state rectification should therefore have a delayed HT power supply to allow the heaters to warm up first. Solid-state diodes, on the other hand, have far lower voltage drop and resistance, which is advantageous in class A/B circuits where the load current varies with the signal. [Some guitar players like amps with tube rectifiers because they “sag” (the supply voltage drops) when pushed hard, giving the amps a more “compressed” response and a “softer” feel. – Ed.]
Some audiophiles like to substitute different tube rectifiers, claiming that some sound superior than others. Be aware that even though all the "modern" tube rectifiers have the same pin-out configuration, they might not be interchangeable. Make sure at the very least that the tube has at least the peak voltage and current ratings of the tube it substitutes, can withstand the same amount of filter capacitance, and does not draw a significantly higher heater current. The difference in sound quality is usually due to the different voltage drop, especially if the power transformer is undersized (a common problem for consumer products, even so-called high-end products) for cost reasons. A lower supply voltage means a lower load current, which reduces transformer saturation and hence distortion at the expense of a lower power output. The difference in output resistance between rectifiers should not matter if the power supply is well-designed with adequate filter capacitance, but it is not always the case.
A $1,500 Western Electric 274B might be better made than a $30 RCA 5U4 and hence will last longer, but I can assure you that any perceived improvement in sound quality is a figment of your imagination. The magnitude of the placebo effect is positively correlated with cost, which is why many audiophile components cost such ridiculous sums of money. Some Russian and Chinese tube manufacturers now make rectifier tubes, but these can be plagued by reliability problems. Personally, I would not use tube rectification if I were to design a new piece of equipment, and I would leave the dwindling stock of new old stock tubes to the vintage audio collectors (myself included).
Header image: vacuum tubes in a McIntosh MC240 amplifier. Courtesy of Wikimedia Commons/Sebastian Nizan.
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