Subminiature Tubes: The Future of Audio!
by Phil Taylor
The first practical subminiature tubes were designed and developed by Raytheon in the 1940s. These tubes were sometimes referred to as “pencil” tubes because of their small stature. They’re approximately a quarter of the size of the miniature B9A tube types found in guitar amplifiers, and instead of pins they have flexible leads just like transistors. These tiny tubes were manufactured to meet the stringent MIL-E-1 specification for reliability and designed for long service life under conditions of severe shock, vibration (up to 20,000G), high temperature and high altitude. These tubes were some of the most meticulously built and most rigorously tested of all tubes, as their main intended use was extreme military applications, such as missile guidance. They’re exceeingly tough and will easily withstand a drop test from a height of three to four feet onto a tiled or concrete floor without breaking.
Below is an extract from an old RCA datasheet for their 6021 medium mµ double triode tube that gives an idea of the supreme engineering effort that went into ensuring the reliability and consistency of subminiature tubes:
RCA-6021 is a subminature medium mu twin triode of the heated cathode type having flexible leads. It is intended for is use in oscillator and amplifier at frequencies up to 400 Mc (that’s 400MHz in today’s money and overkill for audio applications). Constructed to give dependable performance under conditions of shock and vibration, this “premium” tubes is especially suited for use in mobile and aircraft equipment and is rated for service at altitudes up to 60000 feet without the use of pressurised chambers.
The design of the 6021 incorporates a compact structure in which special attention has been given to the following features:
- “U” frame construction to keep the mount rigid and prevent distortion of plates.
- Precisely made and accurately fitted tube parts, including new mica design, to lock the parts firmly in place.
- Grid side rods having high heat conductivity to provide cool operation of the grids.
- Pure tungsten heater having high mechanical strength.
- Getter shield to prevent deposit of getter flash on tube elements.
- Pure nickel plate to minimise evolution of gas.
As a result of its structural design this tube is characterised by:
- Small spread in electrical characteristics.
- Reduced microphonic effects.
- Reduced grid emission.
- Long life under frequent on-off switching.
- Low leakage currents and high leakage resistance between the elements (plate, grid and cathode).
In addition, this tube utlises separate terminals for each cathode to permit flexibility of circuit arrangement.
Manufactured under rigid controls, the 6021 undergoes rigorous tests during manufacture to insure its “premium” quality as follows: test reading at the end of 1 hour, 100 hours and 500 hours to ensure that tubes fall within the established tight characteristics limis and that early failures are held to a low percentage.
They weren’t messing about were they. These tubes were to be used in extreme environments and mission critical situations. The result of this wartime driven engineering effort are some of the finest tubes ever made. These little devices represent the absolute pinnacle of tube technology and offer more consistent and reliable performance than the early N.O.S. germanium transistors. The Raytheon datasheet boldly states, “Tubes developed for this purpose proved so rugged that in-operative failures became very rare.”. Impressive stuff, eh. It’s also fascinating to consider that if the development of the transistor had been delayed for just a few more years, these tubes might have become the standard amplification device used in the audio industry today.
Some types were designed to operate at low heater and B+ voltages enabling the development of battery operated equipment such as portable operated radio receivers in submarines, domestic radio and hearing aids. The tubes were supplied with 8-pin subminiature leads suitable for use in subminiature sockets or printed circuits. They have a maximum diameter of 0.400” and a maximum seated height of 1 ½ inches and can be mounted in any position.
The smaller physical dimensions of a subminiature tube do not necessarily guarantee that it will be completely immune to microphonic pickup or exhibit lower microphony than their more commonly used miniature B9A counterparts. Microphonic sensitivity entirely depends on how rigid the elements (plate, grid and cathode) within the glass envelope are relative to one another and how well isolated they are from any sound or vibration sources. This in turn depends on how well the tube was designed (how smart the design engineers were – in the case of RCA, very smart) and manufactured (how well the assembly technican put it together). The theoretical ideal of a tube with zero microphony would require it to be manufactured from parts with infinite precision and materials of infinite rigidity. Even when the tube industry was at it’s zenith during the 1960s, manufacturing tubes for military and laboratory applications using the finest materials and best techniques of the time it wasn’t possible to achieve this. Consequently the only way to find tubes of exceptionally low microphony is to hand-select them from a given batch by audio testing.
Digressing a little, this is why silicone o-ring tube dampers are ineffective – they only have a small effect on the mass of the glass envelope but do not stop vibration being transmitted into the elements through the pins on the base. Additionally tube dampers do absolutely nothing to minimise the inter-electrode movement. However, this is a redundant point as the marketeers aren’t selling dampers for subminiature tubes…yet. And digressing just a little further, there is a remedial solution for reducing micophonic pickup in tubes (that actually works!), for miniature B9A tubes such as the 12AX7 – a N.O.S. shock-proof socket. The one certain thing that can be said of any microphony in subminiature tubes is that it is typically very low and centered around higher frequencies because the small elements resonate (ring like a bell) at higher frequencies.
The Effectrode Fire Bottle boost pedal utilises the 6112 subminature tube type which is a high mµ double triode. The first batches of PC-2A compressors were fitted with the medium mµ 6021 double triode – current models utilise a 6111WA medium mµ double triode (manufactured with black plates). In both pedals the second triode section makes up a cathode follower – a low impedance tube buffer stage.
I’ve also tested the Russian subminiature tube types: 6N16B-V and 6N17B-V dual triodes, medium mµ and high mµ respectively, with a 400mA heater. These tubes are manufactured with gilded gold grids which is supposed to improve longevity – the gold content is 5.4 mg in grids of both triodes. Military quality, “OTK” and military rhombus signed on the body – manufactured by MELZ plant (Moscow city, Soviet Union). The 6N16B can be susbtituted for U.S.A manuafactured 6021 and 6N17B for 6112 subminature tubes.
These Russian tubes generally tend to be a little more microphonic than their western counterparts because the getter is only secured at one point and also because of slacker part tolerances. All things being equal, a tube with a getter fixed at two points exhibits lower microphony than a tube that has only one getter fixing point. However, all things are not equal. A tube is a very complex device with many other parts that need to be kept fixed relative to one another if it’s sensitivity to external vibration (find out much more about the physics of microphony in this technical article published by Mullard in 1962) is to be reduced to an absolute minimum. Even though very tight tolerances are used in vacuum tube manufacture, it is not possible to avoid slight differences in practice, even in tubes from one batch of the same type. These differences do not have any significant effect on the electrical characteristics of a tube but they can affect microphony considerably. That said, the Western and Russian subminiature tubes represent the pinnacle of tube technology and are made to a very high standard – as good as practically possible – and these tubes sound exceptional. A 6N17B tube is used in the Helios tube fuzz pedal with very good results.