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.
The subminiature tube represents the pinnacle of tube technology and offer more consistent and reliable performance than the early NOS 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 they 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 electrodes (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) and manufactured (how well the assembly technican put it together).
As a side note, this is why silcone o-ring tube dampers are not very effective – they only have a small effect on the mass of the glass envelope but do not stop vibration being transmitted into the electrodes through the pins on the base. Additionally tube dampers do nothing to minimise the inter-electrode movement. However this is a redundant point is that the marketeers aren’t selling dampers for subminiature tubes…yet. The one certain thing that can be said of microphony in subminature tubes is that it is typically centered around higher frequencies because the small electrodes resonate (ring like a bell) at higher frequencies.
The Effectrode Fire Bottle pedal utilises the 6112 subminature tube type which is a high mµ double triode. The PC-2A compressor utilises the low mµ 6021 double triode. In both pedals the second triode section makes up a cathode follower – a low impedance tube buffer stage. More recently I became interested in Russian subminiature tube types: 6N16B-V and 6N17B-V dual triodes, medium mµ and high mµ respectively, with a 400mA heater. These tubes have gilded gold grids 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. I’m using the 6N16B in the latest PC-2A compressor build – with outstanding results – and am now evaluating the 6N17B for the Helios tube fuzz pedal.