The 955 is one of about a dozen types of "acorn valve", so called because their size and shape is similar to the acorn (nut of the oak tree), introduced starting in 1935 and designed to work in the VHF range. The 954 and 956 types are sharp and remote cut-off pentodes, respectively, both also with indirect 6.3 V, 150 mA heaters. Types 957, 958 and 959 are for portable equipment and have 1.25 V NiCd battery heaters. The 957 is a medium-μ signal triode, the 958 is a transmitting triode with dual, paralleled filaments for increased emission, and the 959 is a sharp cut-off pentode like the 954. The 957 and 959 draw 50 mA heater current, the 958 twice as much. In 1942, the 958A with tightened emission specs was introduced after it turned out that 958s with excessively high emission kept working after the filament power was turned off, the filament still sufficiently heating on the anode current alone.
The 955 is an indirectly heated triode with heater electrically isolated from the cathode. The heater has a 6.3 volt rating, which it shares with many other common thermionic valves/electron tubes, and it draws about 150 mA.
An acorn tube, or acorn valve, refers to any member of a family of VHF/UHF vacuum tubes starting just before World War II. They were named after their resemblance to the acorn, specifically due to the glass cap at one end of the tube that looked similar to the cap on an acorn. The acorn tubes found widespread use in radios and radar systems.
High-frequency performance is limited by (1) parasitic lead inductance and capacitance and skin effect, and (2) electron transit time (the time required to travel from cathode to anode). Transit time effects are complicated, but one simple effect is the phase margin; another one is input conductance, also known as grid loading. At extremely high frequencies, electrons arriving at the grid may become out of phase with those departing towards the anode. This imbalance of charge causes the grid to exhibit a reactance that is much less than its low-frequency "open circuit" characteristic. Acorn- as well as Lighthouse tubes and Nuvistors attempt to minimize this effect by arranging cathode, grid(s) and anode as closely spaced together as possible.
The original range included about half a dozen tubes, designed to work in the VHF range. The 955 is a triode. The 954 and 956 types are sharp and remote cut-off pentodes, respectively, all with indirect 6.3 V, 150 mA heaters. Types 957, 958 and 959 are for portable equipment and have 1.25 V NiCd battery heaters. The 957 is a medium-μ signal triode, the 958 is a transmitting triode with dual, paralleled filaments for increased emission, and the 959 is a sharp cut-off pentode like the 954. The 957 and 959 draw 50 mA heater current, the 958 twice as much. In 1942, the 958A with tightened emission specifications was introduced after it turned out that 958s with excessively high emission kept working after the filament power was turned off, the filament still sufficiently heating on the anode current alone. After the introduction of the miniature 7-pin base, the 954, 955 and 956 were made available with this base as 9001, 9002 and 9003. Other acorn tubes include:
The introduction of the EF50 was the first serious competition for the acorn design, and replaced the acorns in many roles, especially post-war when millions of surplus EF50s were dumped on the market.
By the mid-1930s valves were been produced in their millions at very low cost and for most receiver-type applications were pretty much standardised on octal bases and were not exactly compact in size. This size and the base format (and the socket they plugged into) set a definite upper limit on the frequency of operation. In 1933/34 RCA's developers B J Thompson and G M Rose published articles describing experimental triode and tetrode designs. These became the basis of the production valves that saw the light of day from 1935 onwards, when RCA took the bold step of introducing the so-called 'acorn' valve (or 'tube' as they would have called it), which resulted in a quantum leap in the high frequency performance of these new devices.
These new valves were well positioned as the Second World War drove the need for high frequency devices for use in VHF/UHF transmitters and receivers, radars, radar altimeters, beacons, IFF (Identify Friend or Foe) transponders and interrogators, and all the associated test equipment. The war also drove further valve developments and in fact the acorn format didn't have a long 'design-in' life time, and became obsolete probably about half way through the war, though of course it's impossible to pin down an exact date for this.
I'll try to use the word 'valve' consistently rather than 'tube', even when I'm dealing with devices that are of US origin, as were most of the early 'acorns'. Also of course the word 'anode' is interchangeable with the US term 'plate'.
Most of the hardware shown in the photos was provided by Mike Lewis of the BVWS. My thanks go to Mike for being so generous with his time in tracking down the acorn and Nuvistor 'bits and pieces', which I hope really bring the article to life.
The first commercial acorn valve released by RCA was the 955 triode, announced in March 1935, and promoted as usable up to 500MHz. RCA sold it initially under the 'RCA-De Forest' brand, which it was then using for amateur-market transmitting valves and CRTs. Initial versions of the acorn triode had the glass at the rounded, just like an acorn, but later versions (and the ones you can see in my photos) have a flat top. Pentodes, where the anode connection passes through the glass at the top, retained the rounded shape throughout their production lifetime.
I couldn't resist the temptation of photographing a few of these valves alongside a real acorn. The picture shows a JRP-957 triode (military version of the 957) and commercial 956 pentode flanking a real acorn. I've also seen these valves referred to as 'peanuts', which is maybe understandable, but someone must have grown some big peanuts!
So what was the problem that the acorn format was trying to solve? The upper operating frequency of any valve is limited by the combination of many factors, including the size of the elements inside the valve; the inductance of the connections inside the valve and to and from its base; the inter-electrode capacitances; the losses in the construction (especially the base and the socket that it's plugged into); and of course the electrical and mechanical design of the external circuit it's connected to.
The acorn valve's construction addressed all these issues and the solution was a new miniature glass envelope design with correspondingly small internal elements and short, direct connections to the pins, which were arranged radially around the valve and were themselves simply extensions of the internal connections, rather than a separate soldered-on base. The designs achieved very low grid to anode capacitances, of the order of 0.007pF for the 954 (compared to typically greater than 3pF for many other valves of the day), and correspondingly low values for the other inter-electrode capacitances.
Above: a cut-away diagram of the 955 triode showing what the internal construction looks like.The 955 and 956 pin connections. The 956 has its control grid connection, which is the lead coming out of the bottom of the envelope, unmarked on this diagram.Below: an acorn valve family line-up. Left to right, they are the 954, 955, 956 and 957, some in various military guises. As you can see from the presence or absence of the top lead, we have pentode-triode-pentode-triode in the line-up.
The shape of these valves necessitated the introduction of a new socket format. The image below shows a Hytron VT-121-955 triode sitting on a ceramic socket. The five pins need to be pushed into the clips in the socket. The second image shows a couple of styles of clip-on connectors for making contact to the top and bottom pins of the acorn pentodes.
A note on the 954, 955, etc. numbering of these valves: at the time these valves appeared RCA were following what would soon become the standardised US RMA valve coding system, which gives us the 6V6, 12A6, etc. However valves could also be numbered with an all numeric code. These all numeric codes were normally reserved for 'special purpose' valves, which shows the experimental nature of these acorn valves. Later use of the standard codes of '6F4' (acorn oscillator triode, for use up to 1200M, or '6L4' (acorn oscillator triode, capable of running with 500V on its anode) for some acorn valves indicates that the technology had then become more 'main stream' and no longer merited a 'special' code number.
Several US manufacturers made acorn types, particularly 955s, during WW II. As a result, most tubes found today are ex-military, and carry Joint Army-Navy (JAN) designations in either a long form or an abbreviated version ahead of the tube type number. The known makers are: GE (JAN-CG or JG); Hytron (JAN-CHY or JHY); Raytheon (JAN-CRP or JRP); RCA (JAN-CRC or JRC); Sonotone (JAN-COZ or JOZ); Tung-Sol (JAN-CTL or JTL); Westinghouse (JAN-CWL or JWL).
Two acorn diodes were added to the line-up for WWII military equipment: the 9004 and 9005. The 9004 was quoted as having a 'resonant frequency of 850MHz' on the 1942 data sheet, and found use in radar altimeters. To aid in matching pairs of 9004s, each valve carried a 'zone' number, between 1 and 6, marked at the top of the glass envelope to indicate its relative emission, as verified under test conditions. The 9005 was quoted as having a 'resonant frequency of 1500MHz', had the strange heater voltage of 3.6V, AC or DC. I presume some special application was in mind when this valve was designed.
Although promoted as UHF detectors (that is rectifiers of low level signals) 9004 and 9005 acorn UHF diodes/rectifiers were rated at 117V PIV, so I suppose in theory at least one of these valves could have been used as the AC mains rectifier stage in US-mains powered equipment. The non-acorn 9006 (see Table 2), again though publicised mainly as a UHF diode had a maximum PIV rating of 750V (!) at 5mA. 041b061a72