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955 acorn triode

Thermionic valve for VHF operation

Field	Value
image	955ACORN.jpg
caption	Right: R.C.A. 955 U.H.F. Triode tube

classification	V.H.F. Triode
service	Class-A amplifier or oscillator
height_in
diameter_in
weight_lbs
cathode_type	Indirectly heated
heater_voltage	6.3 V
heater_current	150 mA
max_dissipation	1 W
max_voltage	250 V
class_a_amplification_factor	20-25
class_a_anode_voltage	180 V
class_a_anode_current	4.2 mA
class_a_bias_voltage	-5 V
class_a_anode_resistance	12.5 kΩ
reference	RCA Datasheets of 954, 955, 956, 957, 958A, 959
Philips Valve Data Book, Australia, 1954

Left: TO-72 Transistor for comparison

Philips Valve Data Book, Australia, 1954 The type 955 triode "acorn tube" is a small triode thermionic valve (vacuum tube in USA) designed primarily to operate at high frequency. Although data books specify an upper limit of 400–600 MHz, some circuits may obtain gain up to about 900 MHz. Interelectrode capacitances and Miller capacitances are minimized by the small dimensions of the device and the widely separated pins. The connecting pins are placed around the periphery of the bulb and project radially outward: this maintains short internal leads with low inductance, an important property allowing operation at high frequency. The pins fit a special socket fabricated as a ceramic ring in which the valve itself occupies the central space. The 955 was developed by RCA and was commercially available in 1935.

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.

Pin connections

When viewing the device from above (the end without the exhaust tip), the pins are arranged in a group of three and a group of two, starting with the centre pin in the group of three and going in a clockwise direction, the pins are cathode, heater, grid, anode, heater.

Ratings

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.

The maximum anode voltage is 250 V, with an anode current of 420 microamperes and anode load 250 kilohm, and the maximum anode current is 4.5 mA at a voltage of 180 V with an anode load of 20 kilohm. The 955 is designed to be used in the frequency range of 60–600 MHz (5-0.5 metres wavelength). The amplification factor obtained is between 20 and 25 depending on details of the specific stage design and operating voltage.

References

''Popular Science'', January 1935, {{Google books. lyoDAAAAMBAJ. New Dwarf Tube Ready for Amateur
"957 • 958 • 959 (Acorn Types)". [[RCA Corporation.
Ludwell A. Sibley. "The Acorn Tube".
Philips Valve Data Book, Philips Electrical Industries Pty. Ltd., 69-73 [[Clarence Street Sydney]], Radio Valve Application Division, 1954.

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