STC B14 Glass Vacuum Indirectly Heated Bead 10kΩ Thermistor

On offer are a number of new (old stock) pieces of a high quality 'type B' indirectly heated, bead-type thermistor in a glass envelope under high vacuum. Hand-made at the Standard Telephone & Cable company around 1963 in Footscray, Kent, UK.

They comprise a tiny NTC thermistor of nominally 10kΩ resistance at 20°C, surrounded by a small coil of wire with ~100 ohm resistance, mounted in an evacuated glass tube. As current is passed through the coil, the thermistor heats up and its resistance drops. The coil is electrically insulated from the thermistor. A maximum current of 20mA through the coil causes thermistor resistance to drop to 50 ohms.

Applications include automatic gain control, wien-bridge oscillator amplitude stabilisation, remotely controlled variable resistors, etc.

Suitable for signal power measurement up to 40mW, this style of indirectly heated device seems also to have been known as a "thermal converter", though my references mention that these are considered obsolete devices and no longer manufactured.

To quote from the datasheet:

Type B are indirectly heated, bead-type thermistors.

The thermistor element consists of a small bead of resistance material, about 0.02 in, (0.5mm) diameter, which is integrally formed on two parallel platinum wires that are both carried right through the bead and, in assembly, connection is made to both ends of the wire. This gives the assembly greater rigidity and increases the reliability of the thermistor.

The heater surrounding the element is a rigid coil of a special design which ensures maximum concentration of heat on the resistance bead. The resistance of the heater (which has a negliible temperature coefficient) is 100 ohms.

The thermistor sub-assembly comprises the bead, an insulating cement and the heater coil. Careful check-testing of bead and heater, both before and after assembly, maintains a high standard of reproducibility between thermistors of a type. The assembly is sealed in a bulb, evacuated and gettered and thus provides a high vacuum thermistor.

General data:
Standard Telephones and Cables Limited
Components Group, Semiconductor Division (Transistors),
Footscray, Sidcup, Kent
Tel : Footscray 3333
November 1963
These parts outdate me ... just. They are well travelled; though made in the UK, I purchased these in Australia a couple of decades past and eventually repatriated them, still in their original packaging.

I've never seen them elsewhere ... I guess opto isolators and the like prevailed at a much lower cost. The craftsmanship is excellent, the materials are high grade. If nothing else these are historically interesting ... a timely reminder that we used to make things in this country ...

Here at a price that lets everyone own a piece of history ... only while they last of course ....

About the Seller : RFPlus

I'm Jon, a lifelong hobbyist and occasional seller on Ebay, having sent over 10,000 items over the past 15 years with 100% positive feedback.

Recently I've been revisiting my interest in electronics, and have rediscovered a lot of parts I'll likely not get around to using. Consequently I've decided to auction them off in small lots for those who might find them interesting. Some I'll be able to relist; others are one-offs. Where I have larger quantities I will list with a Buy Now option.

Parts are New unless otherwise stated and are sold in good faith as fully functional. Many parts are 'old stock', meaning they are unused but may have been made decades ago.  Should you find them unusable in your application I'll gladly provide a refund. All parts for sale are my private property, in stock in Cornwall, UK and ready to post to the world via Royal Mail.

I'm hoping there's a payment option which will suit you. If not, please get in touch.

If there's a problem with anything, I can send a replacement order, refund your money or do whatever it takes to make you happy.

Thank you for reading ...   Jon @ RFPlus