Tungsram LU250 with Double Wire Seals

This lamp illustrates the type of arc tube seals employed on Tungsram lamps during the 1970s and 1980s. The niobium wire seal was the preferred method of the company during this period, but curiously, multiple wires were often employed.

Low wattage types of 35-100W employed just a single niobium wire at each end of the arc tube. This was sealed into a PCA plug, which itself was frit sealed into the PCA tube.

The 150W and 250W products have a seal type as illustrated here. The structure is essentially quite similar, but two niobium wires are employed at each seal. By contrast the 400W and 1000W lamps employed four wires at each seal.

The reason for the multiple seals is not entirely clear. It is unlikely to be for handling the higher electric current, since competitive lamps are made up to 1000W with only one single wire feedthrough. It is perhaps more likely to be for  reasons of increased mechanical strength. The electrodes in the higher wattage lamps are relatively more massive, and would apply a large bending moment to the niobium feedthroughs in the event of external mechanical shocks and vibration. With an increased number of sealing wires, the electrode mount assembly can be made somewhat stiffer.

A negative consequence is, of course, that with a greater number of seal points, the chance for seal failure and leakage of the arc tube is also very considerably increased. This unusual construction was abandoned in 1992 following the takeover of Tungsram by GE Lighting of USA. Lamps made since then employ only a single wire feedthrough at one end, and the niobium tube amalgam reservoir at the opposite end.
Manufacturer: Tungsram Rt.
Lamp Power: 250 Watts
Lamp Current: 3.0 Amps
Lamp Voltage: 100 Volts
Cap Type: E40s/45 Ni plated brass with vitrite
Bulb Finish: Clear Hard glass
Bulb Type: ED-59 ED-18 in eighths/inch
Overall Length: 257 mm 10 1/8 inches
Light Centre Length: 160 mm 6¼ inches
Arc Length: 90 mm 3½ inches
Electrodes: Backwound Tungsten Barium Calcium Tungstate
Atmosphere: Na,Hg | Xe Outer: Hard Vacuum
Luminous Flux: 27,000 lm (@ 100 hrs) 26,500 lm (@ 2,000 hrs)
Luminous Efficacy: 108.0 lm/W (@ 100 hrs) 106.0 lm/W (@ 2,000 hrs)
Colour Temperature & CRI: CCT: 2100K CRI: Ra 23
Chromaticity Co-ordinates: CCx: 0.526 CCy: 0.418
Burning Position: Universal
Rated Lifetime: 24,000 hours to 50% survival
Warm-up & Re-strike Time: 4 mins (to 90% lumens) 15 minutes max
Factory: Budapest Hungary
Date of Manufacture: November 1985 Date Code 1185
Original & Present Value: £50.00 (1991) £71.91 (2005)
References: 1) Tungsram Discharge Lamp Brochure, c. 1985.
2) Tungsram General Lamp Catalogue 1990.
3) GE-Tungsram UK Price List, June 1991.