SOX lamps are very sensitive to heat loss which will affect how well the sodium vapourises. In a vertical lamp lantern such as Gamma 6, especially if the lamp is cap down this can cause a red/yellow lamp in a number of ways:

1. Sodium will run down behind the electrodes preventing full sodium vapourisation as the area behind the electrodes is cooler.
2. Heat loss will be aggravated from the lamp end - the u bend is the coolest part of the lamp and the outer arc tube is not coated with heat reflective coating at the end. Philips and Osram tubes reduce the tube end diameter and use a mica disc for insulation to reduce this effect. GE tubes don’t and make the heat loss worse by using a metal support which will conduct heat out of the arc tube.

The GEC 5670 series is generally cap up although I have seen some where the lamp holder has been moved to the bottom of the bowl and turned over. The Gamma 6 I believe is supposed to be cap up as well to reduce these effects.

The other cause of red/yellow lamp is of course a duff lamp. Sodium will tend to migrate from solid to vapour at the colder u bend at switch off but that should not prevent a full yellow lamp. Osram tubes are designed to collect the sodium at the u bend end in sort of "grooves" at the transition of the arc tube from straight to curved.

If burnt vertically, SOX lamps should be "cap up", so any lanterns which run them cap down must have been in-house conversions where the person doing the conversion did not understand this concept, as I'm confident manufacturers always burnt SOX lamps cap up.

Originally SO and early SOX lamps had to be burnt cap up as hot sodium attacks the electrode pinch seal of the arc tube. Later Philips changed the design of its lamps to overcome this problem.

Over 90W, should be operated in the horizontal plane as the longer arc tube will have a greater temperature gradient across its length and which will prevent proper vapourisation if the arc tube is vertical. The u bend end may never get hot enough and sodium is likely to collect in the u bend resulting in insufficent sodium vapour in the upper part of the lamp.

I forgot to take my camera today (but I will remember next week,) and 'old faithful' as I am nicknaming it was as pink as ever.  

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"I used to rule the world... Now I sweep the streets I used to own."
Remembering GEC.

Sorry to use an older thread, but I have a question regarding SOX lamp life. Recently I bought an MI26 which was taken down about 2 months ago, which came with the original SOX-E lamp. According to the date the lamp was put in the lantern in 2007 in March. My question is can anyone estimate maybe how much life it would have left in it?

A SOX lamp should have a life of around 3 years minimum, a "plus" lamp (red cap) should do about 4 years. E lamps tend to be better made than bog standard ones.

You should be able to get another year of continuous nightly use out of it. If there is any blackening of the outer envelope then the life may only be months. If there isn't much of the electrode coating which has sputtered onto the inside of the arc tube then that is a good sign.

Philips lamps last the longest, GE lamps the shortest and Osram somewhere in between.

Ah, many thanks Phosco. There is some coating sputtered onto the side of the tube but not much. The lamp in question is a Philips.

You should be OK with phillips ones (I've got two brand new boxes of 35w phillips SOX lamps and haven't yet had to use them because of the part-used ones I've got with my lanterns from the depot have also been phillips and have worked)... it's the GE ones to be weary of.

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It makes you wonder why councils would buy GE lamps if they're so poor... unless they're very cheap of course! (which would be false economy but there you go...)

You get what you pays for and quite often all they worry about is materials cost and not the labour. As you say false economy. In the 1980s the Osram lamps were also of quite high quality. However when the Osram lamp line was shut down and bought up by Philips, the quality reduced. It wasn't in Philips best interest to have Osram branded lamps lasting as long as their own.

I never did understand why Philips kept the Osram brand for SOX lamps alive, presumably they thought there was a certain amount of brand loyalty to the Osram name that would make keeping the line worthwhile.