Thorn Alpha 4

Lantern acquired in February 2004.

New and unused, this lantern entered the collection minus its bowl and two clips, as these were used to replace a missing bowl from an existing Alpha 4; however, a new bowl (and clips) were sourced separately. Along with its 135 W versions (both with the lamp control gear housed in the lantern and remotely), the Alpha 4 was a popular lantern in Derbyshire, with the 90 W version often being used as a replacement for sister lantern, the Alpha 1. Following the Street Lighting PFI in Derby itself, numbers declined considerably, but odd examples remain. Within the wider County of Derbyshire, examples are still relatively common, but all are on borrowed time, as LED replacements will see their numbers reduced significantly within the next few years.

The Alpha 4 comprises a rather angular GRP canopy, aluminium rear casting, and a GRP cover for this. In the case of the 90 W version, the ballast and ignitor are housed on a dedicated gear tray within the lantern's optical area, whilst the capacitor resides in the rear casting. The lamp support is encased with a tube of insulating material as a means of providing a tighter fit for the lamp.

A close-up of the gear reveals that the ballast is not a Thorn product, but a Transtar. This lantern is narrower than the Philips MA 90, and so there is not enough space to fit all of the gear components together, as there is in the gear-in-lantern version of the Philips product. Looking again at the capacitor, notice that its incoming live (orange) wire is connected directly to the live terminal for the supply cable, rather than to the NEMA socket. This would mean that, had the lantern entered service, the capacitor would have remained live, even if the rest of the lantern wasn't operational.

The label is located at the very front of the lantern and uses all kinds of coding, but the most important is the 'QA4' bit, as this is the Alpha 4 identification code. The manufacturing date code is '2503', which may represent the 2nd May 2003, or week 25 of the same year. If the latter is correct, the lantern was made on the week of the 16th - 20th June.

This picture was taken after the two replacement bowl clips had been fitted to the lantern. The clips seemed to be a rather loose fit, and so a dab of glue was applied to all four of them, as a way of making them more secure.

When the glue dried, the bowl was fitted. Although all the clips are all the same, they have to be fitted differently to both sides of the bowl. One side opens completely, whereas the other acts as a hinge. The bowl is only able to be attached one way around in the lantern - the front has more curved radii than the rear does.

These pictures demonstrate the size difference between the 90 W and 135 W versions.

The 90 W version is 1067 mm [3.5 ft] in length, whilst the 135 W version is 1258 mm [4 ft 112 inches] along the same dimension. Both lanterns are 146 mm [534 inches] in depth when the semi-cut-off bowl is employed, and the same figure in width.

 

The apparent colour difference between the two bowls is the result of the 90 W version being somewhat cleaner than the 135 W version is!

 

This view shows the difference within the lamp area - the shorter 90 W lamp meaning that the lampholder is situated further forward in this version.

 

The lantern was fitted to an AC Ford AC872 wall bracket and powered up on Sunday, 28th August 2005.

Lantern warm-up video:


A cut-off bowl is also available for the Alpha 4, though it isn't as common as the regular semi-cut-off one is.

Thorn Alpha 8 | Urbis ZX1


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