The MW41-1 CRT

At the start of the 50's, the vast majority of sets utilised CRT's between 9" and 12" diameter, the main limitation on size being due to the available glass manufacturing technology.

At the time, two alternative large-screen methods were devised. The first of these was the projection set, which projected a very bright image from a very small tube onto a flat screen. The second method was to make the cone of the CRT from metal, of which the 16" MW41-1 was the the main example. Like the projection solution, the metal cone tube also gave a fairly flat screen. However, the cone had to carry the full EHT of the set, making insulation from the chassis and surrounding components a major problem, to say nothing of having to service such sets.

However, glass technology was developing quickly and within a couple of years larger screen normal CRT's became available, and both projection sets and the metal cone CRT rapidly became obsolete.

This rapid obsolescence meant that by the end of the 50's spare tubes were almost non existant. the following artical from Practical Television (October 1959) descibes replacement of an MW41-1 with a later MW43-69 and whilst focussing on the Ferguson range of receivers applies equally well to many other sets, including the Bush TUG26.

HAVING had considerable difficulty in obtaining a new 16in. metal tube (MW41-1) for one of the above receivers, and bearing in mind the very high cost of these near obsblete tubes, it was decided to try to modify the set to take a 17 in. rectangular type.

The advantages to be gainedwere numerous; firstly, there are large quantities of rebuilt and re-conditioned 17in. tubes on the market and these cost less than the 16in. metal type. Secondly, an aluminised tube could be chosen to give a far brighter picture, and thirdly, the dangers of servicing a chassis with a metal tube are eliminated. (The metal cone of the 16in. tube is connected to the final anode and therefore to the EHT supply- some 15 or 16 kV.)

It was decided to use an MW43-69, 17in. rectangular tube which is aluminised and also has a filter glass screen. Electrically, it is interchangeable with the 16in. type, except that its external conductive coating must be earthed and that pin 7 must be connected to pin 11 (cathode). The pentode gun gives better overall focus than the tetrode type—another advantage over the 16in. tube.

Fig. 1 (above).— Dimensions of the new support and its position above the original support (shown dotted).

As will be seen from the illustrations the mechanical considerations are small. A new tube rest is made as shown in Fig. 1 and lined with sponge rubber. This is bolted to the original rest by means of the two existing bolts. Next, the tube clamp is shortened by about 2in. and a piece of sleeving threaded on.

As the 17in. tube bulb is considerably longer than the 16in. type, two measures have to be taken: the deflection coil securing bolts are loosened and the coils pushed back as far as they will go. The Perspex mask must be removed from the inside of the cabinet and mounted on the outside with four chrome headed-screws.

It was mentioned earlier that the external coating of the 17in. tube must be earthed and this is most easily done by a spring clip. Mount it on the H.T. smoothing electrolytic fixing screw. This coating acts with the inner coating to provide an EHT smoothing capacitor, and the original smoothing capacitors can be removed, a lead now going directly from the EY51 EHT rectifier to the top cap of the tube. It will be necessary to change the existing top cap connector.

Finally, a rubber dust-sealing band should be slipped over the tube (as shown in the illustrations [below] ) and sprung in position between the tube face and mask when the chassis is bolted in the cabinet .