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The 6/4 or 6/6 and an EMI/Marconi Advert for the CRT

The video signal in these first generation EMI sets is fed directly from the detector to the grid (or modulator) of the cathode ray tube (CRT) without requiring a video amplifier stage. This was made possible by the high sensitivity of the CRTs. Experiments with the gun assembly had shown that by increasing the length of the modulator the sensitivity could be increased but this had the undesirable side effect of defocusing the electron beam.

Further experiments showed that it was possible to gain sensitivity without defocusing by altering the design of the aperture in the modulator. The first improvement was achieved by effectively thickening the aperture plate or arranging multiple plates and this was patented in GB447493A. Yet more experiments showed that designing the aperture to present a flat face towards the cathode but a concave face towards the anode also gave improvement and this was patented in GB452589A.  These modified modulators reduced the beam cut-off voltage by about 40%. The guns in the earlier type of CRT shown in the upper picture below may be using the concave aperture. Those in the later type clearly have more than one aperture.

These CRTs are labelled as Pyrex glass which offered greater strength than normal soda glass. The earlier production units were blown to shape but later units were constructed with a blown cone welded to a moulded face plate that offered a more uniform optical performance. The glass maker Corning was granted a patent (GB470885A) for the use of combined blown and moulded parts.

Other differences are the construction of the electron gun and the presence of a seal nipple opposite the EHT connector on the early tube.