1966 Mag: Build NE-2 Neon Bulb Computer - scan available

valve that gave off the most Xrays wasn't the HV rectifier (a GY501 in most UK sets) but the 'Shunt Stabiliser' (often a PD500). This was a big triode connected between the EHT line and chassis, and formed an adjustable load on the EHT supply. The idea was to keep the total load (and thus the EHT voltage) constant as the CRT beam current changed. The valve had about 25kV acorss it and passed 1mA. The anode would, I beleive glow a very dull red. This valve, along with the valve EHT recrtifier, disappeared when semiconductor-based triplers came into use. These things had a much lower intenral impedance, so you didn't need to stabilise the load current. Tottally OT, but I worked on a colour TV that had 5 valves in the EHT cage, none of them being a shunt stabiliser, and it did use an valve rectifier, not a semiconductor. Apparently, the manufacturers wnated to get away from that hot-running X-ray-producing, PD500 There were effectively 2 horizontal output stages. The first used a PL504 (a valve mroe commonly found in monchrome TVs) with a PY800 booster diode (damper to you) alongside it. This circuit drove the deflection yoke, and the flyback votlage was rectified by a semiconductor rectifier to give the 5kV focus voltage for the CRT. The other output stage used a PL509 ouptut pentode witha PY500A booster (the conventional valves used in a colour TV horizontal defleciton stage). The flyback from this was rectified by a GY501 and used to provide the EHT to the CRT. The drive to the PL509 was controlled based on the beam current and/or EHT voltage to provide stabilisation (since this stange did not drive the yoke, the picture width was unaffected). -tony