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

[Tony Duell]

> I've long wondered about using a HV rectifier tube as a source of 
> soft X-rays, though.

>From my days in repairing _old_ colour TVs, I seem to remember that the 
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