GEC BT310 (BT1156)

[Murphyv310]

Am I being thick, but what's the 100n cap for? The 1n is across the rectifier to reduce switching transients but I can't see what job a 100n would do.

Hi Jeffrey.
The BY127 although it has a reasonable rating does fail, there was never any real proof but it was believed that spikes in the mains was the reason. Many of us would fit a 0.1uf 1kv cap between the anode and chassis, this obviated the 1000pf cap across it as the cathode was usually connected to the reservoir cap.
Personally I don't use BY127's or 1N4007's these days, I use 1N5408 Diodes.

[Geordie McBoyne]

Might be a case of belt and braces.   Looking though the 1963/64 R & T servicing book we find Pye group dual standard receivers employed two capacitors to protect the silicon diode from transient spikes. Also Rediffusion in the Mark 8 cable receiver.
Some makers did not fit a small value capacitor across the diode but nevertheless employed a 0.1mfd capacitor between the anode of the diode and ground.  
As Trevor states, these days the 1N5408 is the diode of choice as a TV high tension rectifier.  Mono or colour.

Geordie McBoyne.