[Electrical]

Hello Roger
Please see attached data sheet on the K5/80 rectifier. It has a PIV of 3400 volts and is rated at 5mA up to a temperature of 40C. From the data sheet I would estimate the RMS input voltage at around 1250 volts AC. The DC output voltage measured across the smoothing capacitor should be between 1700 to 1800 volts. As mentioned in the other posts you can use three or four series connected 1N4007 silicon diodes to replace the old selenium K5/80 rectifier. But always connect an old carbon composition resistor in series with the silicon diodes to limit the switch on surge, which will not damage the new silicon diodes, but may over stress the EHT winding on the mains transformer. Try a value around 4.7 to 10k to start with, and supply the scope with a variac if possible and monitor the EHT voltage at the output of the new silicon rectifier. If the voltage is too high or low adjust the value of the series resistor to obtain as near as possible the correct EHT voltage. The resistor compensates for the high forward voltage drop of the original K5/50. Sometimes on old, equipment I fit a thermistor in series with mains transformer primary to control the switch on surge.
With reference to metrosil resistors, If you are referring to the types as fitted to TV’s you will find a list of types and colour codes in the old Mullard Valve data books, in these books they are listed as VDR's, Voltage Dependant Resistors
Regards Stan.