The EY51 is a wire-ended EHT rectifier with a 6.3 V 90 mA heater which is normally fed from a well-insulated winding on the line output transformer of a television receiver. This is the original EHT rectifier, which in 1948 made the AC/DC technique possible in the television set. The EY86 is effectively the EY51 on a B9A base.
Over time its peak inverse voltage rating has been raised to 17 kV in order to cope with larger tubes and higher final anode potentials and its physical size has been slightly reduced.
General symptoms of its failure are blackening around the glass just below the bell and a picture, which 'blows up' in size as the brightness control, is advanced (the latter fault may also be due to a displaced ion trap magnet).
The high voltages present when running make the valve difficult to check by conventional means and the filament colour provides a useful means of ascertaining the operating conditions by comparing it against the filament colour of a similar valve run from a 6 V battery.
In many cases a dim filament will indicate a low-line output stage and it is not always appreciated that unless the line output stage is working correctly the EY51 may not light up at all.
An EHT short or excessive current drain by the CRT will also dim the filament and this property is useful to enable the brightness control to be set to a working level before adjusting the ion trap magnet when a new tube is fitted. The author found that if the brightness control is set to just cause the valve filament to dim slightly a bright raster is ensured when the ion trap passes its correct position.
If the filament is too bright, short life usually results, and this may be overcome by fitting a small resistor in series with the heater lead to absorb some of the surplus. A 22 Ω or 27 Ω carbon resistor, 0.25 W, is usually adequate.
The valve is thus designed for pulsed operation. The anode PIV is 17 kV and the rectified current has a maximum average value of 350 µA and a peak value of 80 mA. The reservoir capacitor can take a maximum value of 5,000 pF.
The EY51 from the Kradlepak box seen looking into the anode bell. The heater extends beyond the oxide coated cathode tube.
A closer look at the cathode sited at the centre of the anode.
The thin glass tube envelope is 13 mm in diameter and, excluding the base wires is 48 mm tall.
References: Datasheet & 1040. Type EY51 was first introduced¶ in 1948.
Absolute Maximum Operating Conditions¶
Thanks to Frank Philipse for supplying the above PDF datasheet