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New Rectifier for Television Receivers

Mullard Outlook, Vol. 11 No. 11, April 1961.
    
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The PY33 is a new mains rectifier valve for use in television receivers and is now recommended as a direct replacement for the PY32. It incorporates a new Mullard technique in the manufacture of its cathode.

The new Mullard rectifier, type PY33, is capable of supplying output currents of up to 325mA at higher values of output voltage than the PY32. This has been made possible by the introduction of an entirely new type of cathode, the Pressed Powder Cathode. The new cathode, permits the use of a lower anode-cathode spacing, thereby reducing the forward impedance of the valve. It also makes it possible now to use a lower limiting resistance and higher smoothing capacitance.

Pressed Powder Cathode

The cathode of the PY33 uses normal barium strontium oxide as its source of emission but a new technique is adopted for producing the emissive coating. This is prepared in such a way that a hard shiny surface is produced. The smooth character of the coating eliminates 'high spots' on the cathode surface and the method of manufacture ensures that the internal resistance of the cathode coating is low, thereby reducing local heating. Because of these features the likelihood of a flash-over occurring is greatly reduced, and if it does, the damage to the coating is much less than that experienced with a conventional cathode. Sputtering too is less likely to occur as there is little chance of cathode particles becoming detached, either during assembly or operation.

Cathode Ratings

The PY33 is rated for a maximum output current of 325mA and a peak current of 2.6A. The hot switching surge current is 9.5A; that is, the current that flows when the anode supply is switched on at the peak of the AC waveform with the heater on and the cathode at working temperature is 9.5A. The higher surge current rating enables the use of a larger reservoir capacitor, say 200μF, in the smoothing circuit giving better smoothing.

Limiting Resistor

The wide range of mains input voltages likely to be encountered in practice makes it necessary to include a limiting resistor in the rectifier circuit of a television receiver. The minimum value of the limiting resistor is that required to keep the valve within its ratings on the 200V tap, the lowest declared mains voltage. On other inputs, the limiting resistor is increased beyond its minimum value to give the same output voltage as under the 200V condition.

With the PY32 a limiting resistor of 23Ω is required with a 200V input, the reservoir capacitor being 100μF. At the full output current of 325mA, the output voltage is 182V. With the lower impedance PY33 used under the same conditions, the output voltage is 193V, an increase of 11V. For the same input voltage as the PY32, under the same conditions, the value of limiting resistor required is now only 15Ω. This lower value of series resistance is 35% less than with the PY32, giving a corresponding reduction in resistive heat loss. This point is of value mainly to the set designer. For replacement purposes the PY33 can be used in place of the PY32 without impairing the safety or performance of the set.

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