As was announced in Amateur Wireless when the first details of class-B amplification were announced, a new type of class-B valve was on the way.
And now the Mullard Co. has brought out a very economical class-B valve the PM2B. The Way in which this differs from previous ideas of class-B working will be obvious from the accompanying characteristic curve. The valve is designed to give a reasonable maximum undistorted output of 1.25 Watts, and, of course, great economy is effected.
The 2 Volt filament circuit consumes only 0.2 Ampere the recommended maximum anode voltage is 150.
The valve is reasonably small, being only 110 mm, long and 46 in diameter. The standard seven pin base connections are used.
An idea of the economy effected by this new valve can be obtained from the accompanying figures.With the anode voltage at 120 V and the grid voltage 0 V the 'rest' anode current is 3 mA. At 20% modulation the approximate anode current is 4.2 mA, rising to a maximum of only 20 mA at 100% modulation. A high efficiency detector valve is recommended for use as a driver, and a good valve of this class with 120 Volts on the anode and biased at -4.5 Volts will take only 1.5 mA.
The total quiescent current of a PM2B valve used with a suitable driver is therefore only 4.0 mA.
An input transformer for use with this very new Mullard valve should have a step down ratio of 3 to 1 to each half of the secondary (1.5 to 1 overall ratio) and the maximum voltage to each grid of the PM2B is 14 Volts RMS. So far as the output transformer is concerned, the optimum plate to plate load for the PM2B is 14,000 Ω and the DC resistance of the primary should not exceed 1,000 Ω.
This valve has pentode (constant current) characteristics and it is recommended that a high-note filter be used to control the amount of top.
This valve is now generally released and will be available to the public to coincide with the publication of this announcement. Amateur Wireless tests made with advance samples of this type of valve have shown that it is highly satisfactory and as the result of this experience it is not only possible to recommend the PM2B for general use in Class B circuits, but it is intended at an early date to incorporate the products of the Amateur Wireless experiments in a receiver using the new type of Class B valve.
Characteristic curves of the new PM2B.
From Amateur Wireless May 27, 1933.