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The U17 is an EHT rectifier and suited to providing the anode voltages in CRT based circuits. An oscilloscope CRT normally has electrostatic deflection plates which, especially in older designs, required quite large signal swings (+ hundreds of volts) in order to achieve deflections up to + a diameter. It was therefore necessary to provide horizontal and vertical deflection amplifiers capable of very large output voltage swings and this in turn would mean that the deflection amplifiers would require special valves operating from an anode supply of, perhaps, 1,500 V. Additionally, potentiometer chains consuming several mA would be needed to move the zero trace up and down the screen.

Since the deflection sensitivity of an electrostatically-deflected CRT varies according to the EHT voltage applied to it, it was essential to use the same rectified supply for the deflection circuits as for the EHT applied to the CRT itself. Thus, the EHT rectifier must supply enough current for two deflection amplifiers and two potentiometer chains in addition to the cathode current of the CRT. With this in mind it can be appreciated that the rectified current rating (30 mA) of the U17 was by no means excessive.

As heater-cathode insulation would have posed design problems, the valve is directly heated. The filament passes six times through the anode cavity as a continuous wire making the shape of three inverted V's. The reservoir capacitor can be 1 µF. The normal current for a CRT power supply would be less than 5 mA.

The faint U17 Type designation enhanced. The other side of the glass carries the AM of the air ministry and the crown.

Side view of the anode with the supplementary lettering on the glass.

The lettering enhanced reveals the old stores code of 10E/19B as well as the crown and AM. Below are the words Made in England by the M.O. Valve Co. Ltd. The stylised GEC letters are below this other wording.

The bright anode face view.

The coated filament is a flat ribbon.

The classic envelope is 49 mm in diameter and, excluding the B4 base pins, is 119 mm tall.

References: Data-sheet & ¹⁰⁴⁰. Type U17 was first introduced in 1936. See also 1936 adverts.

Pin Connections

Absolute Maximum Operating Conditions^¶

Thanks to Frank Philipse for supplying the above PDF datasheet.