Electronic Design

Double-Frame-Grid Pentode Yields High Gain-Bandwidth

With both control and screen grids using frame-grid construction, a new pentode boasts performance features that far surpass those available in conventional pentodes. Compared with some of the better pentodes using conventional construction, the new tube, type 7788, is said to offer a gain-bandwidth figure-of-merit of 400 mc (vs. 130 mc), a transconductance of 50,000 µmhos (vs. 10,000) an equivalent noise resistance of 100 ohms (vs. 200 to 300), and a much higher plate-to-screen-current ratio.

Available from Amperex Electronic Corp., Hicksville, L.I., N.Y., the new tube minimizes the shift in characteristics when one switches from low to high anode currents. The rigidly constructed, precisely positioned control and screen grids yield a very low spread in characteristics from tube to tube, as well as very low levels of microphonics.

The sturdy construction provided by the grid frames allows the control grid to be made of wire only five microns in diameter—about half the thickness of the finest grid wire normally used. This allows more turns to be wound per inch while it minimizes grid current.

The 7788, with 6.3-V, 340-mA filament requirements, is rated at 35-mA plate current with a 135-V plate supply. The nine-pin miniature tube sells for $7.95 in quantities of more than 50. In smaller quantities, the price is $10.50. Delivery is from stock. (Electronic Design, April 12, 1961, p. 56)

Vacuum-tube technology was advancing despite the rapid growth of transistor sales. And sales of tubes were far from dead. The April 12, 1961, issue of Electronic Design carried many ads for tubes, from CBS, Mullard, Raytheon, RCA, Sylvania, and Tung-Sol, which were all major players in the tube market. It should also be noted, however, that the balance had already shifted: the issue carried about twice as many ads for transistors.

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