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Zahl Tube model |
| The prototype tube was invented in 1939 by Dr. Harold A. Zahl to operate at 250,000 watts of 600 MHz pulsed
power. The VT-158 has four triodes grouped in two pairs, each in a push-pull arrangement. The anodes and grids of the pairs are tied together by inductor loops. The inductors of the anodes and grid are in close proximity, magnetically coupling to each other causing a regenerative feedback link, making the tube an oscillator. The cylindrical portions of the anodes have horizontal fins to help cool the anodes. Glass stems around the connections brought out of the tube are doped with uranium (yellow/green areas) to decrease the coefficient of expansion of the glass, increasing the ability of the glass seals to withstand heat. The grids are biased via one connection center tapped to one of the internal inductors. Each of the anode and cathode pairs are brought out to connections so that an additional, external loop can be used for tuning, using a shorting bar. The model shows a slider-tuned loop connected to the cathode pairs. A simple ultraviolate light brought near an of the glass stems causes them to fluoresce a green color. Many hand magnifiers have an L.E.D. "black light" on them to conveniently obtain the glow. This is also true with many old power tubes of the time, such as the "250TH" and other "TH" and "TL" tubes manufactured by Eimac. The tube is about 190mm long by 90mm in diameter. |
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View from inside, looking up through anode barrels. |
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Envelope showing model topology. Original envlope shape was done by reshaping a cylinder vertically with a reference image, then rotated to Horizontal. Symmetry modifier was used twice. |
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Four Barrel Anode assembly model with inductor loops. |
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Grid and heater assembly model with inductor loops.. |
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Combined Anode, Grid and Heater assemblies. |
| References: · “Radar Spelled Backwards”, Harold A. Zahl, Vantage Press 1972 · “The Secret Tube that Changed the War” by William Orr, W6SAI · http://n6jv.com/museum/articles/vt158.pdf |
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