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Vacuum Tubes above 100 MHz

July 20th Fact-of-the-Day

Several factors degrade the performance of vacuum tubes with increasing frequency. Interelectrode capacitance, lead-to-lead capacitance, lead inductance, and lead mutual inductive coupling tend to be the most obvious problems. Electron transit time from cathode to plate is another significant problem that is often overlooked. Cathode-to-plate electron transit time is an insignificant fraction of cycle periods at low frequencies, but electron transit can take ten-percent or more of a cycle a 100 MHz and the transit time percentage increases with increasing frequency. Electron transit time can be reduced by moving a plate and cathode closer together, but that increases interelectrode capacitance and reduces the internal breakdown voltage. Transit time can be reduced by using higher plate-to-cathode voltage, but the maximum voltage is limited by the internal breakdown voltage. In general, vacuum tube performance above 100 MHz is improved by using a plate-to-cathode voltage that is just below the internal breakdown voltage. ©2005 Martek, International. All rights reserved.


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