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  Cathode Interface
Cathode interface resistance caused circuit problems for years, and was a direct result of the manufacturer's efforts to produce high-emission cathodes at low cost. This effect is caused when a thin resistive layer forms between the cathode's base metal and its barium-oxide coating after the tube has been in service for some time. When the resistive layer expands, a capacitor is formed. The base metal becomes one plate, and the coating the other. In a new tube, the interface resistance is nearly zero, but over time it can increase to as much as a few thousand ohms. The total capacity can approach .01uF. This capacity acts as a load on circuit constants, and partially explains why a particular tube might have differing performance in different pieces of equipment. The newly-developed resistance and capacitance has an associated R-C time constant, which will affect an amplifier's performance by changing its rise time and frequency response.
When a fast rising signal is applied to a stage having cathode interface resistance, the interface capacity acts as a direct short across the interface resistance for a short interval. The resultant RC time constant causes a loss of low frequency gain, while the high frequency gain is nearly unaffected. A fast-rise square wave will show cathode interface problems as overshoot on the leading edge. It is most noticeable in broad-band amplifiers and in circuits where plate current is cut off for long periods. A typical broad-band amplifier is found in the vertical channels of your trusty tube-type 'scope. A tube that spends long periods under cut-off might be found in transmitters, noise blanker pulse formers, or other equipment using gating and multivibrator circuitry.
This page is derived from an article originally published in Electric Radio magazine, Issue 54, October 1993. The article, "Thermionic Mysteries", by Ray Osterwald, NØDMS, explains various vacuum tube phenomena, including "cathode interface", "blackout", "dc shift", "stray emission", "signal grid emission", and "mica charge". This extract is reproduced with the permission of Electric Radio.
Back issues of Electric Radio, including issue 54,
are available at $3.00 per issue (U.S.) from:
Electric Radio,
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Hesperus, CO 81326

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Last modified on  2/29/00 10:08:06 PM