Power tubes protected against flash-overs and arcing

The Rocky Point effect identifies the flash over that may occur in high-power transmitting vacuum tubes. The name derives from the Long Island site, Rocky Point, where a big RCA transmitting station was installed since the days of GE Alexanderson Alternators. The flash over occurs in the tubes probably because of hot spots, due to the same RF fields or to soft X-rays, with gas emission inside the envelope. The flash over could trigger an arc sustained by the energy stored inside the filter capacitors, that is proportional to the square of the operating plate voltage. The huge energy available, in the order of thousands joules, cause heavy damage in the tubes, from cathode poisoning up to the vaporization of parts inside!

Usually a stabilization process was run on tubes intended to be operated at high voltage: anode voltage was slowly raised to the maximum specified value, carefully watching for sparks or arcing inside. When faults occurred, the tube was stabilized for a while at a slightly lower voltage, before any further voltage ramp-up. This process ‘cleaned’ the new tube from internal burrs or contaminating particles, but of course could not prevent flash overs due to different causes through the tube life.

A simple way to prevent arcing was the use of a series current source, as a tungsten-filament temperature limited diode, but efficiency dropped to unacceptable or barely acceptable levels. The crowbar circuit, first proposed by RCA in the late forties, was an electronic switch, as a thyratron, in parallel to the high-voltage power supply, that shorted the filter capacitors in case of faults. The switch was triggered by an overcurrent sensing circuit. Several thousands joules were instantaneously discharged, but expensive transmitting tubes were safe, before ordinary protections could remove primary power! The drawback of this solution was the possibility of damages to the same protecting components, even due to electrodynamic effects of the huge peak currents involved.

A modified ‘crowbar’ was proposed by Roger G. Wenner of ITT, Federal Telecommunication Laboratories. The article appeared in Electronics, June 1957. In this case the switch, a 3W5000F3 power triode, is connected in series between the power supply and the transmitting tube. The 3W5000 is normally biased for a heavy conduction, its voltage drop being dynamically adjusted to some 2 percent of the supply value. The overcurrent sense circuit fires a small 2D21 thyratron, that drives the series switch triode well into cutoff region. In case of faults, power to transmitting tubes is switched-off within microseconds, preventing dangerous arc build-up. The article also suggests possible circuit variants to fit different design constraints.

The circuit can be easily modified and adapted to protect audio power tubes from overcurrents due to shorts or failures of the grid bias circuitry.

Here is the basic circuit:

The full article, scanned from Electronics, is available below as attachment.

• Crowbar from Electronics, June 1957 (467 KB)

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