Vacuum tubes
| Vacuum tubes | |
|---|---|
| Also called | Valves (British usage), tubes |
| Principle | Electrons boiled off a heated cathode flow through a vacuum to a positive plate, controlled by one or more grids |
| Main types | Diode, triode, tetrode, pentode |
| Triode introduced | 1906, Lee de Forest's Audion |
| In amateur radio today | |
| Main uses | HF linear amplifiers; restored vintage equipment |
| Common amplifier tubes | 811A, 572B, 3-500Z, 8877 (3CX1500A7) |
| Usual amplifier circuit | Grounded grid |
| Main hazard | Plate supplies of several thousand volts |
Vacuum tubes are the devices that made radio practical, and for decades almost every amateur transmitter and receiver depended on them. A tube controls a flow of electrons through a vacuum inside a sealed glass or ceramic envelope, so a small voltage on one electrode can control a large current to another. Transistors and other semiconductors have since taken over almost all amateur equipment, but tubes survive in two places: in high-power HF amplifiers, and in the classic tube radios known as boat anchors.
How a tube works
The cathode is heated, either directly, as a glowing wire filament, or indirectly by a separate heater. Heat gives some of its electrons enough energy to escape from the surface, a process called thermionic emission. The plate, or anode, is held at a high positive voltage and attracts those electrons, so current flows from cathode to plate. Because only the cathode emits, current flows one way, so a two-element tube, the diode, works as a rectifier.
A grid, a mesh or coil of fine wire between cathode and plate, controls the flow. Making the grid more negative repels electrons and reduces the plate current. A small change in grid voltage produces a large change in plate current, and that is amplification.
- A triode has a cathode, one grid and a plate.
- A tetrode adds a screen grid between the control grid and the plate. It shields the input from the output, greatly reducing feedback through the tube, and gives more gain.[1]
- A pentode adds a suppressor grid next to the plate, which stops electrons knocked loose from the plate from reaching the screen.[1]
The 6146, common in the final stages of tube-era transceivers, is a beam power tube, a tetrode that does the suppressor's job with shaped electrodes.
Tubes in the shack today
Most tube HF amplifiers use triodes in a grounded-grid circuit, with the grid connected to ground and the drive applied to the cathode. Popular choices include the 811A, used in sets of three or four in lighter amplifiers, the 572B and the 3-500Z.[2] Many of these are high-gain triodes that need little or no bias voltage in this circuit, which keeps the design simple.[1] The 8877, also sold as the 3CX1500A7, is a larger ceramic triode used in legal-limit amplifiers.[2] Tube amplifiers are rugged, tolerate a moderate mismatch and can be repaired, so many decades-old models remain in use. When buying an older one, note that before the early 1980s US power limits were stated as input power, so a "1 kW" amplifier of that era usually means 1 kW of input, and many older designs cannot run at today's limit of 1,500 W PEP output.[2]
In vintage receivers and transmitters, supplies of new old stock tubes keep many sets on the air.[3]
Neutralization and tuning
In a tube whose grid is driven directly, the small capacitance between grid and plate feeds some of the output back to the input, and that feedback can turn an amplifier into an oscillator.[4] Neutralization cancels it by feeding back an equal signal of the opposite phase, adjusted once when the amplifier is set up. A grounded-grid stage normally needs none, because the grounded grid shields the plate from the cathode, one reason the circuit became standard in amateur amplifiers.[4]
Tuning a tube amplifier means adjusting its output network, usually with two controls labeled tune (or plate) and load. The usual procedure is to start with low drive, peak the output with the tune control, then raise the drive in steps, adjusting load and tune for maximum output while keeping grid and plate current within the manual's limits.[5] The manual of one widely sold amplifier advises erring toward heavier loading, and reducing output by lowering the drive rather than by misadjusting the load control.[5] Tune into a dummy load and note the settings for each band.
Safety
A legal-limit tube amplifier may run its plate at well over 3,000 volts,[5] and its filter capacitors can hold a lethal charge after the power is off. Never defeat the cover interlock; unplug the amplifier, wait for the capacitors to discharge, and confirm that the high voltage reads zero before working inside.[5] Some large ceramic tubes contain beryllium oxide, which is toxic if the ceramic is broken or ground.[6] The restoration precautions described for boat anchors, including the shorting stick and the one-hand rule, apply to all tube equipment.
History
John Ambrose Fleming patented a two-element tube, his "oscillation valve," as a detector of radio signals in 1904[7] and published his findings in 1905.[8] Lee de Forest presented his Audion on October 26, 1906, and within weeks a grid was added between the filament and the plate, creating the triode, a tube that could amplify and, with feedback, oscillate.[8] In the 1920s tube transmitters replaced spark in amateur stations, reducing interference and greatly increasing range.[9] Tube equipment then dominated amateur radio until solid-state designs took over receivers and transceivers, and later most amplifiers.
See also
References
- ↑ 1.0 1.1 1.2 The Care and Feeding of Power Grid Tubes, Eimac, via ARRL.
- ↑ 2.0 2.1 2.2 HF Tube Amplifiers, ARRL.
- ↑ Tube Transmitters/Receivers, ARRL.
- ↑ 4.0 4.1 Grounded-Grid Power Amplifiers, E. E. Spitzer, RCA, April 1946.
- ↑ 5.0 5.1 5.2 5.3 AL-82 instruction manual, Ameritron.
- ↑ Operating Hazards, Communications & Power Industries, Pub. 3386, Rev. J.
- ↑ Milestones: Fleming Valve, 1904, IEEE Engineering and Technology History Wiki.
- ↑ 8.0 8.1 The Vacuum Tube Celebrates 100 Years, James E. O'Neal, Radio World, February 2007.
- ↑ Amateur Radio Through the Decades, IARU.