USE OF AN AMPLIFIER PART 2

The October article described some basics of a solid state RF amplifier. This time, a few facts about tube amplifiers.

Tube amplifiers are still in common use. It appears that the preference for tube amplifiers will be with us for a while.

Several of the tubes used are very rugged and durable devices. The life of many of these lasts for years even decades. This of course is if you operate it within its limits.

The tubes are mostly operated in a "grounded grid" configuration. What is a grid?

The grid would correspond to the gate on the MOSFET described in the last article. It turns the tube on, off or somewhere in between. It is the most delicate part of the tube. The current going through the grid should be monitored so it does not exceed its limits. Most amplifiers have a grid current meter, but not all.

The grounded grid amplifiers have the input at the cathode. This is normally the bottom of the tube and is closest to ground potential. This corresponds to the Source on a MOSFET. Biasing is usually at the cathode. There are other variations.

Bias is a set voltage that turn the tube partially on. When biased, there is normally a small current flow through the tube that can be seen on the Plate current meter. In standby, this current is off.

The RF from your transmitter is applied to the cathode through a matching circuit. Therefore, the power from the transmitter is added to the power of the amplifier.

The plate is where the high voltage is applied. This is a lethal voltage source. Extreme caution should be exercised when working on this part of the tube. The range of voltage is from 1200 volts and up. Common voltages are 2400 to 3500 volts and can deliver 1 amp. It can be fatal. This part of the tube corresponds to the Drain on a MOSFET.

From the Plate, the RF is sent through a matching circuit and then to the antenna. The impedance of the tube is high, therefore the matching circuit is used for the 50 ohms needed for the antenna.

If it a glass tube, it glows in the dark. Tubes need a heater called a filament to work. The filament can be also the cathode or it can heat the cathode independently.

The current through a tube is much lower than that of a MOSFET. The voltage is much higher. Therefore the tube is considered a voltage amplifier and the MOSFET a current amplifier.

Tube amplifiers are usually a bigger piece of equipment than a solid state amplifier.

A tube amplifier is more forgiving to operator error. Especially when over driven or SWR is not real low. The tuned input and output circuits help.

It is personal choice which type of amplifier to use. Both types, solid state or tube, have advantages and disadvantages.

73,

Ralph WD0EJA

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