How the Tube Converts DC to RF Power

A typical vacuum tube radio-frequency amplifier has a high voltage power source. This power source supplies the energy for the RF output power. The vacuum tube acts like a non-linear variable resistance in series with a diode. The electrical equivalent of the anode and output system is:

R1 represents the time-varying anode to cathode resistance, as grid-cathode and anode-cathode voltage changes, R1’s value changes.

 D1 represents the directional characteristics of the tube anode-to-cathode path. It is always “on” when the voltage at “A” is more positive than ground. D1 would also behave as a current limiter, the limit being somewhat less than the saturated emission current of the cathode. 

Let’s examine how this circuit converts DC to RF power. We will assume +HV is 3000 volts.

In the initial state, C1 has been charged through the following DC path:

Hopefully the designer was smart enough to include RFC2, to prevent charging or discharging C1 into the load, which might be a sensitive piece of gear or the operator!

The voltage across C1 is the anode-to-ground voltage at A.  Everything past A, at B and VL, is zero volts in the resting state because of the low-dc resistance of RFC2 and the nearly infinite resistance of C1. RFC1 establishes a fixed magnetic field that is set by the quiescent current through R1.

As a positive-going grid-to-cathode voltage perturbs R1, the magnetic flux in RFC1 tries to hold supply current steady. RFC1 does this by increasing the voltage across its terminals. Energy stored in the electric field of C1 tries to hold the voltage steady, and it sources any additional current the tube (R1) requires.

The anode, in effect, “tugs” or pulls point A towards ground. The initial peak current can be quite high, because C1 has a low-reactance path to ground through C2. L1 also builds up a magnetic field, as C2 develops voltage from the current flowing through C2 and C1 back to the tube. On the initial RF cycle, the voltages hardly change before the grid is moving back negative.

At a time interval equal to 1/(4F) (where F is the RF frequency), the positive grid voltage has peaked. The grid begins to swing negative (or less positive) with respect to the cathode. Anode-cathode resistance (R1) decreases.

The collapsing field in L1 (and to a much lesser extent RFC1) tries to hold current the same, and in doing so the collapsing fields supply a small amount of additional anode voltage. Since there is no forced upward swing (the tube cannot source anything), the tank components are left to pull the small voltage change back up to the original anode voltage and slightly beyond. L1 (and to a lesser extent RFC1) actually pull the anode voltage higher by an amount nearly equal to the amount the than the tube pulled the voltage down. The actual amount the tank pulls this voltage higher than +HV depends on the amount of energy transferred to the load compared to energy stored in the tank system! This is VERY important in understanding where arcs come from!

This cycle repeats over and over, and assuming the tank system at C2, L1, and C3 is resonant, the voltage at point A increases in swing while the peak current through R1 (the tube) is gradually reduced. Eventually, in a fully loaded amplifier, equilibrium is reached. In a properly tuned amplifier running at maximum available power, equilibrium occurs when point “A” swings up and down an amount just under twice the anode DC voltage. 

Consider a class AB 3-500Z using a 3000-volt supply. When the amplifier is tuned into the load properly at full rated power, and driven to full power, anode voltage will swing between 5500 volts maximum and 500 volts minimum at point A. This would be a total anode voltage swing of 5000 volts. Peak anode current in continuous carrier operation would typically be 1.12 amperes, with a minimum R1 value of 446 ohms. This is typical of continuous CW power out of 750 watts with a single 3-500Z tube with a 190-degree conduction angle running at 400mA anode current.

Let’s assume (we actually CAN have this condition, if the amplifier and exciter relays are not sequenced correctly) we have an envelope that rises instantly. At the peak of first positive grid-cathode cycle, the anode resistance would drop to someplace well under 400 ohms (anode resistance is non-linear with anode voltage, and is lower with higher anode voltages). The saturated anode current would easily reach 7.5 amperes, if tube emission permits.

(Emission in a directly heated thoriated-tungsten tube is typically in the range of 50-100mA per watt of heater power, large transmitting tubes being at the upper end of that range. A full-emission 3-500Z has a saturated emission current of about 7.5 amperes.) 

If we have infinitely fast envelope rise and fall times from the exciter, amplifier RF-envelope rise and fall times are determined by the operating Q of the tank system. C2/L1/C3 dominate the high frequency energy storage. RFC1 and C1, being larger values, dominate lower-frequency energy storage, while stored energy in the power supply dominates long-term energy demands. The voltage across C2, upon initial application of RF drive, is a rapidly expanding sine wave. It reaches maximum steady-state swing many dozens or hundreds of RF cycles later than the initial tube excitation (and decaying in a similar fashion).

The peak voltage across C2 is equal to the peak anode voltage swing, and is slightly less than twice the HV supply voltage in NORMAL operation. The voltage across C3 is a function of the load resistance, and power delivered to the load.

Incorrect Loading or Load

We know the tank circuit stores energy. We now understand the conversion process where DC is converted to AC (or RF) power. We also must understand energy must be transferred out of the tank at a rate equal to or exceeding the rate at which it is supplied by the downward “pull” of the tube. If we do not remove energy at a sufficient rate, voltages and currents increase until a new point of equilibrium is reached.  Voltage at point “A” can actually swing well beyond twice +HV on upward excursions, and below zero volts (becoming negative) on downward excursions.

The maximum voltage with a load or drive fault can be tremendously higher than the typical working voltage of the tank system when energy is being removed at the proper rate.

We have the same basic tank system as discussed earlier, but with light loading compared to drive level the tank is pulled down very hard by the tube. Minimum Rp is reached early in the cycle, before the tank voltage reaches it’s minimum swing.  This is easy to do, since the tube only pulls the system down and the tank stores the energy of that downward tug.

As the plate voltage swings below zero (negative) from the tank energy, the tube is already cutting off. Nothing clamps or prevents point A from going negative. D1 effectively takes the tube out of the circuit. By the time the tank reaches it’s minimum, the tube’s grid-cathode voltage is already on its way positive. The tank free wheels positive, and can overshoot the +HV supply by several times the supply voltage. If loading is light enough and Q is high enough, this continues until the energy stored in the tank reaches equilibrium with energy transferred to the load, or a component fails and the arc dampens the tanks gyrations.

See the practical demonstration page.