FROM

Issue #110
Scanned and OCR'ed by Grant Youngman, NQ5T

SCREEN GRID TUBES AT FIXED SCREEN VOLTAGE

A typical amateur modulator uses a pair of push-pull beam tetrodes operating at a fixed screen voltage. Unfortunately, this does not make a very linear amplifier. The dynamic characteristic curves of plate current versus control grid voltage show an upward curvature, followed by a gradual flattening off as they approach saturation. The result is distortion.

Generally, screen grid tube linearity is best at lower fixed screen voltages and worse at higher fixed screen voltages, but in all cases the curve shows that general upward curvature, followed by saturation.
 

AN EASY IMPROVEMENT FOR SCREEN GRID TUBES

Just place a resistor in series with the screen grid. In a push-pull stage, place one resistor in series with each screen grid, as shown in Figure 1. These resistors should be film or composite types of suitable power rating. If large dissipation is necessary, a series combination of lower value units should be used. Don't use wire-wound resistors, because they may introduce enough inductance to cause parasitic oscillations.

Full output power can still be obtained from the tubes if you raise the screen supply voltage enough to compensate for the voltage drop in the resistors at maximum screen current peaks.

I'm using push-pull 8417 beam power tetrodes in my modulator. This tube is similar to EL34 and 6550 and not very different from any of the 6L6 - 807 – 6146 class of tetrodes. I was running regulated 300 volts to the screens; I took two 820 ohm, 2 watt resistors, and placed one in series with the screen grid in each 8417, and raised the screen regulator output to 380 volts. Distortion went down!

On audio peaks, there is about a 60 volt drop in the screen resistors. By raising the screen supply voltage, the screen voltage on peaks is approximately the same as it was before the change. Otherwise, there would not have been as much output power from the 8417s as there was before the change.
 

THEORY: HOW DOES A SCREEN RESISTOR HELP?

Consider the screen current over the linear range of the tube. The screen current is generally low at control grid cutoff, and it increases until saturation.

Next, consider the effect of screen voltage on plate current. Generally speaking, the higher the screen voltage, the higher the plate current.

Now consider the effect of a resistor in series with the screen grid. The screen voltage will pull down below the screen supply voltage, by an amount which depends on the value of the screen resistor, and the current drawn by the screen grid.

When the control grid is near cutoff, the screen current is low, and so the screen voltage is close to its source voltage. As the control grid voltage increases, the screen current rises, and so its voltage drops further below the source voltage. The main effect of the resistor in series with the screen is to smoothly reduce the screen voltage at higher drive levels, which compensates for that troublesome upward curvature in plate current.
 

IT HELPS FROM CUTOFF TO SATURATION

The screen current characteristic of a screen grid tube tends to match the plate current characteristic. This causes the compensation introduced by the series screen resistor to tend to match the plate characteristic, further improving the compensation from cutoff to saturation.
 

SCREEN IMPEDANCE NEGATIVE FEEDBACK

The series screen resistor also produces negative feedback. For any given control grid voltage, any variation in plate voltage will affect the screen current in such a way that the resulting voltage drop in the screen resistor will increase the screen voltage if plate voltage increases, and decrease the screen voltage if plate voltage decreases. This produces negative feedback which further reduces distortion, and also reduces the plate resistance of the tube.
 

THE OPTIMUM SCREEN RESISTOR VALUE

There will be some optimum value for the series screen resistor.

The characteristics of certain tube types may produce a distortion minimum with some specific combination of operating conditions and series screen resistor value.

A desired plate resistance may determine the optimum screen resistor value in some cases. Output transformers usually respond best with some specific value of plate resistance. The larger the screen dropping resistor, the lower the plate resistance will be.

Available supply voltages and output power requirements may place a limit on the maximum screen resistor value. If output power is critical, and the plate and screen supply voltages can not be increased, then the optimum value would be just less than the value which begins to reduce output power.

For tubes in the 6L6-807-6550 class, try a screen resistor value around 1000 ohms. If you experiment with this resistor value, remember that P=I²R. In this circuit, a higher value resistor will drop more voltage and therefore dissipate more power.
 

TOO MUCH OF A GOOD THING

If a very high screen dropping resistor is used, output power will be greatly reduced unless the screen supply voltage is raised to a very high value. This can cause many problems: low gain, high input capacitance, excessive screen voltage around cutoff, screen secondary emission runaway, etc.

For tubes in the 6L6-807-6550 class, I would think the maximum value for the screen resistor would be around 10K.
 

THE WILLIAMSON/HAFLER CIRCUIT

I haven't tried the Williamson-Hafler circuit, so I'm not sure how different series screen dropping resistor values would affect its operation. However, I suspect that any number of balances could be struck between plate voltages, screen dropping resistor values and screen tap percentages.

Amateurs don't always have access to driver or modulation transformers with taps at exactly 40%, so a little experimentation with the resistor values and supply voltages may be in order.

Figure 2 shows screen dropping-resistors in the Williamson-Hafler circuit. Try using a screen tap percentage below 40% and adding screen dropping resistors; if the output is too low, try increasing the supply voltage. Be careful, because this will raise both plate AND screen voltage; note that the earlier warnings about excess voltages still apply.

GROUNDED-CATHODE SCREEN GRID TUBE RF LINEARS

Grounded-cathode screen grid linears offer high gain, but always generated high intermod distortion as well. A screen dropping resistor can reduce distortion here, too.

Figure 3 shows how the screen dropping resistor is applied to a grounded-cathode screen grid tube RF linear amplifier. The resistor simply goes between the screen and the screen supply, The screen grid is bypassed just enough for RF, because it must be allowed to swing in voltage according to the instantaneous value of the RF envelope.

Again, full output can be maintained by raising the regulated screen voltage somewhat to compensate for the drop in screen voltage caused by the resistor at maximum screen current.

In fact, a higher screen voltage can actually permit higher output in class AB1.

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