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A space-saving and very practical idea was recently suggested by W2SGJ, using a miniature tube (6AK6, 6AQ5, etc.), connected as a tetrode, in the protective circuit for a screen-grid amplifier. After trial, we can safely say that for a given tube the arrangement provides more effective clamper action than does the more commonly used low-μ triode circuit. Referring to the characteristics of a pentode or a tetrode tube, it will be seen that the plate current is more dependent on screen voltage than it is on plate voltage. Therefore, if the screen voltage of a multigrid tube is maintained as some suitable value, the normal plate current of the tube will not be too greatly affected by reduced plate voltage. Naturally, this feature is ideal for clamper-circuit operation because it means that the protective circuit will continue to draw heavily through the screen-dropping resistor even after the plate voltage (screen voltage for the r.f. amplifier) has been reduced to a very low value.
Fig. 1 is the schematic diagram of a clamper circuit which uses a Type 6AQ5 tube. The circuit differs from the standard low-μ triode layout in that the screen of the tube is fed from a fixed voltage source. R1, R2 and RFC1 are all normal r.f. amplifier components. The voltage applied to the screen grid of the 6AQ5 may be obtained from the screen circuit of one of the exciter stages or it may be taken from the low-voltage supply through a dropping resistor. In any event, the applied voltage must be less than the value which will cause the screen-grid dissipation rating to be exceeded. A potential of approximately 130 volts appears to be maximum for the 6AQ5.
Fig. 1. Circuit diagram of the tetrode damper using a miniature tube.
It is logical to assume that the screen-grid voltage for the tetrode clamper could be obtained from the amplifier high-voltage supply if the latter is one of the low-power jobs. If this system is employed, it may be necessary to tap the screen onto a voltage divider connected between the h.v. supply and ground. A simple series-dropping resistor may be used between the supply and the screen if the voltage does not rise too high when the clamper tube is cut off.
The above circuit will be of special interest to anyone who wants to clamp a 6146 - a tough tube to hold down. The writer has used both the 6AK6 and the 6AQ5 to clamp a 6146 to 15 mA when the amplifier was operating with 360 volts on the plate. Under the same conditions, a conventional triode clamper held the current to no less than 100 mA. - R. B. Haner, WIFBA
Morrow's Nova: Here in the ARRL lab we found that the 6AQ5, operated as a tetrode clamper, will draw approximately 30 mA plate current under the following conditions: Ep 500 volt; plate-dropping resistor 15 kΩ; Eg2 130 volts; g2 13 mA; Eg1 0 volt.