Rf Stage Grid

advantage of this system's capabilities. If your voltages axe radically different, the value of the cathode resistor should be varied to achieve the best compromise between satisfactory voltage delay and effective avc action. The purpose of the voltage delay is, of course, to prevent avc action on weak signals, permitting maximum sensitivity until a certain signal level is reached.

The operation of the circuit is quite simple. On weaJk signals, the rectified signal voltage is not great enough to overcome the positive voltage in the grid circuit, so no avc voltage is developed. When the rectified signal voltage exceeds the positive voltage, the triode is biased in the direction of cut-off, the cathode goes negative in proportion to the signal strength, the attack gate is forward biased, and a proportional degree of the negative bias is applied to the avc line through the "hang" gate. Should the rectified signal voltage be great enough (as when operating full break-in), the triode will cut off, and full bias voltage will be applied to the avc line, muting the receiver.

An additional form of delay is introduced by the voltage divider through which the avc voltage is applied to the rf stage. This keeps the sensitivity of the rf stage high enough to preserve a satisfactory signal-to-noise ratio on moderately weak signals which activate the avc but need some "help'1. This is not a technique to correct a design flaw, but rather allows the avc to act early enough to maintain satisfactory control over the output level without masking the signals in noise. Of course, as has been pointed out, on weak signals, no avc voltage is applied at all.

The divider sets the decay time constants of the system. As it is shown in Fig, 1, decay times of 140 milliseconds, 500 milliseconds, and 1 second are provided in the Fast, Medium and Slow positions respectively. Attack time is less than 75 milliseconds in any position.

RF gain control is achieved by biasing the avc line through a silicon diode, which prevents further loading of the line. The diode is reverse biased until the negative voltage applied through the rf gain control exceeds the avc voltage, so the control has no effect until that point Beyond that point, the avc is inoperative unless an extremely strong signal would cause the avc voltage to exceed the rf gain bias. This would, or course, reverse bias the diode again, permitting the avc to take control. This method of control offers some inter esting possibilities, Turning the avc switch to the off position bypasses the diode, loading the avc line heavily enough that the avc voltage is "killed". After you've used this system for a while, you'll probably find that you never have occasion to use the rf gain control at all, I guarantee, this is one avc that will be left on.

The superiority of this system is evident. As installed in my 75 A2, it holds the output within 6 db on all signals, and 1 have yet to encounter any signal strong enough to overload either the AM detector or the product detector. Because of the delay, weak signal work on 6 meters is greatly facilitated. Signal levels which previously would not yield copy now give Q-5 copy, in many cases.

One final word . , . a high noise level can render the delay function of the system completely ineffective. However, the great benefits of amplified avc action are no disturbed. A good if noise blanker will make it possible to derive the full benefit of this avc system, and you should definitely consider adding this feature to your receiver. I'm presently developing a simple blanker, and when it's finished, 73 will be the first to know. . # . W8RHR

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