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The short answer as to way I like them...

Posted by Nickel Core (A) on November 8, 2014 at 14:25:36

In Reply to: RE: Btw Dave... posted by dave slagle on November 8, 2014 at 13:59:41:

Is that your amp sounds way better using a grid choke then a grid to ground resistor ;)

However, this is what I learned from Andy Grove on this subject:

The DC resistance of the grid circuit must also be kept low to control the effect of another rather annoying bugbear, grid current. Unfortunately, the vacuum in many modern valves is far from perfect, so there are quite a few gas ions floating around inside the bottle. Some of these ions will collide with the grid and draw electrons from the grid circuit. If the grid resistance is high, the grid bias will be modulated in tune with the signal, a real no-no in my book.

Also the grid may occasionally be driven positive on signal peaks, causing the grid cathode diode to conduct, rectifying the input voltage in the manner of a shunt diode supply with the decoupling cap as the reservoir. This action makes the bias voltage more negative, reducing the quiescent current through the tube, sometimes to the point where it will only conduct on peaks (Class C). In fact, a severe peak can cause the amp to cut off altogether, resulting in a total loss of output.

Worse still, the grid resistor/coupling cap combination as an RC time constant, so the effect lasts for some time after the overload has passed in sort of a time-delay distortion mechanism.

Reducing the grid resistor to combat these effects is no solution. We want a DC grid resistance similar in magnitude to the impedance of the driver valve, i.e. a few hundred ohms, not a few hundred kilohms.

Making your grid resistor 600 ohms will likely kill the driver stage and, anyway, would require a coupling capacitor so big that the RC time constant would put us right back where we started.

To cure the voltage swing problem requires a circuit element which has low DC drop but a high AC impedance. Plus, we need a low DC resistance in the grid of the output valve.

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Sonic Craft

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