=CB= said:Patrick from Davis said:Sorry, forgot to mention that the schematic I put up there has nothing to do with the amp that started the post. Just an example of the 68k resistor in question...
Well considering how 12AX7's dont have screen grids... no wonder they took it out!
That 68k resistor is usually called a grid resistor or grid-input resistor or isolation resistor. It serves two purposes really, or maybe three. What it does is provide a constant relationship between the grid input, pickup (or mic) and the grid load. This is important in many designs in order to maintain bias on the input stage, depending on the pickup used. In practice it isolates the grid itself from the pickup. The second thing it does, is isolate one input from another, such that the instruments controls don't interact (too much). You will still get some interaction if say... you have one guitar played with max treble cut and the other not. In that regard, its a mixing resistor as well. The third thing it does, maybe... is provide some protection in the rare case of grid shorting to plate. This "has" been known to happen, and of course, when it does... you get B+ on the pickup for a brief moment right before it blows the windings..... a capacitor input would be great protection, but there are other problems with that... however the resistor input does offer some degree of imperfect protection under such conditions.
But its no screen grid resistor.
Another reason for this resistor is that it interacts with the capacitance with the grid of the first input stage to create a low pass filter. The knee of this filter is in the RF range, which should prevent you from picking up the local police radio. It also aids in the stability of this stage by moving the pole so as to not create a positive feedback loop and thus oscillation in this stage, again up in the RF range.
Having said all that, I'm glad that the amp is working for you. BTW - how does it sound?