Roger,
Could you explain to an idiot (me) how infinite resistance for input impedence is better for the source load? Since lowering the input impedence (from infinite to whatever) can load down a source then how come infinite input impedence doesn't REALLY load down a source? Is it because the input grid resistor is tied to ground? How would things change if you removed the grid resistor altogether? Am I even right in assuming the grid resistor is the load presented to the amp's source?
We are talking about the grid resistor that goes from the grid to ground. It is often called the "grid return resistor". It is in parallel with the input jack and determines the input impedance of the amplifier. Transistor amps also have return resistors or bias resistors that determine their input impedance which is typically much lower. A FET (field effect transistor) can have very high input impedance as it acts like a tube and needs no input current. Bipolar transistor inputs require some DC current so the bias resistors cannot be very large unless the input current is very small.
When the impedance is infinite there is no current drawn from the source. Perhaps numbers will help. If the load is one ohm it will draw one amp of current at one volt. A power amp can supply one amp but a preamp can't. If the load is 1,000 ohms the current drawn is only one milliamp and most preamps can supply that. We don't want to load the source at all because drawing current causes more distortion than not drawing current. Almost all amplifiers will have lower distortion into a higher impedance load. Thus I wrote the paper on "light loading" for the RM-10. It applies to all amplifiers.
Most tube preamps can put out 20-40 volts but not into 1,000 ohms because that would require a current of 20-40 mA. Because the output stage of most preamps is class A the output current is limited to the idle current which can be as low as a milliamp and rarely more than 5 -10 mA. There is really no need for more current unless the amp being driven has a very low input impedance.
As to the grid resistor. It is the load presented to the source as you said. It can be very high for tube amps as the grid draws no current and just needs a return path to ground so that it sits at zero volts DC. Most signal sources have a resistor to ground at their output so in those cases the grid can just go to the the input jack with nothing to ground. I use 100 Kohms to one megohm for that resistor just in case there is no DC path to ground supplied by the signal source.
Perhaps it will help to consider the impedance of things in your home. A 10 watt light bulb is about 1000 ohms when lit. A 100 watt light bulb is about 100 ohms. A 1500 watt resistance heater is about 8 ohms and makes a great speaker load. Bruce de Palma told me that and I've used it. Resistance heaters tend to keep their resistance constant with heat where light bulbs don't. If you measure a 100 light bulb with an ohm meter it will measure about 10 ohms and the resistance will rise as you warm it with your hand.
