Snafux,
The 5 Watt resistor drops the approximately 15V AC from a 12 VAC @ 1A plugpack,down to about 12.6V AC to run the heater in a 12AX7 Valve,so it should be connected between where the 12V AC connects to the circuit board,and pin 5 on the Valve socket,in Lutors layout,it doesn't have the 5Watt resistor,but in Bajaman's version,there is one,I personally think Bajaman's version is the best with regards to the hum issue....
I'll explain how a Valve works,using a Triode in this explanation,but first,I'l make this statement,since it will help explain what happens,Like charges repel,unlike charges attract....
Okay,when you apply power to the heater pins of a 12AX7 type valve (pins 4 and 5),a filament inside the valve gets hot and glows,like an incandescent light globe,now around this heater filament is placed a thin metallic cylinder,this is called a "Cathode",the heater filament heats the cathode up so it glows a dull orange colour,now,the cathode is coated with a chemical compound that emits electrons when it is heated,an electron is a very small atomic particle that has a "negative charge",these electrons boil off the cathode much like water vapor boils off boiling hot water,around the cathode is placed a mesh of very fine wire,this is called a "Grid",I'll explain what it does shortly,finally around the cathode and the grid,is placed a third electrode which is made of thin metal sheeting,this is called an "Anode",when you put a positive charge on the anode,by connecting it to a positive point in a circuit,the electrons which are negative in charge,are attracted to the positively charged anode,we then have an electric current flowing from the cathode to the anode(I'm using what's called the "electron-flow"convention here),this electron flow is in itself,not very useful to us if we want to use the Triode Valve as an amplifier,so we need some way to control the current flow from the cathode to the anode,this is what the grid is for,if we put a steady negative charge on the grid,the electrons boiling off the surface of the cathode,are repelled back to the cathode,this has the effect of reducing the current flow from the cathode to the anode,or in other words,we are turning the current flow off,the more negative we make the grid,the more we reduce the current flow from the cathode to the anode....
Just a little bit of Valve Theory,but I hope it helps you to understand how Valves work better,Snafux....
Genius is not all about 99% perspiration, and 1% inspiration - sometimes the solution is staring you right in the face.-Frequencycentral.
It is probably there to protect against unregulated power. Pete may have dropped it because the power in David's board is regulated, so it's not really needed.