by Bob Latino Wed May 22, 2013 3:28 pm
Hi Gordy,
There are those who say to use a relatively small (1/2 watt) bias resistor on the theory that if a tube goes "bad" that resistor will blow, cut off current flow to the tube and "save the tube". If the tube is BAD then the tube should be replaced. I use 10 ohm 2 watt bias resistors on all the VTA amp kits. A 2 watt bias resistor can withstand a "transient" issue inside a tube. Sometimes a (usually NEW) tube experiences a momentary "blip" or transient short between two elements of an output tube that are not normally connected. A 2 watt resistor can usually withstand this transient whereas a 1/2 watt resistor may blow. Many times the tube is perfectly fine after this momentary "blip".
In your case, I would just get some 10 ohm 2 watt resistors for your two Mark IV's. Changing out this resistor will have no effect on the sound of the amp since this resistor is not directly in the signal path. This resistor is there to give each output tube a reference to chassis ground. Without this resistor in place, the tube will still light up but will not "work" without the reference to chassis ground. I would not change this resistor to a 1 or a 100 ohm resistor (although you could) because then your bias voltage would have to be different by a factor of 10 in order for each output tube to draw the required 40 milliamps of current.
Bob
