6V6 and 6F6 fixed bias grid No1


2009-09-09 2:55 am
I was reading the datasheet for the 6F6 tube checking whether with small adjustments could try it in place of a 6V6 in a Fender Princeton and I noticed that it "requires" for fixed bias operation a grid resistance of no more than 100K (some datasheets 50k) to prevent a runaway condition, while for auto-bias can use up to 500K.

The same applies for 6V6 tube, accordingly to the datasheet.


But checking at schematics most of the times, the two resistors that connects the control grid to the bias source is about 220K, for other tubes like EL84 it's about 470K (when the datasheet states 300K) and so on. SO my question is should I stick to the datasheets in this regard ? what's the benefit (with fixed bias) to have a higher grid resistor apart from the input impedance.