Cathode Biasing a 6L6 quartet power stage

[Dave Z]

I have a tube guitar amp that has 4 6L6's in the 100W power stage. I want to convert it to a Cathode Bias configuration. I have heard two ways of doing this. The first is that ALL of the power tubes would have the cathodes connected together and then have a SINGLE cathode resistor with bypass cap to GND. The second method I've heard is to have the two tubes which are in parallel on the same phase of the output transformer, to run through a common cathode resistor with a bypass cap, and the other two tubes on the other phase run though another common cathode resistor with bypass cap. I was hoping for comments on each method. Are both valid? Tone differences? Matching difficulties? I would be checking the output with a scope and adjusting the bias based on visual illimination of crossover distortion. I want to a point of reference what different levels of crossover distortion sounds like for the dirty channel of the amp

Thanks
Dave

[SY]

Common cathode resistors generally work best with matched tubes. Otherwise, one tube does more work than the other (push and pull are not symmetric). And idle current mismatches will tend to increase output transformer distortion. This may or may not be desirable in MI amps. You could either have one resistor in common for all tubes, two resistors separate for each side, or even four resistors individual to each tube.

One thing is for sure- the size of the cathode resistor's bypass cap will very much affect tone and overload. That might be the most interesting thing to experiment with.

[tubewade]

Dave,

S.Y. may correct me. Going to cathode bias will cut your power nearly in half. May be a good thing. It will also move your operating point close to Class A. In Class A both valves are always on and there will be no crossover distortion. If you use a single cathode resistor with nearly perfectly matched valves you can probably dispense with the cathode bypass capacitor. I prefer the sound this way. On the other hand, if you use a separate resistor for each pair of valves, you must use a bypass capacitor on each, but you can use a configuration similar to that in a Williamson amplifier to balance the output stage even with less than perfectly matched valves. As for whether that is necessary in an instrument amp, only you can decide. As S.Y. said, the bypass capacitor and the amount of unbalanced current saturating the core of the transformer will probably have the greatest effect on the tone.

Best,

Wade