vvr bias differences

[andrew]

Hi, I put in a vvr in my cathode biased expressish clone. At 350v and a 300ohm cathode resistor I get 26v across the resistor = bias at 44. I lowered the voltage to 250 and got 18 across the cathode resistor = bias at 28. According to a bias chart I am using the bias at 350v is in range but at 250v it is way too low and should be about 60. Please help me understand this. Thanks

[andrew]

The better question is why does it still sound good when the bias is so low at 250 volts ?

[martin manning]

That sounds about right- all the voltages will come down proportionally, and the current a bit faster. It's biased pretty cold for cathode bias at 350V: I get 26/300/2*350=15.2W per tube, including the screen current.

[printer2]

The tube has its own internal resistance and it needs voltage to develop across it. Just because you reduce the voltage does not mean the voltage the tube needs across it will go down in step.