Explanation of Self-Biasing/Cathode Biasing

[Stan Malinowski]

Can anyone give me an explanation of how cathode biasing becomes self-biasing? I understand how cathode biasing works...the current running through the cathode resistor creates a voltage which is negative relative to the grid voltage.

What I don't understand is how tubes of different types with different currents can be self-biasing. It seems like the more current flowing through the tube the greater the negative bias voltage would become. I'm confused....

Thanks For Any Input!,
Stan

 

[Old Fuzzface]

Have a look at Randall Aiken's web site.

Under "Tech Info" there's lots of good stuff including an explanation of biasing.

Aiken amps

 

[Stan Malinowski]

Ian,

Thanks for the input, but I have already been at Aiken's site. He explains cathode biasing to the degree I already understand it. He unfortunately does not explain the self-biasing ability of cathode biasing.

The search for knowledge continues........

 

[PaulC]

The problem with cathode bias IS the notion that it's self biasing. Sure it will generate it's own bias voltage, but that doesn't mean it's the right voltage! You can have over/under biased conditions just like fixed bias. This happens alot when you run into designs saying you can swap tube types without changing the bias resistor. Alot of times in those cases the bias is set to a medium point that will work for the different tubes the amp claims you can use. What you could wind up with is something like seeing 6V6's running cold class A/B, EL34's running about "right", and 6L6's running really hot. They might all sound cool, but one might sound great if the amp was biased up for just that tube. PaulC

 

[John Phillips]

Originally posted by Stan Malinowski
It seems like the more current flowing through the tube the greater the negative bias voltage would become.

That's correct. The more current that flows, the greater the bias voltage, which in turn reduces the current, so it is self-stabilising... for SMALL variations.

It is 'self biasing' provided the tube is within the right range to begin with - a slightly hotter-running tube will bias itself a little cooler, and vice versa. This means that if you're replacing tubes with the same type, you don't need to rebias.

But as Paul says, the same cathode resistance will not cope with wide variations, eg very out-of-spec tubes, or different tube types entirely.

For each resistance value and tube type, there will be a stable operating point, so by changing the resistor, you can easily rebias the amp.

 

[Stan Malinowski]

OK Paul, you're thinking along my thought lines! I was thinking you really would need to change the value of the cathode resistor as you change tube types! I guess setting the resistor value to a mid-point "safe value" will allow self-biasing to work, but probably not optimally. Makes sense that my Carr Mercury has an adjustable bias setting to support different tube types. My Carr Rambler supports only 6L6s and its bias is not adjustable. I think you have verified the conclusion I had come to!