Re: Fixed bias vs self bias
Posted: February 20th, 2019, 3:01 am
Cogito wrote:dberning wrote:Traditional self bias requires the amp to draw more or less constant power regardless of the signal level. This implies class A operation. If you try to violate this by driving the tubes harder, the bias voltage will be unstable. Fixed bias allows Class A-B or B, and much greater power output for the same operating conditions for the tubes.
David
David, Can you explain the operational details of Fixed and Cathode bias? My question is specifically centered around the bias topology and how it effects the operational characteristics of the tubes.
For example, the grid voltage can be set to -5V and Cathode 0V which gives -5V grid bias
or
the grid voltage can be held at 0V and the cathode at +5V which again gives -5V grid bias voltage.
What exactly happens in the tube which makes the first one operate in AB mode and later in A mode?
Thx.
Whether the grid is kept at -5V or the cathode at +5V makes no difference. The difference comes in how that voltage is achieved. In fixed bias, the difference between the cathode & grid is fixed (at DC), hence the name. It makes no difference how hard the stage is driven, the DC difference between the grid and cathode remains constant. In a self-biased circuit, the bias is achieved via a voltage drop in a (usually bypassed) cathode resistor. During class A operation, the average current through the cathode resistor remains constant, so the DC bias remains constant. When a PP stage is pushed into class AB, the average net current in both tubes increases since the tube that's drive with an increasingly positive grid voltage continues to increase it's current flow, but the tube that is cutoff by the negative grid voltage can't have the current go less than zero. When this happens, the bias voltage is increased.
Roscoe