RobBozic
09-29-2008, 10:39 PM
This is taken from Randall Aiken's website where he talks about biasing. I built a cathode biased AB763 type amp with 6L6GC's, and I want to calculate what the bias shift will be at the maximum signal flow.
"Cathode-biased class AB amps are usually exempt from the "70% rule", because their cathode voltage rises when a signal is applied, effectively reducing the bias, and shifting the amp further into class AB operation. This means you can bias them hotter than a normal fixed-bias class AB amp and the tubes will still survive. Having said that, you have to experimentally determine how hot you can bias them by finding out how far the bias shifts during signal flow.
If the cathode-biased amp is "true" class A, there will be no bias voltage shift seen on the cathode when signal is applied, so you can bias at max dissipation and not worry about it. If the amp is actually class AB, you might still be able to get away with biasing at max dissipation because of the large bias shift at full power that pushes the amp into the class AB region, but you should check the tube dissipation at all signal levels. Note that max dissipation may not occur at full power, rather at somewhere between idle and full power (usually around halfway), so you have to carefully determine the safest max idle current to avoid exceeding the dissipation at any point in the tube's operation."
My amps readings are as follows: Plate Voltage = 404V, Pin 4 = 404V, Pin 8 = 28V, Cathode resistor = 250R. My dissipation at idle is 21W per tube, now how do I calculate what the max dissipation will be at maximum signal flow?
thanks in advance
Rob
"Cathode-biased class AB amps are usually exempt from the "70% rule", because their cathode voltage rises when a signal is applied, effectively reducing the bias, and shifting the amp further into class AB operation. This means you can bias them hotter than a normal fixed-bias class AB amp and the tubes will still survive. Having said that, you have to experimentally determine how hot you can bias them by finding out how far the bias shifts during signal flow.
If the cathode-biased amp is "true" class A, there will be no bias voltage shift seen on the cathode when signal is applied, so you can bias at max dissipation and not worry about it. If the amp is actually class AB, you might still be able to get away with biasing at max dissipation because of the large bias shift at full power that pushes the amp into the class AB region, but you should check the tube dissipation at all signal levels. Note that max dissipation may not occur at full power, rather at somewhere between idle and full power (usually around halfway), so you have to carefully determine the safest max idle current to avoid exceeding the dissipation at any point in the tube's operation."
My amps readings are as follows: Plate Voltage = 404V, Pin 4 = 404V, Pin 8 = 28V, Cathode resistor = 250R. My dissipation at idle is 21W per tube, now how do I calculate what the max dissipation will be at maximum signal flow?
thanks in advance
Rob