tmbg
Well-known member
Ok... now that this amp is doing what I expect, I'm trying to draw some conclusions about its biasing... I'm reading the Aiken paper on biasing, trying to become more learn-ed.
The amp is an AB push-pull 50W amp using EL34s, I think this is what you'd call fixed-bias, in that the cathodes sit at ground and the grids are fed through a bias network from a -34V source (adjustable via the bias trimpot.
I measured the idle current using one of Aiken's methods, by measuring the open resistance of each tap of the output transformer, then measuring the voltage of each leg relative to the CT, and finding the current that way. I measured 108.1R, 3.76V ~= 35mA on one side, and 72.6R, 2.55V ~= 35mA on the other, and those values seem more than reasonable, so I'm confident in that measurement.
Now, Aiken's page says "The plate load impedance is extremely important, as it sets the maximum average plate dissipation under signal conditions, and thus, the maximum idle current that can be used." I don't have any good way to know the load impedance, as the documentation I have does not mention the ratio of the Output trafo at all, so I had to resort to measurement.
I fed the secondary a 1KHz, 1VRMS sine wave, and observed a 21.47VRMS sine wave on the primary, so 21.47:1 would be the turns ratio, making the impedance reflection ratio 461:1. As the load speaker is 8R, that makes the reflected load 3688R. (hopefully I'm still sane at this point).
His paper then goes on to show a bunch of graphs taken from a piece of software that does calculations based on the plate load, plate voltage, and idle current, but he doesn't actually give the method to calculate such, and just uses that as proof that 70% of the max plate dissipation is a good bias point.
ANYWAY, so I figure that 35mA at 420V is 14.7W, which is only 59% of the maximum 25W dissipation for the EL34, and that 70% would be 17.5W, which at 420V is 41.67mA. Is that a reasonable value, and should I tweak it up?
Thanks folks!
The amp is an AB push-pull 50W amp using EL34s, I think this is what you'd call fixed-bias, in that the cathodes sit at ground and the grids are fed through a bias network from a -34V source (adjustable via the bias trimpot.
I measured the idle current using one of Aiken's methods, by measuring the open resistance of each tap of the output transformer, then measuring the voltage of each leg relative to the CT, and finding the current that way. I measured 108.1R, 3.76V ~= 35mA on one side, and 72.6R, 2.55V ~= 35mA on the other, and those values seem more than reasonable, so I'm confident in that measurement.
Now, Aiken's page says "The plate load impedance is extremely important, as it sets the maximum average plate dissipation under signal conditions, and thus, the maximum idle current that can be used." I don't have any good way to know the load impedance, as the documentation I have does not mention the ratio of the Output trafo at all, so I had to resort to measurement.
I fed the secondary a 1KHz, 1VRMS sine wave, and observed a 21.47VRMS sine wave on the primary, so 21.47:1 would be the turns ratio, making the impedance reflection ratio 461:1. As the load speaker is 8R, that makes the reflected load 3688R. (hopefully I'm still sane at this point).
His paper then goes on to show a bunch of graphs taken from a piece of software that does calculations based on the plate load, plate voltage, and idle current, but he doesn't actually give the method to calculate such, and just uses that as proof that 70% of the max plate dissipation is a good bias point.
ANYWAY, so I figure that 35mA at 420V is 14.7W, which is only 59% of the maximum 25W dissipation for the EL34, and that 70% would be 17.5W, which at 420V is 41.67mA. Is that a reasonable value, and should I tweak it up?
Thanks folks!