Kind of hard searching for results, so many questions on bias, like trying to find a needle in a haystack. Now this is not a Instrument question, just came across this when killing some time watching videos. In this video Uncle Doug adjusts the bias on the amp from 40% to 70% and in each increase in bias current there is an increase in gain and an increase in power out. Is this real or an error in measurement? We will skip ahead a little.
And the final result.
Is this right and why would there be more power with more bias current? Inquiring minds want to know.
And the final result.
Is this right and why would there be more power with more bias current? Inquiring minds want to know.
The more bias, the more the output can swing negative before it gets to zero current.
Suppose you have 50mA, then the output can swing from 0 to 100mA for a sine wave undistorted.
With 80mA bias you can swing from 0 to 160mA.
So with more bias you have more output swing.
The gain doesn't change, because to get higher output swing you also need higher input swing.,
Jan
Suppose you have 50mA, then the output can swing from 0 to 100mA for a sine wave undistorted.
With 80mA bias you can swing from 0 to 160mA.
So with more bias you have more output swing.
The gain doesn't change, because to get higher output swing you also need higher input swing.,
Jan
So I tried it on my class AB amp. No real difference same clipping level 120W rms @ 1KHz. However the difference here Jan is this amp goes into class B.
That is Class A, right? But for Class AB at 350V plate with the idle current at 15 mA per tube the max is 147 mA to 408V @ 4 mA. At 25 mA idle, 147 mA to 453 @ 4 mA,
So the higher bias gives you a greater range where you are in Class A, but I do not see where any extra power gets generated.
I suspect the amplifier is clipping in the driver stage so that if you turn down the bias, the output never actually clips. Tubes amps often have gross inefficiencies. I fixed the bias in the phase splitter/driver of an old ~Bogen amp once and it went from 60W clipping to 100W. Listening to Uncle Doug, it's plain his understanding is "basic". 🙄
I think part of the confusion lies in the RMS increasing as the waveform shape changes with increased bias. The peak seen on the scope does not change much, but the waveform becomes more sinusoidal as the bias is increased. At 40% bias, the waveform stays at baseline longer due to severe crossover distortion. Such a waveform has less RMS power even tho the peak clipping level may have changed little.
Have you looked at the amplifier circuit before suspecting the driver stage is clipping? The amplifier is suppose to make 20W and it seems around there.
UD also said that the output distorting sounds bad, something I read in an amplifier repair book in the 70's saying distortion was a passing fad.
Ahh there may be a difference between fixed and cathode bias here. As the amp enters class B with cathode bias the static bias will drop and the amp may start to produce crossover distortion. Cathode bias needs to be biased hotter to avoid heavy class B operation.
https://www.vtadiy.com/loadline-calculators/loadline-calculator/
This online loadline calculator is very useful to determine the output power and distortion with different idle current. Also, it is easy to find the right amount of idle current within safe margin.
Johnny
This online loadline calculator is very useful to determine the output power and distortion with different idle current. Also, it is easy to find the right amount of idle current within safe margin.
Johnny
It is my most used link in my Amplifier links. The power out sometimes is not realistic. A fun tool though, wish he would add a few more models but it seems he is no longer updating it.
With Push-Pull class AB amps it makes no difference, because you get maximum power when in class B, whatever bias you use, and the loadline in class B is independent from the bias.
The difference in the opening post is below the audibility limit. Double power would be just noticable. The distortion behavior will change, though.
In 50+ years of making tube amps I have seen it go both ways, or not make a difference at all. The actual "power limit" is reached when the signal at the plates of the output tube swings from nearly twice the B+ voltage to the saturation voltage of the tube being used. Bias current can shift the swing up or down a bit to possibly get a tiny bit more power, but the effect is minimal on an amp that is optimally loaded.
Of course, speakers are not an optimal load since their impedance varies with the frequencies, power level, and even musical content that is applied to them. Cranking up the idle current generally does improve the amplifiers damping factor by dropping the tube's plate resistance, especially in a non feedback amp. This will often sound like more power and sometimes actually boost the power a tiny bit. What do you think the impedance of a large heavy woofer looks like when a huge bass drum hit tries to instantly reverse its cone trave? I wasted nearly a year chasing the rabbit down this hole.......don't go there.
Of course, speakers are not an optimal load since their impedance varies with the frequencies, power level, and even musical content that is applied to them. Cranking up the idle current generally does improve the amplifiers damping factor by dropping the tube's plate resistance, especially in a non feedback amp. This will often sound like more power and sometimes actually boost the power a tiny bit. What do you think the impedance of a large heavy woofer looks like when a huge bass drum hit tries to instantly reverse its cone trave? I wasted nearly a year chasing the rabbit down this hole.......don't go there.