Grid bias value

G

Giordydiver

Inspired
Hello, someone can explane how set it? I dont understand if there is a relationship betwee the value and the %of bias voltage in real amp. For example the standard value for tubes is about 70% but for example the value for el34 is set to 0.5, my question is if they are matched or 0.7 would be the correct value
 
From the Manual:

Power Tube Grid Bias – Sets the bias point of the virtual power amp. Lower values approach pure Class-B operation. Higher values approach pure Class-A operation.
 
There is not a linear relationship between the grid bias value and quiescent dissipation as a percentage of maximum. 0.5 is roughly 66% IIRC but I'd have to run the numbers again to be sure.

With a real tube amp there is no hard-and-fast rule. Some say 60%, some say 70%. But that's a simplistic view of the problem. The optimum value is dependent on many things: B+ voltage, transformer turns ratio, etc. For example, if the transformer is overmatched you can run the bias hotter and this is indeed how some amps are designed (i.e. Trainwrecks).

The reason for the "rule" in tube amps is to reduce crossover distortion while also preventing premature wear to the power tubes. The hotter the bias the more linear the response. However too much bias and there's the danger of the tubes momentarily operating outside the SOA which shortens tube life. Even if you stay within the SOA if you operate near the limits you'll shorten tube life. If you overmatch the transformer you move the load line and increase the SOA margin so you can increase the bias.

The Axe-Fx is immune to tube wear so there is no danger in running the bias hot if you like that sound. However the hotter the bias the less dynamic the response becomes. Also some people like a bit of crossover distortion. Legend has it that EVH liked his amps biased cold. Whether that was because of the added dynamics or the extra grit due to crossover distortion is unknown.

As always the correct answer is what sounds best.
 
Last edited:
FractalAudio said:
There is not a linear relationship between the grid bias value and quiescent dissipation as a percentage of maximum. 0.5 is roughly 66% IIRC but I'd have to run the numbers again to be sure.

With a real tube amp there is no hard-and-fast rule. Some say 60%, some say 70%. But that's a simplistic view of the problem. The optimum value is dependent on many things: B+ voltage, transformer turns ratio, etc. For example, if the transformer is overmatched you can run the bias hotter and this is indeed how some amps are designed (i.e. Trainwrecks).

The reason for the "rule" in tube amps is to reduce crossover distortion while also preventing premature wear to the power tubes. The hotter the bias the more linear the response. However too much bias and there's the danger of the tubes momentarily operating outside the SOA which shortens tube life. Even if you stay within the SOA if you operate near the limits you'll shorten tube life. If you overmatch the transformer you move the load line and increase the SOA margin so you can increase the bias.

The Axe-Fx is immune to tube wear so there is no danger in running the bias hot if you like that sound. However the hotter the bias the less dynamic the response becomes. Also some people like a bit of crossover distortion. Legend has it that EVH liked his amps biased cold. Wether that was because of the added dynamics or the extra grit due to crossover distortion is unknown.

As always the correct answer is what sounds best.
Click to expand...
Thanks for your answere.
From my personal experience marshall amps sound better with hotter bias, around 72 75%. What can be a correct value to match that in fractal modelers?
 
Giordydiver said:
Thanks for your answere.
From my personal experience marshall amps sound better with hotter bias, around 72 75%. What can be a correct value to match that in fractal modelers?
Click to expand...
From your personal experience your particular Marshall amp(s) sounds better to you with hotter bias.

The question is which amp? A 1968 100W Plexi with over 500V on the plates or a 50 watter with around 360V on the plates? The response of those two amps will be completely different at the same quiescent dissipation due to the different plate voltages. The 100W amp will require around 35 mA for 70% dissipation. The 50W amp will require 48 mA. Since the transconductance of a tube is fairly independent of the plate voltage the 50W amp will be operating in a much more linear region. However, it will clip the grids sooner. If you load up, say, a 1959SLP model in the Axe-Fx you'll notice the bias is lower than the 50W versions. This is why. This is also why a lot of people prefer the 50W Plexis.

Then we need to ask what type and brand of power tubes? NOS? The response of the tubes is dependent upon the exponent in the tube equation. The original Childs-Langmuir law says that the exponent is 1.5 but real tubes exhibit an exponent somewhere between 1.2 and 1.5. The greater the exponent the less linear the response.

Finally we need to ask what is the output transformer primary impedance. Is it 4K per pair? Is it 3.6K? Is it 3.2K? Primary impedance was all over the map in those days.

All these things interact. The hotter the bias the more linear the response, the higher the gain, the sooner the grids clip, etc. A good amp designer will balance all these things along with the transformer matching to get the desired response.

The correct answer is turn it up/down until it sounds how you want.
 
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