Properly Biasing a Tube Amplifier (The Bias Mod Debate)

[BeZo]

I have had this debate with a few people recently, and I wanted to post it here. I'll just open the thread with the question before I share my thoughts.

Does the bias mod alone improve the tone of the amp, compared to simply biasing the amp properly by replacing the resistor with one of the correct value?

To explain further, every amp needs to be biased to whatever tubes are in it, and every tube is different. Does the bias mod give the amp any better results that could not be achieved without replacing the resistor with an adjustable pot.?

GO!!

 

[rockstah]

Bezo,

I think i understand what you're asking so ill give it a shot.
in a fixed bias amp, that is adjustable with a pot rather than just placing a resistor in the bias circuit, would technically lead to better tone due to the fact that one could bias the tubes cold or hot depending on how they like it.

where a set resistor would yield different results depending on how much current the given set of tubes draws, cold or hot, a resistor AND pot combination would allow you to dial them in exactly to where you want them.

so yes, better results in that you can fine tune the bias of a given set of tubes.

Mark

 

[jonl]

good info thanks!

 

[BeZo]

Bezo,

I think i understand what you're asking so ill give it a shot.
in a fixed bias amp, that is adjustable with a pot rather than just placing a resistor in the bias circuit, would technically lead to better tone due to the fact that one could bias the tubes cold or hot depending on how they like it.

where a set resistor would yield different results depending on how much current the given set of tubes draws, cold or hot, a resistor AND pot combination would allow you to dial them in exactly to where you want them.

so yes, better results in that you can fine tune the bias of a given set of tubes.

Mark

To bias a fixed bias amp, you take measurements the same way you would with an adjustable bias, and use a math equasion to figure out the proper resistance. The only difference between the two is that an adjustable bias has a pot to adjust the resistance, where a fixed bias has a resistor that needs to be replaced. You can still properly bias an amp with a fixed bias by replacing the resistor with one that will have the correct value for the bias setting.

 

[rockstah]

i think you may not be understanding what i said.

With a pot you will be able to bias an amp that after you calculate and need 52k total resistance, or a total of 49.9k. good luck finding those values in a resistor or resistor combo.
Where, as an example, a Marshall has a 47k bias resistor in series with a 22k pot. We can go from 47k to 69k and every value in between.

in your mod, assuming the amp comes with just a resistor (probably something like a 68k resistor or around there), you wouldn't replace the resistor with just a pot anyway. You would do like a Marshall and replace the resistor that's in there with a resistor AND a pot in series giving you a range you can work with.

Mark

 

[mboogman]

To clarify, I think Mark is talking about a fixed bias amp, which includes amps like Marshall's with a bias pot, as apposed to a cathode bias amp.

 

[rockstah]

To clarify, I think Mark is talking about a fixed bias amp, which includes amps like Marshall's with a bias pot, as apposed to a cathode bias amp.

I didnt think he was talking about Cathode bias'd amps with doing such a mod.

 

[guitup too]

Mark,
Love all of your clips. Can you explain cathode biased to me and would you know for sure if my Budda SD30 is cathode biased?

 

[O'Rourke]

Fixed biased doesn't have anything to do with whether it is adjustable or not, just means there is a fixed value of negative voltage being applied to the power tubes to set the bias. Whether that fixed value is adjustable or not doesn't matter, a 5150 without a bias trim pot and an amp like say a JCM 800 with a bias trim pot are both fixed bias amps.

To answer the original question doesn't matter whether there's a pot or a resistor, the bias pot is just a variable resistor in this application anyway. It will sound the same "if" the same amount of bias voltage is being applied. The advantage to having a trim pot is you can bias towards the hotter side or colder as well as properly bias different sets of tubes that require different amounts of negative voltage to bias up where you want them.

 

[BeZo]

Okay, so I never claimed to be an amp tech. If I were, I would do some experiments to prove or disprove this. But, I am really just an amateur gear enthusiast.

That said, the point I have to make is that the difference in tone that occurs from the bias mod comes from the amp being biased, and not from the mod itself. The bias mod only makes it easier to bias your amp and get the most out of your tubes.

A lot of people seem to think the bias mod makes all the difference in the tone. The point I would make is that if you were to take out the adjustable pot and replace it with a resistor of the same value, you wouldn't notice a difference. The adjustable pot just makes it easier and more efficient to bias your amp, but the results really come from the flow of the juice and not the parts involved. In other words, I think the bias mod can help your amp sound good, but it doesn't make your amp sound good. If you really wanna change your tone, you gotta take your preamp tube covers off.

 

[mboogman]

Okay, so I never claimed to be an amp tech. If I were, I would do some experiments to prove or disprove this. But, I am really just an amateur gear enthusiast.

That said, the point I have to make is that the difference in tone that occurs from the bias mod comes from the amp being biased, and not from the mod itself. The bias mod only makes it easier to bias your amp and get the most out of your tubes.

A lot of people seem to think the bias mod makes all the difference in the tone. The point I would make is that if you were to take out the adjustable pot and replace it with a resistor of the same value, you wouldn't notice a difference. The adjustable pot just makes it easier and more efficient to bias your amp, but the results really come from the flow of the juice and not the parts involved. In other words, I think the bias mod can help your amp sound good, but it doesn't make your amp sound good. If you really wanna change your tone, you gotta take your preamp tube covers off.

I get what you're saying, and I think you're absolutely correct. Having the tubes biased correctly would make the tonal difference, not the adjustable resistor (the pot) in the circuit.