Any consensus on the bias mod?
- Thread starter wolfman
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Done it. Worth it. Every tube amp should be able to be biased.
Not a huge change in tone but clarity. Fizz I was trying to dial out prevented me from making the ideal eq settings. Now I can set the eq and gain more to how I want it and have some.....clarity???? in the gain. If that makes sense.
I did it about a month ago but bought another amp on the same day I did the bias mod so my interest has been else where. But Im back to the LS now and loving it more.
I did the other mods but switched them back to stock pretty fast. Just not what I was looking for. The inclusion of adjustable bias was.
My eq and gain are now set a little differently to how they were. I think many of the differences between having biased the amp and what it was before are just fine tuning and not huge tone/sound differences. Im just able to zone in on the way the eq and gain effect each other with out having to fight the cold bias ...fizz???
Did I just say the same thing 3 different ways? :?
Not a huge change in tone but clarity. Fizz I was trying to dial out prevented me from making the ideal eq settings. Now I can set the eq and gain more to how I want it and have some.....clarity???? in the gain. If that makes sense.
I did it about a month ago but bought another amp on the same day I did the bias mod so my interest has been else where. But Im back to the LS now and loving it more.
I did the other mods but switched them back to stock pretty fast. Just not what I was looking for. The inclusion of adjustable bias was.
My eq and gain are now set a little differently to how they were. I think many of the differences between having biased the amp and what it was before are just fine tuning and not huge tone/sound differences. Im just able to zone in on the way the eq and gain effect each other with out having to fight the cold bias ...fizz???
Did I just say the same thing 3 different ways? :?
messaboog1e
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Is there any way to determine how much sooner the tubes would need to be replaced by doing the biad mod? I know everyone plays their guitar at different volumes and for different amounts of time. Is it safe (or stupid) to say that if the tubes run at twice their original bias, that you might have to replace them twice as often? I'm on the verge of doing this mod as well.
Thanks,
Thanks,
There is no way to tell exactly how long your tubes will last .If you do the bias mod and run the tubes twice as hot then yes they will wear out alot quicker . Probably quicker than twice as often. I usually change my tubes about once a year on an amp I play daily . I'm sure you can get more time out of them but I don't want a tube blowing and taking something else with it .messaboog1e said:
messaboog1e
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Very helpful Splatter, thank you!
Ive never seemed to go through tubes like others seem to on any amp. Dont know why that is. I always have spares and change when I need to. Not very often it seems.messaboog1e said:
Having said that, my mesa nomad went through its 1st set quite quickly(6mths). I was surprised. The set I replaced them with lasted much longer. Still going strong last time I spoke to the guy who bought it. 3yrs now.
To be honest, being able to dial out that cold bias fizz that I was hearing and set the eq closer to how I like it is worth any tube swap. I probably would still have the Nomad if I had thought to put a bias control in it. I think the cold bias distortion was a plague in that amp. I could never set the eq and gain controls right. Always a harsh underlying fizz/distortion. I knew the sounds I wanted were in there, just couldnt get to them.
It has been a very long while since I've posted here. I hope all is well with everyone, that you all had a great Thanksgiving, and that your Lonestars are all doing fine. For the past year or so I have been involved with a number of projects and dealt with a number of family issues, so time has been scarce. I have however been working on a project that hopefully will result in supplying a number of national acts with older and vintage Fender amplifiers. That being the case, and having read the other posts concerning this subject I thought I might dispell some myth.
All amplifiers, actually all tubes in an amplifier circuit, must be properly biased. Not a myth, a fact. Even the pre-amp tubes are biased. We, human beings, can not change the laws of physics any more than we can know what tomorrow will bring. We, human beings, can however APPLY the laws of physics to meet our needs, so I think it important to understand a few things before we go on.
1. Almost all class AB amplifiers, which includes the Lonestar and all Fender amplifiers, are dependent on a BIAS circuit to control the flow of charged electrons.
2. The bias voltage for the output tubes will always be a negative voltage in a class AB circuit, generally in the neighborhood of -51 volts.
3. The bias supply will be common to all of the output tubes and apply the same negative voltage to all tubes equally.
4. Not all tubesconduct in the same manner, and this is the reason an adjustable bias supply is desirable.
Bias defined;
"In Class AB fixed-bias the cathode is grounded, and a constant negative voltage is applied to the grid. The tubes draw current directly from a grounded cathode and the voltage remains stable. Fixed-bias power tubes do not have a cathode resistor—the only way to make the grid more negative than the grounded cathode is to apply negative voltage to the grid. So, the power supply has circuitry that generates a negative voltage. As you make that grid bias voltage more negative, you “choke” the idle current flow more and more, and as you make it less negative you allow more and more idle current to flow. A negative bias voltage on the grid repels some of the electrons and prevents them from reaching the plate, resulting in less current flow. In this way a changing negative charge on the grid modulates the plate current."
Simply put, the 6L6 in your amp is at idle because the negative voltage will not allow the flow of electrons through the grid to the plate.
http://musicalilluminism.wordpress.com/2009/04/27/vacuum-tube-operation/
The circuit being sold for the Lonestar and other amps is also simplistic. You replace the fixed bias resistor with a variable resistor. The variable resistor is a valve of sorts allowing either more or less negative voltage to flow. This change in voltage results in a change in current at the tube, and this change is measured in millivolts.
Because no tube can be identical to another, the term "matched set" has evolved. This match is nothing more than measuring a pair or more of tubes conductance. It is ideal to use tubes which conduct equally.
This was not much of a problem in my day gone by, however, tubes are made cheaply today, and specs vary widely from manufacturer to manufacturer, hence the need to match tubes.
In 1968 Fender saw this problem beggining to exist, and that is why many post CBS amplifiers had a modified bias circuit which balanced but did NOT adjust the bias voltage. Very handy for tubes that are not matched, but not great for tone. Here are examples of those circuits for reference.
http://www.el34world.com/charts/bias_conversions.htm
Adjusting the bias is a simple matter if you use the correct tools, and some more voodoo needs to be clarified here.
A 1 ohm resistor does not "change" anything other than the amount of voltage. It is attched to the output tube in place of the ground and allows for a point at which you can easily measure the voltage without using any math.
If you are going to apply changes to the bias then a bias probe is the correct instrument to use. You can adjust the bias safely, quickly and accurately. I use the one below;
http://cgi.ebay.com/Meter-ed-Dual-Config-QuadStage-Amp-Tube-Bias-Probe-Tool-/220701453888?pt=LH_DefaultDomain_0&hash=item3362d4ee40
Running the amplifier without the resistor or bias mod in place can destroy your amplifier.
All tubes have specifications and limits to the voltages they can withstand.
A 6L6 run with a bias set too high will "red plate" and the tube(s). The power transformer and output transformer can also be destroyed. A safe bias starting point is 35ma, measured between -40 and -51 volts at the supply output depending on tube manufacturer.
A 6L6 with a bias set too low will suffer from blocking distortion. A very unpleasant distortion typified by low note farting and poor high end definition. Blocking distortion can be measured with a scope, but your ear should be your guide.
Typical bias settings for a 6L6 are from 25ma (cold) to 55ma (very hot), depending on plate voltage.
With few exceptions the majority of amplifier manufacturers bias their amplifiers cold. biasing an amplifier cold results in fewer calls for tube replacements, due to the use of the cheaply made tubes supplied by the manufacturer.
Good quality tubes can be biased "hotter", resulting in better tone, depending on your style of play. Tube life will be shortened, but not by any amount that anyone reading this post will notice.
FAIR WARNING: Running the tubes to extremes WILL shorten thier life, and you will notice. "Red plating" an output tube results in either a 50% shorter life span or immidiate destruction of the output tube(s) and associated components.
Tubes which measure within 5ma are considered matched, within 10ma usable, over 10ma and you probably won't like the tone.
And finally, here's a great site for calculating the correct bias for your output tubes.
http://www.webervst.com/tubes1/calcbias.htm
And 6L6 data site;
http://tdsl.duncanamps.com/show.php?des=6L6GC
Have fun with the mods. An adjustable bias can only be a good thing.
BL
All amplifiers, actually all tubes in an amplifier circuit, must be properly biased. Not a myth, a fact. Even the pre-amp tubes are biased. We, human beings, can not change the laws of physics any more than we can know what tomorrow will bring. We, human beings, can however APPLY the laws of physics to meet our needs, so I think it important to understand a few things before we go on.
1. Almost all class AB amplifiers, which includes the Lonestar and all Fender amplifiers, are dependent on a BIAS circuit to control the flow of charged electrons.
2. The bias voltage for the output tubes will always be a negative voltage in a class AB circuit, generally in the neighborhood of -51 volts.
3. The bias supply will be common to all of the output tubes and apply the same negative voltage to all tubes equally.
4. Not all tubesconduct in the same manner, and this is the reason an adjustable bias supply is desirable.
Bias defined;
"In Class AB fixed-bias the cathode is grounded, and a constant negative voltage is applied to the grid. The tubes draw current directly from a grounded cathode and the voltage remains stable. Fixed-bias power tubes do not have a cathode resistor—the only way to make the grid more negative than the grounded cathode is to apply negative voltage to the grid. So, the power supply has circuitry that generates a negative voltage. As you make that grid bias voltage more negative, you “choke” the idle current flow more and more, and as you make it less negative you allow more and more idle current to flow. A negative bias voltage on the grid repels some of the electrons and prevents them from reaching the plate, resulting in less current flow. In this way a changing negative charge on the grid modulates the plate current."
Simply put, the 6L6 in your amp is at idle because the negative voltage will not allow the flow of electrons through the grid to the plate.
http://musicalilluminism.wordpress.com/2009/04/27/vacuum-tube-operation/
The circuit being sold for the Lonestar and other amps is also simplistic. You replace the fixed bias resistor with a variable resistor. The variable resistor is a valve of sorts allowing either more or less negative voltage to flow. This change in voltage results in a change in current at the tube, and this change is measured in millivolts.
Because no tube can be identical to another, the term "matched set" has evolved. This match is nothing more than measuring a pair or more of tubes conductance. It is ideal to use tubes which conduct equally.
This was not much of a problem in my day gone by, however, tubes are made cheaply today, and specs vary widely from manufacturer to manufacturer, hence the need to match tubes.
In 1968 Fender saw this problem beggining to exist, and that is why many post CBS amplifiers had a modified bias circuit which balanced but did NOT adjust the bias voltage. Very handy for tubes that are not matched, but not great for tone. Here are examples of those circuits for reference.
http://www.el34world.com/charts/bias_conversions.htm
Adjusting the bias is a simple matter if you use the correct tools, and some more voodoo needs to be clarified here.
A 1 ohm resistor does not "change" anything other than the amount of voltage. It is attched to the output tube in place of the ground and allows for a point at which you can easily measure the voltage without using any math.
If you are going to apply changes to the bias then a bias probe is the correct instrument to use. You can adjust the bias safely, quickly and accurately. I use the one below;
http://cgi.ebay.com/Meter-ed-Dual-Config-QuadStage-Amp-Tube-Bias-Probe-Tool-/220701453888?pt=LH_DefaultDomain_0&hash=item3362d4ee40
Running the amplifier without the resistor or bias mod in place can destroy your amplifier.
All tubes have specifications and limits to the voltages they can withstand.
A 6L6 run with a bias set too high will "red plate" and the tube(s). The power transformer and output transformer can also be destroyed. A safe bias starting point is 35ma, measured between -40 and -51 volts at the supply output depending on tube manufacturer.
A 6L6 with a bias set too low will suffer from blocking distortion. A very unpleasant distortion typified by low note farting and poor high end definition. Blocking distortion can be measured with a scope, but your ear should be your guide.
Typical bias settings for a 6L6 are from 25ma (cold) to 55ma (very hot), depending on plate voltage.
With few exceptions the majority of amplifier manufacturers bias their amplifiers cold. biasing an amplifier cold results in fewer calls for tube replacements, due to the use of the cheaply made tubes supplied by the manufacturer.
Good quality tubes can be biased "hotter", resulting in better tone, depending on your style of play. Tube life will be shortened, but not by any amount that anyone reading this post will notice.
FAIR WARNING: Running the tubes to extremes WILL shorten thier life, and you will notice. "Red plating" an output tube results in either a 50% shorter life span or immidiate destruction of the output tube(s) and associated components.
Tubes which measure within 5ma are considered matched, within 10ma usable, over 10ma and you probably won't like the tone.
And finally, here's a great site for calculating the correct bias for your output tubes.
http://www.webervst.com/tubes1/calcbias.htm
And 6L6 data site;
http://tdsl.duncanamps.com/show.php?des=6L6GC
Have fun with the mods. An adjustable bias can only be a good thing.
BL
