biasing/plate voltage
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JMP/HBE
Well-known member
You need an O-scope to really know if you wanna get that technical.
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Depending on who you get the data from, you will hear stories of people saying 120 watts of clean power, some will say 90 watts clean power. Really depends on how you measure it. Without having a THD measurement, that 120 watts of clean power may be with well over 10% of THD but still " look clean " on the O-scope. The amount of THD is probably the only thing that differs between one reading and the next on many of these " Clean Power " amps. The theory would be that if you have 4 tubes capable of 30 watts max each, you can't possibly have more than 120 watts total output. If you have them biased to 70% of peak dissipation ( 21 watts roughly ) you can't possibly have more than 84 watts of total power. I would venture an argument that even still, 100% of that output will not be clean power. So perhaps you only get an actual 70 watts of true, clean, distortion-free output? Without knowing the THD, there is no way to really tell. Most just turn the level up until they can see the O-scope trace start to squish and then take the voltage measurements and do some math. The real question is does it matter if you have 120 watts of actual clean power or 90 watts of clean power?
The math says not really. The difference between 120 watts and 60 watts is -3db. So your output in measurable SPL will only be -3db lower than what you would achieve with 120 watts. It's about the headroom though right? Well, you could argue that the extra 3db of output potential does equate to more headroom, technically, and it would hold water. Does 3db matter though? I say no, but there is more going on than just a 3db difference in potential. This is why biasing an amp hot or cold can make a big difference to the player, but the amp may not care.
The 70% rule is not hard and fast. It is simply a best practices and standards idea. I prefer biasing my amps a little more to the hot side. I think that cold biasing an amp makes it sound thinner, more sterile, and anemic. Biasing too hot and you get a lot of noise and the sound can get too warm. I shoot for the point where you can hear the amp shift from cold and sterile to warm. I want it to just sit right there in between the two thresholds. This is usually right around the 70% point. As you crank the amp, it will bias warmer and as you play quiet, it will still just be warm enough. While I bias by ear, I still measure and do the math. If it's more than 60% and less than 80% it's a pass.
Most large output amps don't have cathode-biased outputs. There are a few 50-60watt amps that do, but there aren't any commercially available 100-120watt amps that I know of that have cathode-biased outputs. Why I thought for even one second that the JTM-45 was cathode biased escapes me? Brain fart there, don't drink and type kids
The long and short of biasing any amp is that the number doesn't really matter. As long as you acquire the tone you want from the amp and it doesn't red plate, you are fine. I don't think that there are any appreciable gains in terms of clean headroom vs onset of distortion from biasing either way. In any amp that is fixed bias, if you crank it, you will likely pull the bias up enough to take it out of crossover distortion if you're biased too cold. Some people like the sound of crossover distortion ( Fluff of Riffs beards and gear is a likely candidate for liking cold biased amps ) as it does sound like mild distortion while even at low gain settings. It can add a level of complexity to your actual overdriven sound.
Many people think of an amp in terms of dynamics, compression, and feel. Truth be told, fixed bias amps have more compression and less dynamics than cathode bias variants. Fixed bias amps have more clean headroom though, all else being the same. A cathode bias amp will have less headroom and begin to break up sooner but will have more dynamic range. The way to think of it is how long the tube is on and conducting for. The less it is on, the more efficient the circuit becomes reducing heat and allowing more time for the power supply to recharge, adding more available power to convert into watts of output. A cathode-biased amp is working the tubes pretty hard most of the time and the power supply is working just as hard to keep up. It produces less overall output wattage as a result of its operational standard, but because it acts more like a class A amp, it has better dynamic range and more feel as a result. The tubes are on for much more time in a cathode bias amp, so there is no real " handoff " as the signals from the PI cross through zero.
The bias circuit works because the signals coming from the PI ( to the power tubes ) are the same signal but out of phase with each other. The negative bias on the tubes ( or reference to ground ) is what turns the tube off when the incoming signal from the PI swings negative. The bias on the output tubes places the grid at just below ground potential ( that negative voltage number that you read is the voltage that pulls the grid further away from ground potential) and as the signal from the PI swings positive, it turns the tube on causing it to conduct. As the signal from the PI swings negative, it pulls the bias voltage more negative, turning the tube off. Since all the tubes are essentially the same, the altering signal ( one side negative, while the other is positive ) is what creates that handoff I mentioned in another post. This is where crossover distortion occurs. If the power tubes are biased too cold, both sets of tubes turn off as the signal from the PI starts to cross through zero volts. If you bias too hot, both sets of tubes are conducting signal at the same time adding noise and heat. To recap:
1. The PI sends two identical A/C signals that are out of phase with each other. One is swinging positive, while the other is swinging negative.
2. The power tubes are biased so that the grid is below zero volts in reference to ground, turning the tube " more off " when a negative voltage signal is present.
3. The voltage swing from the PI turns the power tube on and off as the voltage from the PI swings positive and negative respectively.
A cathode-biased amp sets its bias by bringing the cathode up and away from ground potential in relation to the input grid. As you make the cathode go further from ground potential, it sets the bias colder. I.E. The larger ( higher resistance ) the cathode resistor, the colder the bias, the lower ( less resistance ) the cathode resistor is, the hotter the bias. The power tubes input grid is referenced to ground through its grid leak resistors ( usually 220k to 440k ) between these two references to ground a point of equilibrium is created and the tube settles in at a running point.
A fixed bias amp uses a negative voltage supply to create the bias voltage. The cathodes of the power tubes are connected directly to ground, so the only way to turn the tube off would be to apply a negative voltage to the input grid of the power tube. That is what the bias adjuster on a fixed bias amp does. It simply sets where in relation to ground the input grid is. If you make it more negative ( higher negative number, or further away from zero volts ) the tube will bias colder, as you bias it less negative ( lower negative number, or closer to zero volts ) it will bias hotter. Ideally, you are shooting for a bias that turns the tubes on and off at the same time as the signal from the PI swings through zero.
The math says not really. The difference between 120 watts and 60 watts is -3db. So your output in measurable SPL will only be -3db lower than what you would achieve with 120 watts. It's about the headroom though right? Well, you could argue that the extra 3db of output potential does equate to more headroom, technically, and it would hold water. Does 3db matter though? I say no, but there is more going on than just a 3db difference in potential. This is why biasing an amp hot or cold can make a big difference to the player, but the amp may not care.
The 70% rule is not hard and fast. It is simply a best practices and standards idea. I prefer biasing my amps a little more to the hot side. I think that cold biasing an amp makes it sound thinner, more sterile, and anemic. Biasing too hot and you get a lot of noise and the sound can get too warm. I shoot for the point where you can hear the amp shift from cold and sterile to warm. I want it to just sit right there in between the two thresholds. This is usually right around the 70% point. As you crank the amp, it will bias warmer and as you play quiet, it will still just be warm enough. While I bias by ear, I still measure and do the math. If it's more than 60% and less than 80% it's a pass.
Most large output amps don't have cathode-biased outputs. There are a few 50-60watt amps that do, but there aren't any commercially available 100-120watt amps that I know of that have cathode-biased outputs. Why I thought for even one second that the JTM-45 was cathode biased escapes me? Brain fart there, don't drink and type kids
The long and short of biasing any amp is that the number doesn't really matter. As long as you acquire the tone you want from the amp and it doesn't red plate, you are fine. I don't think that there are any appreciable gains in terms of clean headroom vs onset of distortion from biasing either way. In any amp that is fixed bias, if you crank it, you will likely pull the bias up enough to take it out of crossover distortion if you're biased too cold. Some people like the sound of crossover distortion ( Fluff of Riffs beards and gear is a likely candidate for liking cold biased amps ) as it does sound like mild distortion while even at low gain settings. It can add a level of complexity to your actual overdriven sound.
Many people think of an amp in terms of dynamics, compression, and feel. Truth be told, fixed bias amps have more compression and less dynamics than cathode bias variants. Fixed bias amps have more clean headroom though, all else being the same. A cathode bias amp will have less headroom and begin to break up sooner but will have more dynamic range. The way to think of it is how long the tube is on and conducting for. The less it is on, the more efficient the circuit becomes reducing heat and allowing more time for the power supply to recharge, adding more available power to convert into watts of output. A cathode-biased amp is working the tubes pretty hard most of the time and the power supply is working just as hard to keep up. It produces less overall output wattage as a result of its operational standard, but because it acts more like a class A amp, it has better dynamic range and more feel as a result. The tubes are on for much more time in a cathode bias amp, so there is no real " handoff " as the signals from the PI cross through zero.
The bias circuit works because the signals coming from the PI ( to the power tubes ) are the same signal but out of phase with each other. The negative bias on the tubes ( or reference to ground ) is what turns the tube off when the incoming signal from the PI swings negative. The bias on the output tubes places the grid at just below ground potential ( that negative voltage number that you read is the voltage that pulls the grid further away from ground potential) and as the signal from the PI swings positive, it turns the tube on causing it to conduct. As the signal from the PI swings negative, it pulls the bias voltage more negative, turning the tube off. Since all the tubes are essentially the same, the altering signal ( one side negative, while the other is positive ) is what creates that handoff I mentioned in another post. This is where crossover distortion occurs. If the power tubes are biased too cold, both sets of tubes turn off as the signal from the PI starts to cross through zero volts. If you bias too hot, both sets of tubes are conducting signal at the same time adding noise and heat. To recap:
1. The PI sends two identical A/C signals that are out of phase with each other. One is swinging positive, while the other is swinging negative.
2. The power tubes are biased so that the grid is below zero volts in reference to ground, turning the tube " more off " when a negative voltage signal is present.
3. The voltage swing from the PI turns the power tube on and off as the voltage from the PI swings positive and negative respectively.
A cathode-biased amp sets its bias by bringing the cathode up and away from ground potential in relation to the input grid. As you make the cathode go further from ground potential, it sets the bias colder. I.E. The larger ( higher resistance ) the cathode resistor, the colder the bias, the lower ( less resistance ) the cathode resistor is, the hotter the bias. The power tubes input grid is referenced to ground through its grid leak resistors ( usually 220k to 440k ) between these two references to ground a point of equilibrium is created and the tube settles in at a running point.
A fixed bias amp uses a negative voltage supply to create the bias voltage. The cathodes of the power tubes are connected directly to ground, so the only way to turn the tube off would be to apply a negative voltage to the input grid of the power tube. That is what the bias adjuster on a fixed bias amp does. It simply sets where in relation to ground the input grid is. If you make it more negative ( higher negative number, or further away from zero volts ) the tube will bias colder, as you bias it less negative ( lower negative number, or closer to zero volts ) it will bias hotter. Ideally, you are shooting for a bias that turns the tubes on and off at the same time as the signal from the PI swings through zero.
Demanic
PenultimateTone Member
Well, I'm willing to settle for a good ballpark figure.
Sent from my SM-A115A using Tapatalk
JMP/HBE
Well-known member
Can't really call balls & strikes from here.
Figure maybe -30% from max output maybe, just a guess.
BTW who plays clean in rock guitar lol.
Demanic
PenultimateTone Member
It was really just a curiosity. As I recall Jerry Garcia tried to get the cleanest sound that he could. And loud.
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LLL
New member
So that's why he used an MXR Distortion+ from its invention thru to the late 80s?
I see the acid's got to you, too.
beaubrummels
Well-known member
LLL
New member
The arbiter of truth you have an issue with is called "Google".
Here, let me Google that for you, unbelievers:
(jerry+garcia+distortion+plus)
https://www.google.com/search?q=jerr...dUDCAg&u act=5
:butkick:
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beaubrummels
Well-known member
I never said he didn't use an MXR at all. My comment was he played piercingly bright and clean most of the time. He broke out the effects, like distortion, envelope filters, synths, etc., only for specific songs and for the space jams live. The MXR certainly was not on all the time nor a significant part of his tone across the board. You don't need Google to work that out, if you actually have the records and have listened to them, and had seen him play live multiple times. When I saw him, he used two Fender Twins, a blackface and a silverface (two of the cleanest amps that can be had), mounted sideways in an old Fender Twin cab set on end, with speakers mounted in separate extension cabs stacked vertically. Which is also why he used a Macintosh MC-2300 and JBL speakers, for their clarity and fidelity.
Not sure why you think you are the only one who knows what they are talking about, or that Google is somehow an authority in contradiction to my comment. You get from Google what you search for, not the whole truth about any matter.
LLL
New member
That's ridiculous.
That statement you wrote there says a lot about your own personal issues; allow me to suss it out:
What you're doing is called "psychological projection", and it's what you and your friend-o's do when anyone disagrees with y'all.
In a nutshell, y'all think you're the only ones who know anything.
Then, when someone like myself who isn't a hack and who knows their shit comes along and disagrees from time to time, you throw your own ego trip on them.
The MXR Distortion+ info? He did use them, and frequently - the proof is there. I don't F around when it comes to info or tone; I dig the truth and facts.
You can take it or leave it - I can't stand Jerry Garcia and all that acid-dropping, ham-fisted playing hippie crap. :smoker:
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Demanic
PenultimateTone Member
I never said he didn't use an MXR at all. My comment was he played piercingly bright and clean most of the time. He broke out the effects, like distortion, envelope filters, synths, etc., only for specific songs and for the space jams live. The MXR certainly was not on all the time nor a significant part of his tone across the board. You don't need Google to work that out, if you actually have the records and have listened to them, and had seen him play live multiple times. When I saw him, he used two Fender Twins, a blackface and a silverface (two of the cleanest amps that can be had), mounted sideways in an old Fender Twin cab set on end, with speakers mounted in separate extension cabs stacked vertically. Which is also why he used a Macintosh MC-2300 and JBL speakers, for their clarity and fidelity.
Not sure why you think you are the only one who knows what they are talking about, or that Google is somehow an authority in contradiction to my comment. You get from Google what you search for, not the whole truth about any matter.
Now now, don't upset the Arbiter, otherwise he may delete the universe.
Don't listen to him O Holy One of Tone, You are the only one who can get the Brown Sound, all praise forever.
Yes this is sincere drunken praise,
LLL
New member
Aww geez, that's too bad that the well-documented MXR Distortion+ info has y'all sobbing and crying.
Here ya go:
I think we may have cranked the bias voltage a little high here and the tubes are red plating...
But my 2 pennies about Jerry Garcia's rig is this His rig is a pretty self-indulgent setup. He used only the preamp section of a couple standard Fender options that he fed into a McIntosh amp through what are PA speakers placed in a guitar cab. He is basically running the most expensive rack amp system you could build. It worked for him and dare I say only truly for him. His sound is unique, to say the least. I am not exactly a fan, but there is no denying that he has written more hits, sing-alongs, and jammed more than probably everyone on this forum combined. And his sound didn't suck, it just wasn't conventional. Probably purposefully so.
And his rig has nothing to do with bias settings in an amp...
But my 2 pennies about Jerry Garcia's rig is this
His rig is a pretty self-indulgent setup. He used only the preamp section of a couple standard Fender options that he fed into a McIntosh amp through what are PA speakers placed in a guitar cab. He is basically running the most expensive rack amp system you could build. It worked for him and dare I say only truly for him. His sound is unique, to say the least. I am not exactly a fan, but there is no denying that he has written more hits, sing-alongs, and jammed more than probably everyone on this forum combined. And his sound didn't suck, it just wasn't conventional. Probably purposefully so. And his rig has nothing to do with bias settings in an amp...
I think we may have cranked the bias voltage a little high here and the tubes are red plating...
But my 2 pennies about Jerry Garcia's rig is this
And his rig has nothing to do with bias settings in an amp...
Coach, throw the towel! Now!
