Biasing - 70% vs 90% vs Plate Current vs Cathode Current

Discussion in 'The Workbench' started by Wilder Amplification, Jan 8, 2010.

  1. Wilder Amplification

    Wilder Amplification New Member

    Joined:
    Oct 12, 2009
    Messages:
    6,163
    Likes Received:
    700
    Location:
    Fresno, CA
    Hopefully the mods make this a sticky.

    OK - there seems to be a bit of confusion as to the 70% vs 90% rule. I blame everyone who sells "cathode current" DIY bias probes for this. They have made a bad attempt at "dumbing down" the bias procedure to those who have no knowledge of amplifiers and how they work and in the process have created a mass of confusion via lies and misinformation as to how this shit really works.

    So here I will attempt to explain the "traditional plate current" method, along with the more modern dumbed down cathode current method in hopes of clearing up the confusion and explaining where it comes from.

    Inside of a power valve you have a -

    Cathode
    Control Grid
    Screen Grid
    Suppressor Grid
    Anode (aka "plate")

    In a Class AB amp, a negative voltage from an external supply is "fixed", or "applied" to the control grid, hence the name "fixed biasing". The cathode is grounded. This makes the control grid more negative than the cathode. The plate receives a positive charge from the positive side of the power supply through the output transformer.

    Current flows from the negative side of the power supply to the cathode, using the chassis/ground as a path to get there. This supplies the cathode with electrons, which are negatively charged. Opposite charges attract. The positive charge on the plate pulls electrons from the heated cathode to the plate, where the electrons return to the positive side of the supply through the output transformer.

    The screen grid has a positive charge on it as well. The positive charge on the screen grid helps pull additional electrons to the plate. Since it is a "grid", most of the electrons pass right through it. However, some of them flow to the screen, so effectively you have the screen grid itself drawing current from the cathode as well as the plate.

    Now...with 450 - 500 volts on the plate, if the negative voltage on the control grid didn't exist, so much current would flow through the valve that it would fry it along with the other components that are connected to it (i.e. socket, screen resistor, OT). The purpose of the negative voltage is to control the current flowing from the cathode to the plate/screen grid. It does this because it is more negative than the cathode. Since electrons are negatively charged and like charges repel, this negative control grid voltage creates a repelling force between the control grid and the cathode. This repels some of the electrons that are flowing from the cathode to the plate, thereby reducing the current flow to a safe level that the valve can handle at the plate voltage it runs at. It is this negative "control voltage" that you adjust when you're setting bias. By varying this negative voltage more & less negative, this directly controls the current flow through the valve by increasing/decreasing the repelling force between the control grid and the cathode.

    Plate current vs cathode current -

    As mentioned above, the cathode must supply electrons to BOTH the screen grid as well as the plate. Plate current deals ONLY with the actual current flow to the plate itself. However, cathode current deals with BOTH screen grid AND plate current flow FROM the cathode.

    What does this mean?

    This means that cathode current will read HIGHER than plate current since cathode current deals with current flowing to BOTH the plate and the screen, whereas PLATE current only deals with the current flowing to the plate only.

    Now we get into the interesting part - Plate dissipation vs screen dissipation!

    "Dissipation" is the term used to refer to the ability of the individual parts in the valve to dissipate power as heat as well as how much DC power it can handle as "constant current flow" during no-signal conditions. As electrons flow from the cathode to the plate and screen, there is a resistance to that flow which creates heat when struck by electrons, which must be "dissipated" by the plate as well as the screen grid.

    Power (Watts) = Potential (Voltage) x Electron Current Flow (Amps)

    or

    P = V x I

    If we have 475 volts on the plate with a plate current flow of 35mA, the plate is dissipating -

    475 Volts x 35mA (0.035 Amps) = 16.625 Watts

    This would be well within acceptable limits for an EL34 running in class AB. This is because an EL34's maximum plate dissipation rating is 25 watts. 16.625 watts is 66.5% of 25...just under the 70% limit for Class AB.

    16.625 Watts / 25 Watts = 66.5%

    70% of 25 watts = 17.5 Watts (25 Watts x 0.7 = 17.5 Watts). If we wanted to know what the max plate current for an EL34 running class AB at 475 volts -

    17.5 / 475 Volts = 0.0368 Amps = 36.8mA

    This would be the absolute maximum plate current you would want to see when biasing via measuring plate current.

    This was the 'traditional' method that us 'old school' techs have used over the years. This was also the way I learned to do it and for reasons I'm about to mention the only way I prefer to do it.

    Now's where things get REALLY interesting. If I haven't lost you yet...read on.

    The 70% rule states that you bias your valves in class AB no higher than 70% of the maximum rated plate dissipation. You can bias lower, but no higher than this.

    Biasing via Cathode Current

    This seems to be the more modern way to do things as it gives a safe method of biasing to the novice. This is because when biasing via cathode current, you're looking at a voltage drop across a 1 ohm resistor that is in the millivolts range rather than trying to measure current across the OT that has a much higher voltage on it. The theory is simple Ohm's Law -

    Voltage = Current x Resistance, or

    V = I x R

    Due to this law, any amount of current flowing through a 1 ohm resistor will equal itself as a voltage measured across that 1 ohm resistor, because anything times 1 equals itself. If you have 40mA flowing through 1 ohm of resistance, across the 1 ohm of resistance you will have -

    40mA x 1 ohm = 40mV

    Of course here you are subjected to the resistor being a true 1 ohm and not slightly off. Let's say for instance the resistor was actually 1.2 ohms -

    40mA x 1.2 ohms = 48mV

    There it just threw your reading off by a factor of 8mA, yet the resistor value was only 0.2 ohms off. Let's go the other way and say the resistor is actually 0.8 ohms -

    40mA x 0.8 ohms = 32mV

    In thise scenario, your reading would be LOWER than what's actually flowing through the resistor. Throw in the fact that cheapo multimeters aren't very accurate when reading super low voltages in the mV range and you have a real recipe for disaster. This is reason #1 why I DO NOT recommend the biasing via cathode current method.

    Now we get into where the guys who developed the "dumbed down" method fucked up and created the mass of confusion.

    Cathode current will always read higher than actual plate current because again cathode current includes BOTH the screen grid current AS WELL AS actual plate current. Since we have a 70% rule for plate current, we ALSO have a 70% rule for cathode current. This is how it SHOULD'VE been explained.

    An EL34 has a plate dissipation rating of 25 watts while having a screen dissipation rating of 8 watts. Remember, cathode current deals with both of these, so naturally we would want to add these two figures together.

    25 watts + 8 watts = 33 Watts Plate+Screen dissipation

    Now...the proper way to do this is to take 70% of the above figure -

    33 Watts x 0.7 = 23.1 Watts

    So for a plate voltage of 475 Volts, we should see -

    23.1 Watts / 475V plate voltage = 0.048 Amps = 48mA

    Since we are taking this reading across a 1 ohm resistor, we will see this as 48mV instead.

    This is a MAXIMUM FIGURE. Again you can go lower, but DO NOT go higher. Biasing for a cathode current mV reading of 48mV will put us right at 70% of both the plate & screen dissipation combined.

    Here's where the fuckup came in -

    In an effort to dumb down the procedure, all the valve dealers who favor this method as well as the manufacturers of DIY Bias Kits and Cathode Current monitors took this and tried to relate it to PLATE DISSIPATION ONLY! WTF!? You ONLY deal with plate dissipation by itself when biasing via the PLATE CURRENT method. Cathode current is BOTH PLATE & SCREEN!

    Well this is what happened -

    Lets' take our 23.1 watt figure, which is 70% of PLATE & SCREEN, and apply it to PLATE ONLY dissipation, which is 25 watts -

    23.1 Watts / 25 Watts = ...

    YOU GUESSED IT!

    92.4% Dissipation

    This puts us close enough to THEIR recommended "90%", which gives a 2.4% 'cushion'.

    However, this is flat bullshit. This does not properly explain just how things work inside of an amp and if you're going to teach someone how to properly bias an amp, you should teach them the CORRECT way rather than "dumb it down" in a way to where if they tried to apply it using more traditional methods it won't make sense. Not only that, but applied in the wrong way this can prove to be a very dangerous practice.

    I just had to help a guy who was trying to make sense of "classes of operation" going by a PLATE CURRENT LOADLINE, yet trying to apply the 90% rule used to apply cathode current to plate only dissipation and it just flat out wasn't making sense to him. AND IT WON'T! Because it's not a correct way to explain it and a VERY bad idea to say the least. Throw in the fact that now you have novice guys biasing their own amps going onto public message boards telling people that it's OK to bias to 90% plate dissipation and you can create a whole epidemic of disaster. This is because not everyone biases via cathode current and lots of us bias via the more traditional plate current method and applying the 90% rule here would fuck a lot of shit up.

    It is for this very reason why people who don't have any valve amp tech background should not post up tech info without stating that they're not a tech and have no way to verify the accuracy of a certain bit of info they happen to feel like posting up. For the record, the all too common statement "I know it's true because 'so and so' at 'such and such' shop told me so" or throwing up the defense of "Go tell so and so at such and such shop that...I'd like to see what they tell you" when you get called out on it just simply won't cut it. If you can't back it up with electronics theory and hard facts, either start your post off with "I'm not a tech by any means so I have no way to verify this, but I seem to recall reading somewhere that...blah blah blah" or "...but I happened to hear from so and so from such and such that...blah blah blah". This notifies the many people who could be reading your posts immediately upon reading that what they're reading is not yet verified hard fact info rather than reading a bullshit post of "Yeah you bias it to 90% and call it good" as if it's already verified info, and risking the reader blowing their amp up. Even if it's something you've done with no problems...just because you haven't had problems YET doesn't mean you WON'T. It just means that it hasn't happened yet. If you can't do that, then just simply don't post it or expect to get called out.

    I hope this somewhat clears the air as to the 70% vs 90% vs Plate Current vs Cathode Current debate that's been going on here for quite some time.
     
  2. Ken

    Ken Well-Known Member

    Joined:
    Feb 24, 2009
    Messages:
    9,935
    Likes Received:
    3,116
    Location:
    Arlington Heights, IL
    Nice post!! Half the biasing posts say, "Do it yourself and save money," but you seem to say that most DIY guys won't have the bias they think they do. I have a DSL 50 with the pins in the back. Can I bias this myself and get a good result? Is this plate or cathode? Can't I just use about 43 mv and figure that's okay?

    Ken
     
  3. solarburnDSL50

    solarburnDSL50 Well-Known Member

    Joined:
    Aug 8, 2008
    Messages:
    11,968
    Likes Received:
    4,704
    Location:
    Wetville
    What we weren't taking into account were the extra 8watts added from screen dissipation to the 25watts that an EL34 already has. I was actually biasing mine way cool according to the 70% dissipation using a PV of 470v no screen dissipation(8watts)figured in and on top of that my cheap mm is off about 8mv too.
     
  4. barryg

    barryg New Member

    Joined:
    Sep 6, 2009
    Messages:
    15
    Likes Received:
    0
    As for Jon's well explained post, I don't know where all this shit started, but when I hear people doing this, I wonder just what are they trying to get from their amp. This is why Randall Smith's amps are not adjustable. I'm not an amp tech, but I take care of my own equipment. I've used both methods, transformer shunt, whichever used, there must be some common sense used also. Amps run at a designed voltage range when properly adjusted, which I fine tune by ear within a volt or two. That's it. To me, it should be as the schematic says, or close to it. Your informative post should educate anyone who reads it. Thank You.....barryg
     
  5. robertlatham1

    robertlatham1 New Member

    Joined:
    Dec 22, 2009
    Messages:
    10
    Likes Received:
    0
    In thise scenario, your reading would be LOWER than what's actually flowing through the resistor. Throw in the fact that cheapo multimeters aren't very accurate when reading super low voltages in the mV range and you have a real recipe for disaster. This is reason #1 why I DO NOT recommend the biasing via cathode current method.

    i could not agree with you more!!!! i second the motion on "don't act like a tech if you aint a tech" i have spent many years to learn electronics theory and practices. seeing guys come on to some of these boards and post stupid bullshit like 90% bias methods really makes me smile cause i know my amp repair shop is going to continue to prosper and grow!

    the mass of population will almost always follow rather than lead because joe blow did and joe blows brother did and their amps sound great.

    a very hot bias does sound great in certain applications, but in fact you are playing with certain disaster!

    great post Bro!
     
  6. Wilder Amplification

    Wilder Amplification New Member

    Joined:
    Oct 12, 2009
    Messages:
    6,163
    Likes Received:
    700
    Location:
    Fresno, CA
    Thanks for all the great replies and kind words!

    Ken, the bias points on the DSL/TSL series are in fact cathode current. Assuming a plate voltage of 470VDC, 45mV will place you right at 65% of 33 Watts Plate/Screen dissipation, which is a good safe place to be at. That and Marshall recommends that.

    The key point here was that the 90% rule is a bullshit way of explaining the way that the cathode current method works. Joe is right...I have his DSL50 right now and am in the process of "Wilderfying" it :dude:, and we've gone to great discussion lengths over the phone about this, and he is correct in stating that when measuring cathode current you have to take into account the 8 watt screen dissipation and add it to the plate. When you do this, you will see that the rule is actually STILL 70%. But when you compare it to plate dissipation only, it APPEARS as if you're biasing to 90% when using the cathode current method, but that is a bullshit way to look at it.

    @Barryg...not sure as to the extent of your valve amp tech background, but since you've been able to pull off the transformer shunt method without killing yourself I will assume that you are full and aware of the dangers behind this method and that safety is of utmost importance when reading plate current in the plate circuit. I personally prefer this method, but only recommend that those who know full and well what they're doing as well as the dangers involved with this method. And before anyone asks, no I WILL NOT explain this method to anyone due to the fact that if you don't know what you're doing it's too easy to fuck up, and as such would create a HUGE liability on my part if I did so.
     
  7. MartyStrat54

    MartyStrat54 Well-Known Member

    Joined:
    Feb 21, 2009
    Messages:
    25,999
    Likes Received:
    9,455
    Location:
    Licksville
    Of course you left out the guys who swear there is no harm in "not" biasing the tubes. They figure what could it hurt? Why I answered a thread here recently and the guy was telling everyone that he doesn't bias his tubes and has not had any problems. This prompted another guy to post, "that his tech said as long as the tubes are the same brand and hardness (sic), then biasing is not necessary." Wow! That's some rock solid info. I posted a disclaimer about this info as I didn't want anyone following this really bad advice.
     
  8. rjohns1

    rjohns1 Well-Known Member

    Joined:
    Dec 30, 2009
    Messages:
    1,592
    Likes Received:
    407
    Wilder, great post. I think I really got it now. I have a question though, I under stand how it works for one tube per side, but what about two? I have a dsl100, and Marshall has flip flopped on recommending 90mv per side on the test points. Now I think they recommend 70mv per side. Using the test points, do you double the amount of current calculated for one tube if you have a push pull with 4 tubes? Or is there more to it than that.
     
  9. 00jett

    00jett Member

    Joined:
    Jan 6, 2009
    Messages:
    633
    Likes Received:
    17
    Location:
    Ontario, Canada
    Wow Wilder, excellent read, i really learned alot from that. Its nice to learn how things work.
     
  10. JohnH

    JohnH Well-Known Member

    Joined:
    May 31, 2008
    Messages:
    2,743
    Likes Received:
    657
    Location:
    Wilton NSW
    Thats a great post Jon, my new learning for the day.

    Id like to ask a question on it related to DSL401's (with EL84s) to the tech types familiar with it: On that amp, what plate voltage do you expect? Id like to see where the various bias values that people use relate to the 70% etc rule. The amp has 4 x EL84, cathode biased with bias current for all four together determined by measuring voltage across a 10 Ohm resistor. Early models were set at 1.375V, hence 34mA per tube at the cathode. Newer model were factory set at 0.55V leading to 14Ma per tube. And I set mine at 0.75V, so 19mA per tube. A very wide range!

    cheers

    John
     
  11. Joey Voltage

    Joey Voltage New Member

    Joined:
    Dec 22, 2009
    Messages:
    1,717
    Likes Received:
    113
    Location:
    Brighton, MA
    You gotta cut that shit out Bias Man, people are going to think you have no life... or your a really good typist. don't think I have ever written a post anywhere that is anywhere near the length of that one :D


    to be honest chances are he still doesn't understand.... I bet you it's as clear as mud to him. I might even put a dollar on it. he's trying to relate the dissipation to a bias point on a loadline thinking that they are one and the same i.e 50% dissipation = a symmetrical center bias.. He is having a hardtime distinguishing between the bias point and waste heat. what he is not seeing is the bias point as it looks on the curves when you superimpose and move up a plate load, nor do I think he really understands the classes, although that information is easily pulled up via Saint Google
     
  12. thrawn86

    thrawn86 Well-Known Member

    Joined:
    Aug 6, 2009
    Messages:
    6,722
    Likes Received:
    1,762
    I second John's question.
    Well done Wilder....that is a very good explanation.

    About the screw ups between the two methods: Was this because 800 and 900 models have no bias points, thus needing the 'old-school' method to bias via plate instead of cathode?
     
  13. LuredMaul

    LuredMaul New Member

    Joined:
    Dec 30, 2009
    Messages:
    775
    Likes Received:
    146
    I'll throw my Q in here too....lol

    So, 90MV per side on a TSL is good?????
     
  14. Wilder Amplification

    Wilder Amplification New Member

    Joined:
    Oct 12, 2009
    Messages:
    6,163
    Likes Received:
    700
    Location:
    Fresno, CA
    @rjohns1 - no what happens is that on both the 50 AND the 100 watters you only have two bias controls. On the 50 watters, each bias control only controls the current draw of ONE valve, while on the 100 watters each one controls the current draw of ONE PAIR of valves. As such, on each bias point (again there are only two of them for both models) you are seeing cathode current draw for TWO valves at once on the 100 watters rather than only seeing it for ONE valve like you would on the 50 watter. This naturally DOUBLES the maximum, which is why 50 watters are rated to be set at 45mV while 100 watters are rated to be set DOUBLE that at 90mV.

    @thrawn - The fuckups between the two methods were that people who tried to "dumb down" the bias procedure decided to relate cathode current to PLATE dissipation, rather than relate cathode current to BOTH PLATE & SCREEN dissipation since the cathode current includes both the plate and the screen current.

    @LuredMaul - Required bias is NOT done by MODEL, it's done based on how many power valves the amp has. So -

    For DSL/TSL with two EL34s - 45mV

    For DSL/TSL with four EL34s - 90mV

    @Joey - Go play some Dokken...or something :D
     
  15. Joey Voltage

    Joey Voltage New Member

    Joined:
    Dec 22, 2009
    Messages:
    1,717
    Likes Received:
    113
    Location:
    Brighton, MA
    From a design standpoint, most likely they picked a plate voltage that didn't exceed the maximum for the valve they wanted to use, then drew a loadline on the curves listed on the data sheet of the OEM brand of tube they use, found a bias point that was both suitable for the valve, and would ensure the product would last through warranty they provide for the valves. they then calculated the quiescent current draw of all the valves in the circuit, plus the screens for the pentodes, which gave them a figure for the total expected load on the HT. They then spec'd a PT from this information, that would give a similar result to the calculations.

    So in other words they just picked it. you can figure out what your optimal bias point is, but it does take some juggling since the plate voltage is dynamic in that it will increase, and decrease depending on the total current draw of the entire circuit, although the output valves contribute to most of it.
     
  16. rjohns1

    rjohns1 Well-Known Member

    Joined:
    Dec 30, 2009
    Messages:
    1,592
    Likes Received:
    407
    Thanks for the info. I'm going to re-bias the DSL today.
     
  17. lucidspoon

    lucidspoon Well-Known Member

    Joined:
    Sep 23, 2009
    Messages:
    2,121
    Likes Received:
    1,303
    Location:
    Avon, IN
    Ahhh!!! This is exactly the missing information I've been looking for! Thanks!

    It's science!

    [​IMG]
     
  18. jcmjmp

    jcmjmp Well-Known Member

    Joined:
    Nov 20, 2008
    Messages:
    8,104
    Likes Received:
    1,095
    Location:
    Canada
    You are exaggerating the actual resistor values here. The 1ohm resistors used for biasing are selected to have 1% tolerance.

    1% tolerance on 1Ω is 0.99 to 1.01
    5% tolerance on 1Ω is 0.95 to 1.05

    A good DMM is key when reading low Mv values - you're right about that. As for screen current, most will agree that it can be considered to be negligible although, I like to leave a bit of buffer. some say that when biasing, +/- 10% is good enough. I don't know about that, but knowing about the screen current is definitely something to consider.

    For the average DIY'er, this is just fine IMO

    Another thing I noticed is that you assume 8w dissipation on the screen grid. In practice, it would be lower than that, probably closer to 2 watts, no? Of course, this depends on the plate voltage and you'd need to know what kind of voltage / current is on the screen vs the plate.
     
    Last edited: Jan 10, 2010
  19. American Viking

    American Viking Well-Known Member

    Joined:
    Jan 9, 2009
    Messages:
    4,031
    Likes Received:
    1,181
    Location:
    RICHMOND, VIRGINIA
    so, is there a fatal flaw in
    (plate dissipation of tube / plate voltage of amp) x 0.7 , this is not including screen dissipation.

    What's to guarantee all the formulas will get you out of crossover distortion especially since the ratings like plate and screen dissipation of the valves can vary from different manufacturers?

    Alongside biasing, what else is fed to the valves or is that it? I know that different parts of the electrical system provide current for the actual bias and separate from the valve heaters, is there anything else?

    Just trying to learn as much as possible. Great post.
     
  20. jcmjmp

    jcmjmp Well-Known Member

    Joined:
    Nov 20, 2008
    Messages:
    8,104
    Likes Received:
    1,095
    Location:
    Canada
    With the cathode resistor method, the flaw is that screen current is added to the plate current.
     

Share This Page