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Thread: Range of bias voltages for power tubes in fixed bias?

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    Range of bias voltages for power tubes in fixed bias?

    I'm trying to fill in a bit few of the areas of my ignorance, for my own nefarious purposes.

    What is the range of grid bias voltages you see for 6L6, EL34, 6V6, etc. in Class AB1 fixed bias amps?

    The data sheets talk in terms of -25 to -40V for various tubes, but I suspect that the working techs here know better.

    I know that measuring grid bias voltage is not a good way to bias, and regularly advise people not to do it that way. This info is for other nefarious purposes.

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    Better Tone thru Mathematics bob p's Avatar
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    Quote Originally Posted by R.G. View Post
    for my own nefarious purposes.
    It's no coincidence that I have been having some nefarious thoughts of my own.

    As you know, there's going to be a range of desired bias currents, depending upon factors such as B+ on the tubes. If I were to rely on an automated circuit to perform bias sensing and adjustment duties, I'd want to be able to program it with the desired target bias current for the application. I'm thinking about DIP switches.

    I think we had discussed that I have a 6L6 amp on the bench now that has a B+ of 530V. That exceeds the limits you'll find on the data sheets and the bias suggestions that you'll find in popular tables like Jim Jones' table:

    http://www.webervst.com/tubes1/calcbias.htm

    So I'm going to tag along on this thread to see what kind of feedback you get.
    "Stand back, I'm holding a calculator." - chinrest

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    Quote Originally Posted by R.G. View Post
    I'm trying to fill in a bit few of the areas of my ignorance, for my own nefarious purposes.

    What is the range of grid bias voltages you see for 6L6, EL34, 6V6, etc. in Class AB1 fixed bias amps?

    The data sheets talk in terms of -25 to -40V for various tubes, but I suspect that the working techs here know better.

    I know that measuring grid bias voltage is not a good way to bias, and regularly advise people not to do it that way. This info is for other nefarious purposes.
    I honestly don't even go by the negative voltage. I measure the cathode current and whatever negative voltage is required to get the tube to idle in a safe range that sounds good, that is what I choose. More important to me is the percent of dissipation. I may have written it down the last time I worked on my amp that I made, but that uses 7868's so you may not care about what that is....haha. My amp is also using power scaling which changes things somewhat. Most of the commercial amps that I have fixed in the past get negative bias voltage numbers that are way different than what the schematic shows and in fact, I recently fixed a Sunn for a friend that was cooking the tubes with the bias set to the schematic negative voltage level. It had to be set for more negative voltage, but the actual voltage amount wasn't as important to me as the dissipation so I didn't bother to take note of it.

    Greg

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    Noodle of Reality Steve Conner's Avatar
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    -40 isn't enough for 6L6 type tubes. Something like -60 would be better.

    I came across one tube type for which even -60 wasn't enough. The S11E12, advertised as a "6550" by some optimistic EBay sellers.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    Noodle of Reality Steve Conner's Avatar
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    -40 isn't enough for 6L6 type tubes. Something like -60 would be better.

    I came across one tube type for which even -60 wasn't enough. The S11E12, advertised as a "6550" by some optimistic EBay sellers.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    6V6 fixed bias amps tend to be around 8-10% of the plate voltage, measured at the junction of the 220K grid loads. 400-440vdc range on the plates.

    6L6/EL34 amps might be 11-12%, same procedure. Maybe 13% for upper end of B+ range (520-530vdc) with hot tubes, or at coldest bias. Maybe 10% for low plate voltages (~400vdc). Certain, cooler running, "big" tubes like Sovtek 6550WE & EH KT90 will fit the general 6L6/EL34 profile also (KT88/KT120 & some NOS 6550 will need more negative voltage).

    EL84 amps 3-5%. Lower end for plate voltages ~300vdc, higher for 400vdc+.

    These are ball-park targets, subject to fine tuning by mA.
    Last edited by MWJB; 03-10-2013 at 05:50 PM.

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    Everyone, thanks for responding. As I mentioned, I know that biasing by the grid voltage is not the thing to do, and that biasing to a current is the correct process, even if in the end you're trying to get a certain tube dissipation.

    My "nefarious purposes" involve designing an adjustable bias circuit, and I need to know both how much is enough, and what a rough "typical" value is. My experience is limited to small(ish) numbers of 6L6s and EL84s and very limited numbers of EL34s. Even my limited experience with these has taught me that the nominal G1 voltage in the datasheets is not a good guide for the tubes we can actually get today.

    I very much respect the real-world experience of people who make a living by (among other things) re-biasing amps, so I would like to have any shared knowledge you will share with me about the real-world needs of power tubes that you see.

    So if you can, help me with the grid voltages. Otherwise, I'll guess, and that always gets me into a heap o' trouble.

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    Better Tone thru Mathematics bob p's Avatar
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    On a related nefarious note:

    I've got a 6x6L6 amp here that was spec'd 30+ years ago for B+ = 500V with -61V of non-adjustable bias (balance only). On today's voltages B+ = 530, the non-adjustable bias comes in at -61V and the amp sounds quite cold. Unfortunately it's got 3 pairs of 6L6 that are driven in parallel on a grid bus arrangement, which doesn't simplify the bias process. Before doing anything else I'm going to have to un-bus the output section so that I can get a handle on directly biasing individual tubes, using current sensing resistors on the cathodes. Part of the logistical problem with this project is that at these B+ levels there isn't good data on current production tubes. And the typical bias current tables like the ones I mentioned above don't even list B+ that high for any 6L6.

    I'm thinking that any sort of bias-assist circuit that would provide high/low indications for making adjustments would need to allow the user to specify the desired range of acceptable currents for the specific tube application, because there are so many different operating points that you'd need to be able to specify to cover all the bases. That's why I mentioned DIP switch configuration earlier.
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    Generally accepted as 10% of the SCREEN voltage but obviously influenced by tube gm. When designing your adjustment range allow for up to a a maximum 15% of screen volts and you will be fine.

    Bob P - how are you at solid state - here is a bias servo, use one per tube with a common reference and a common -ve supply. Just set the reference voltage to 10mV per mA desired idle current, e.g. set the reference to 350mV and all the tubes will idle at 35mA.
    This is not my design, its one I borrowed from the HIFI crowd. It is actually a nice scheme, for Class AB bias servo you generally have to clamp the current sense at 2x idle to take account of the natural clamp as the tube goes into cut off (to get a consistent measure of average current). This scheme does a similar thing but clamps in a much narrower window around the desired idle current point.

    The LED and the bottom transistor form a current source load for the actual bias amplifier section which adds great noise immunity from any residual "crap" on the bias supply. It has a +/- 1.5 to 2 mA sink/source capability so is tolerant of tubes with grid current (not that you are likely to be designing say a 300B git. amp but this servo would be fine for that too).

    Cheers,
    Ian
    Attached Thumbnails Attached Thumbnails bservo.jpg  
    Last edited by Gingertube; 03-11-2013 at 02:29 AM. Reason: Added the bias servo

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    Supporting Member Chuck H's Avatar
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    Not knowing exactly what R.G.s end circuit is supposed to do (I'm guessing something like a circuit you can plug ANY octal tube into and it automatically adjusts itself?) I thought perhaps a current sense controled variable voltage type bias circuit (like a power scaling circuit) might work. But there's just too many other variables in the tube types that should be accounted for. If you stay with just big bottle types it should be possible. Though you couldn't get top perfomance from 6550's and kt88's without biasing too hot for el34's. Nominal is the key. But nominal feels too much like mediocrity.

    For octals used in guitar amps (other than the 7591) and a moderate blanket voltage something like 400Vp I'd think a bias range of -30V to -50V should do it. With lower voltages you may need a low end of -25V and higher voltages might need a high end of -70V (sorry to any who want to nitpick how I used the terms high and low WRT negative voltage). But I'm guessing there will be a nominal plate voltage if adjusting the bias for different tubes is the criteria. If just big bottles were employed then a B+ of 470Vp and a bias range of -40 to -55 would probably be enough. If it needs to be a concise range. Otherwise, where's the harm in having more range than you would need? The bias supply itself shouldn't care a tic if it's adjustable from -20V to -70V.

    It would help to know a little more about the project. But I understand if that's not possible yet.
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    Supporting Member Chuck H's Avatar
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    Yeah, something like that.
    We were posting at the same time...
    Funny, when you wrote "servo" I expected to see a motor driven potentiometer. Thanks for the neat circuit.
    "The man is an incompetent waste of human flesh. He should donate his organs now to someone who might actually make good use of them." The Dude re: maybe I shouldn't say, but his name rhymes with Trump

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    Chuck,
    Glad you caught up with my edit to add the bias "servo".
    I always value your insights, opinion, experience. A necessary balance to my to purely technical approach and fairly average musician skills,more times than not.
    Cheers,
    Ian

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    Quote Originally Posted by Chuck H View Post
    Not knowing exactly what R.G.s end circuit is supposed to do (I'm guessing something like a circuit you can plug ANY octal tube into and it automatically adjusts itself?)
    Nothing nearly so fancy. It's a simple individual bias-to-a-current setup with one pot per tube. I'm just trying to make it easily adjustable and customizable. Optimized for users, not for fancy stuff.

    While customizable ought to mean that it can be set to whatever, you have to know the limits on 'whatever' to be able to make sense of things.

    I've done the auto-bias thing. It's more sophisticated than is really needed, and adds unnecessary complexity. This is different, and simpler. It may also wind up being a pile of... er, well, unpleasant stuff ... so the unmentioned nature is more me not wanting to look quite so silly if I have to say in my best Emily Litella voice "Never mind."

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    R.G.
    Making it easy for customers is as much about physical layout as what you actually wire.
    This is one I did fro a friend Neale. Shots during construction
    2mm Banana Plug Sockets which take you multimeter probes directly. Black 0V, Push and Pull side Cathodes, Current Sense 10 Ohm resistors wire right on sockets , Put the multimeter black in black hole red in red hole and adjust adjacent pot, when ready for final tweak put probes red to red and adjust one control for a reading of zero. 20 turn Bourns pots.

    Another hint, Those mains switches are the cathode/fixed bias and triode/pentode switches. Don't throw those keyed to the slot on the bush ON/Off plates they come with away, stick them on the inside, at assembly time, wipe some 5 minute epoxy on the wings and they become an anti-rotation device for the switches.

    Cheers,
    Ian
    Attached Thumbnails Attached Thumbnails na_inside_no_wiring.jpg   na_rear_pa_hw.jpg  

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    That's nice, tidy work, G.T.

    I hope he appreciated the effort.

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    Supporting Member Chuck H's Avatar
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    The way I see it (my wife always says I should ignore my own point of view because I'm not normal) is that players don't know, or want to know that much about amps. They want to think they do. But they don't want to put in the effort to actually know. So they are subjuct to the most visible information that is easily and quickly absorbed. This seems to tax the limits of their ability to familiarize themselves with their gear. So...

    A button to press that activates a meter or led panel that has an indication of "optimum" and an adjustment control right next to it would seem ideal. The voltage range of the bias control would need to be based on the Vp and tube type (to avoid any obscene mis-adjustments). I think even an amp made to work with, say, EL34's or 6L6's could be set up with an an acceptible non catastrophic range.

    JM2C
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    Quote Originally Posted by Chuck H View Post
    The voltage range of the bias control would need to be based on the Vp and tube type (to avoid any obscene mis-adjustments). I think even an amp made to work with, say, EL34's or 6L6's could be set up with an an acceptible non catastrophic range.
    Oddly enough, it has been.

    The Workhorse amps had a switch to set either a "typical" or "hot" setting for each of EL34 and 6L6. From there the "red light/green light" adjuster let you set them in about 10 seconds.

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    Some brands of EL34 and 6L6 bias up identically, but even different brands of 6L6 can be wildly different. So you ideally want to base "typical" settings around your preferred brand(s).

    One pot per tube sounds like a hassle if "easily adjustable" is your target, anyone can buy reasonably matched tubes at no extra cost, what should be a 30 second job becomes a 3-5minute job as each tube see-saws with adjustment...unless you use 2 independent supplies, one for each tube.

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    Good information; thank you. I'd like to sponge up anything I can about the issue.

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    Better Tone thru Mathematics bob p's Avatar
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    Quote Originally Posted by MWJB View Post
    ...unless you use 2 independent supplies, one for each tube.
    Or 6 independent supplies, one for each tube.
    MWJB likes this.
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    Quote Originally Posted by MWJB View Post
    ... what should be a 30 second job becomes a 3-5minute job as each tube see-saws with adjustment...unless you use 2 independent supplies, one for each tube.
    I've been pondering this. Can you explain a bit to me?

    Do you mean a separately adjustable bias voltage per tube, perhaps derived from the same raw DC supply for bias, or an isolated bias supply for each tube, or what else that I may be missing?

    Are you thinking the bias supplies will interact?

    Still gathering data.

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    I've seen bias settings interact in a minor way as the B+ changes with current draw as you adjust tube idle currents BUT if the actual bias supply is changing with individual bias settings than you have a problem.
    That problem may be that there is a fault or it may be that it is an inappropriate circuit which was intended for a single bias adjustment and has just been duplicated by someone with limited knowledge. There is no reason why this should happen with a properly designed bias circuit.
    "High Impedance" bias supplies derived from a capacitive or resistive divider off the HV winding are more prone to these sorts of problems.
    Cheers,
    Ian

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    Better Tone thru Mathematics bob p's Avatar
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    I was about to make exactly that point -- if you design the bias supply so that it's voltage divider's Z is adequately low then it's not hard to make the later replicated sections fully independent of one another. You just have to make sure that the aggregate impedance of the replicated sections is adequately higher than the supply impedance.

    If you've got multiple tube pairs in your output section then making this work also requires that you de-bus the entire drive circuit. I've mentioned doing this in the load line drudgery thread.
    "Stand back, I'm holding a calculator." - chinrest

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    Quote Originally Posted by R.G. View Post
    I've been pondering this. Can you explain a bit to me?

    Do you mean a separately adjustable bias voltage per tube, perhaps derived from the same raw DC supply for bias, or an isolated bias supply for each tube, or what else that I may be missing?

    Are you thinking the bias supplies will interact?

    Still gathering data.
    Every amp I have had "hands on" with a pot per tube/side of the primary has see-sawed when setting idle current, bring one tube (side of the primary up) up & the other drops. Perhaps it's not hard to correct/design out, but it seems rarely done. Like I said earlier, WRT to buying matched pairs/quads (say +/-15%) unless the amp is designed for guys who are rescuing unmatched pulls from old gear, or cherry picking NOS cast offs, I generally think that individual tube bias adjustment on a 2/4 output tube P-P amp, is a fix for a problem that doesn't really exist...or if it exists, goes largely unnoticed.

    So, if I were to do it (I really wouldn't bother), given that I'd need 2 trim pots/dividers, I'd go for an extra diode (& cap if required), a drop resistor and take one supply from a 50-55VAC winding, the other from the B+ secondary via drop resistor feeding 50-55VAC.

    Simple, "user friendly biasing" to me ideally means cathode biasing...and ensuring preamp & power tubes use different sockets (noval vs octal) so that folks aren't stuffing 12AX7 in EL84 sockets or vice versa! :-) Guys who are hip to biasing & tube subs (which require rebiasing) are usually already ordering new matched tubes & will take it up with the vendor if they don't get them. If you're looking at an amp that will take differing types of tube (14W, 25W, 40W...though as mentioned before careful choices in brand will make this easier...but maybe somewhat defeat the purpose of adjustability?) & need a big swing in bias voltage then you are inevitably building in a "self destruct" pot for the reckless...again here, getting creative with no's of tubes used & finding a good average for cathode bias is probably better for experimenters? If you are designing in flexibility then it strikes me that you are compromising on power (which is fine), so why not save on the circuitry & extra controls & spend on more/bigger tubes?

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    Quote Originally Posted by Gingertube View Post
    There is no reason why this should happen with a properly designed bias circuit.
    "High Impedance" bias supplies derived from a capacitive or resistive divider off the HV winding are more prone to these sorts of problems.
    That's my intuition, but I'm proceeding very cautiously here. What I have in mind is a bias voltage per tube where the loading on the bias supply is substantially constant. I've messed with this a bit, thought about it, and simmed it. The current from the raw bias supply varies less than about 2% from min to max bias settings over half a dozen outputs.

    One is always at the risk of a high-impedance raw bias supply sagging under loading or with B+. Is this sag with loaded B+ a good or a bad thing? I'd love opinions and explanations.

    Quote Originally Posted by bobp
    I was about to make exactly that point -- if you design the bias supply so that it's voltage divider's Z is adequately low then it's not hard to make the later replicated sections fully independent of one another. You just have to make sure that the aggregate impedance of the replicated sections is adequately higher than the supply impedance.
    I did a bit of a shift to make the bias adjustment not change the loading on the bias supply. The idea is that if you ever get an adequate bias voltage, tinkering the adjustment doesn't change the bias supply loading (much, anyway) so the bias supply itself is unaffected, except indirectly by the sag of the entire PT output caused by the primary wire resistance. Secondary wire resistance doesn't get into it, as the bias secondary (if there is one) is lightly loaded and doesn't experience the sag of the HV secondary.

    However, bias supplies derived from the HV secondary by capacitive impedance, etc. will still sag. They will sag less with this setup than with others, but still some.

    Keep those cards and letters coming folks. And bless you for sharing!

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    Better Tone thru Mathematics bob p's Avatar
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    If the bias supply is tied to the B+ supply and is too saggy, there's always the option to regulate it. I'm sure that you've already considered the pros/cons of doing this.

    The whole idea of de-bussing the drive on an amp to provide individually adjustable bias supply may seem more like an electronics design exercise than a practical application, especially in those cases where you can find matched tubes. The problem that I've encountered with a 6-tube circuit is that even when you buy a "matched" sextet, the sextet doesn't stay matched very long. The burn in process often leads to de-matching of the tubes, and resultant decreases in the amp's performance. Then you're left dealing with a 6-tube matched set that isn't matched any more. What to do? Buy a new matched sextet and recycle the tubes that are no longer matched into 1-tube, 2-tube or 4-tube amps?

    When it comes to operating an amp that has 6 output tubes driven with a bussed arrangement, I think of the analogy where I release a herd of 6 cats from the house, expecting them to march through the yard in two ranks, three abreast -- it's just not going to happen.

    Given that 6-tube sets aren't likely to stay matched, and that there's higher expense involved in trying to buy a replacement "matched" sextet that may or not stay matched, I'm inclined to think that you'd have better luck herding cats than trying to maintain a matched sextet. To me it sounds like the 6-tube problem is one that merits the added design effort required to de-bus the output stage and design a 6-way non-interactive bias supply. Not a big deal with 2 tubes, not a big deal with 4 tubes either, but for some reason 6-tube amps always seem to give me headaches. The fact that the designs out there tend to bus-up the drive circuits definitely doesn't help the problem. I think the 6-tube amp is a case where bussed-up drive circuits are just bad engineering that's driven more by bean counting than performance or ease of maintenance.
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    Again, I really appreciate the commentary. It spurs some thinking, which is like taking castor oil (if you're old enough to remember this as an option): at first you feel terrible, but after a while you're better than before.

    Quote Originally Posted by MWJB View Post
    Every amp I have had "hands on" with a pot per tube/side of the primary has see-sawed when setting idle current, bring one tube (side of the primary up) up & the other drops. Perhaps it's not hard to correct/design out, but it seems rarely done.
    There was a small amount of this in the Workhorse amps, but it was not so bad that biasing was a problem. You might find that bringing a tube up to green, then the other, that the first would drop out, but this was trivially easy to fix. A couple of back-and-forth cycles was all it took, and since the indicator was a green light, there was no mental processing needed, and it was really fast. True, the match in bias current was not perfect, but as you note, that's not needed. The match in bias current actually reached was about +/-3%, once I dug through the error budget in the overall measure/set/indicate process in detail.

    Like I said earlier, WRT to buying matched pairs/quads (say +/-15%) unless the amp is designed for guys who are rescuing unmatched pulls from old gear, or cherry picking NOS cast offs,
    And again, thank you. I have never had the field experience to know how close "matched" was. I suspect it varies with how many matched sets the vendor has to choose from and how close making payroll and rent is this month.

    I do know that initially matched pairs in these amps, set to the nominal tolerances, drifted out of the initial bias "matching" in about 1 hour of playing time, and again at about 10 hours. Maybe they settle down more after that until wear out sends them out out of the stability well again. In any case, they drift.

    I use the engineering maxim that whatever can't be controlled must be made irrelevant. Whatever can't be controlled OR made irrelevant must be easy to adjust or fix.

    I generally think that individual tube bias adjustment on a 2/4 output tube P-P amp, is a fix for a problem that doesn't really exist...or if it exists, goes largely unnoticed.
    I'd agree with "unnoticed" wholeheartedly.

    So, if I were to do it (I really wouldn't bother), given that I'd need 2 trim pots/dividers, I'd go for an extra diode (& cap if required), a drop resistor and take one supply from a 50-55VAC winding, the other from the B+ secondary via drop resistor feeding 50-55VAC.
    OK. This is to offer two different sources of raw bias voltage? What would you think of regulating the raw voltage with a zener to wash out the variation? This is really what I was incenting people to talk about in one of my posts: how important or deleterious is the bias sagging with the B+ on peaks? Is that good, bad, or indifferent?

    Simple, "user friendly biasing" to me ideally means cathode biasing...and ensuring preamp & power tubes use different sockets (noval vs octal) so that folks aren't stuffing 12AX7 in EL84 sockets or vice versa! :-)
    I'd put it another way: "user friendly biasing" would mean "the user doesn't have to think about it". Cathode biasing is one way, but it suffers from the history that makes not all amps that way already. We're stuck with many amps that are already not cathode biased. Second choice would be to make the amp mystically bias itself. I've done that, but it's not simple, and not un-intrusive for a number of reasons.

    Guys who are hip to biasing & tube subs (which require rebiasing) are usually already ordering new matched tubes & will take it up with the vendor if they don't get them.
    I
    I'd expect the same guys to have the insight to view tube drift as an issue. Matched pairs don't stay matched all that long, at least for the admittedly limited sample I have.

    If you're looking at an amp that will take differing types of tube (14W, 25W, 40W...though as mentioned before careful choices in brand will make this easier...but maybe somewhat defeat the purpose of adjustability?) & need a big swing in bias voltage then you are inevitably building in a "self destruct" pot for the reckless...again here, getting creative with no's of tubes used & finding a good average for cathode bias is probably better for experimenters? If you are designing in flexibility then it strikes me that you are compromising on power (which is fine), so why not save on the circuitry & extra controls & spend on more/bigger tubes?
    Good questions, and I will ponder them in some detail.

  28. #28
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    Quote Originally Posted by R.G. View Post
    I've been pondering this. Can you explain a bit to me?

    Do you mean a separately adjustable bias voltage per tube, perhaps derived from the same raw DC supply for bias, or an isolated bias supply for each tube, or what else that I may be missing?

    Are you thinking the bias supplies will interact?

    Still gathering data.
    This is my work on the topic, a quad bias circuit with a range pot to swing the whole bank up/down: Full wave bridge for bias supply- does this look right?
    I haven't fiddled the knobs in a long while on this setup, so I don't recall how bad it interacts, but it does because the pots are wired as variable resistors, so the total load is not constant. I have been thinking a buffered "range" pot would be the answer to this, but haven't done anything to work on it. I also designed this when I was first starting out, so the way I constrained the range of the "range" pot is kludgy, but it works.

    http://music-electronics-forum.com/a...-quad-bias.png

    It on my MOSFET post PI follower proto board.


    -Mike

  29. #29
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    RG wrote: "I do know that initially matched pairs in these amps, set to the nominal tolerances, drifted out of the initial bias "matching" in about 1 hour of playing time, and again at about 10 hours. Maybe they settle down more after that until wear out sends them out out of the stability well again. In any case, they drift."

    Interesting, what happens when everything cools back down? I'm pretty used to idle current being set...amp taken away used for 6 months to a year, or more...next time I see it, idle current hasn't moved when cold/on power up and tens of minutes, even hours on idle/light use. I have seen idle current appear to drift when measured with probes, but I wonder whether this is due to bias sense resistors in the probes getting hot & drifting, as the same tubes in another amp stay rock solid, measured by other means. I have no doubt that tubes drift significantly over very long periods/hard use...but I also wonder if there is a typical assumption when we see old amps with mismatched tubes, that they "must have" been reasonably matched at some point...?

    MWJB wrote "So, if I were to do it (I really wouldn't bother), given that I'd need 2 trim pots/dividers, I'd go for an extra diode (& cap if required), a drop resistor and take one supply from a 50-55VAC winding, the other from the B+ secondary via drop resistor feeding 50-55VAC."

    RG replied "OK. This is to offer two different sources of raw bias voltage? What would you think of regulating the raw voltage with a zener to wash out the variation? This is really what I was incenting people to talk about in one of my posts: how important or deleterious is the bias sagging with the B+ on peaks? Is that good, bad, or indifferent?"

    Yes, to offer 2 different sources, reduce see-sawing, set each tube independently, matched...mismatched...whatever. I have heard reports that negative bias voltage can rise under load, seems counter intuative to me (I'm not discounting it) & would wonder if a non-sagging bias supply would run colder under load as the bias VAC supply would not track the B+ sag (if your negative voltage is say 11% of plate voltage, then it's 11% at 400v or 500v)? I have no idea as to the answers though.

    I think setting fixed bias can be critical (for reasons of repeatability as much as anything, rather than saying you must hit "#% dissipation" or "38mA precisely") and if you are going to check/set/monitor then you want a repeatable number, for that amp/system, reflective of current. There's often a tipping point where it matters - e.g. Blues Jrs, Peavey C30s & Pro Jr's run around 350vdc on the plates here, bias to 70% and they sound noticably terrible, you need late 20's mA wise, 30mA if at all possible...I deal with a lot of harp players and some run 6-15mA per tube in big fixed bias amps, if it drops below 5mA the amps won't hold a constant note, rises above 15mA you get a lot more feedback, these things are tangible & easily noticed by the player. Setting easy/safe/ball-park/rule of thumb works in a lot of cases, but just as you get the guys who must have a perfect match at 70% idle, you get odd cases where deviating from this can be make or break from a user perspective.

    The other week a guy dropped by for check over on a prospective purchase, I was giving it a general appraisal & suggested we look at the bias, "Oh yeah, what are we looking for? 35mA, a good match?" he asked...well the guy's opening statement, previously, was that this was the best amp he had ever heard (why he wanted to buy it in the first place), when we discovered the amp was cool & mismatched from one tube to the other by a factor of x3 he suggested that this "needed fixing"...but hang on, when he heard it & played it like this, it was the best amp he had ever heard?

    Given that the effect of biasing at idle seems somewhat arbitrary, when we consider what goes on dynamically, it does amaze me that it works so well & is entirely repeatable.

    Sorry, i'm rambling now..

  30. #30
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    Quote Originally Posted by MWJB View Post
    RG wrote: "I do know that initially matched pairs in these amps, set to the nominal tolerances, drifted out of the initial bias "matching" in about 1 hour of playing time, and again at about 10 hours. Maybe they settle down more after that until wear out sends them out out of the stability well again. In any case, they drift."

    Interesting, what happens when everything cools back down? I'm pretty used to idle current being set...amp taken away used for 6 months to a year, or more...next time I see it, idle current hasn't moved when cold/on power up and tens of minutes, even hours on idle/light use. I have seen idle current appear to drift when measured with probes, but I wonder whether this is due to bias sense resistors in the probes getting hot & drifting, as the same tubes in another amp stay rock solid, measured by other means. I have no doubt that tubes drift significantly over very long periods/hard use...but I also wonder if there is a typical assumption when we see old amps with mismatched tubes, that they "must have" been reasonably matched at some point...?
    A lot of my experience is with brand new tubes of new manufacture, less with old manufacture of various amount of use, so take that into consideration too.

    The emission of electrons from a heated cathode depends on the temperature and chemical composition of the surface. It varies over time as the surface gets heated and cooled, bombarded by residual gas ions, and gradually poisoned by various processes. This is why most of the drug store tube testers were emission testers. They applied a nominal heater voltage then checked for how much plate current flowed when the tube was considered to be a diode. Over time, emission dropped. Even CRTs did this. The "picture tube rejuvenators" were devices to boost the heater voltage to get higher temps and more emission, posponing the day when the picture tube needed replaced. Some of them applied a real over temp to shake up the cathode surface and move un-poisoned oxides to the surface.

    So there is at least one well-known way that emission changes, and it's directly related to tube current. I've never seen the scholarly works on exactly HOW tubes age, but they must exist. If they're like most engineering materials and processes, they have an S- or Z-curve. Change happens fast at first, stabilizes to a small change over most of the lifetime, and then rises as wearout sets in. That process could well account for the big drifts I saw in new tubes as well as the stability of biasing in well used amps.

    At one time, tubes were manufactured in such large quantity that the manufacturing processes were tweaked in and produced very, very consistent products. Tubes produced in the same run on the same line by the same people were highly, highly consistent because every possible inconsistency and variation had been ruthlessly hunted down to maximize the yield of good parts. Today, we deal with remnants from different runs, and with what amounts to a cottage shop manufacturing process in out of the way corners of the technical world. I suspect that this may account for the larger variation, as well as the fact that amp makers used to get such consistent tubes that biasing in general was done with soldered-in, fixed resistors.

    To me, that seems to make sense of tubes that drift a lot at first, then less later. The heat/cool cycle does cause drift, but its a drift from dead cold towards the fully warmed-up value that the tube's heater and cathode conditions have drifted it to. I would expect a single tube to warm up to nearly the same current each time after first being drifted-in over some tens to hundreds of hours. So an amp coming back months later at essentially the same current doesn't surprise me.

    Yes, to offer 2 different sources, reduce see-sawing, set each tube independently, matched...mismatched...whatever. I have heard reports that negative bias voltage can rise under load, seems counter intuative to me (I'm not discounting it) & would wonder if a non-sagging bias supply would run colder under load as the bias VAC supply would not track the B+ sag (if your negative voltage is say 11% of plate voltage, then it's 11% at 400v or 500v)? I have no idea as to the answers though.
    Interesting. I'd like to find out more about that. With only the information at hand, I'd expect the bias supply to sag (that is, get less negative) by less than half the ratio that the B+ sags (that is, get less positive) because of the workings of the transformer, rectifiers and filters under load. If you can point me to any info on that, I'd be grateful.

    I think setting fixed bias can be critical (for reasons of repeatability as much as anything, rather than saying you must hit "#% dissipation" or "38mA precisely") and if you are going to check/set/monitor then you want a repeatable number, for that amp/system, reflective of current.
    I agree. Hitting the same point every time with different tubes or the same tubes over their life is the critical thing. The gain of a tube depends on the physical arrangement of the metal structures inside it. The emission of the tube depends on the chemical composition of the surfaces and the temperature. Bias sets an operating condition. Gain for a given tube just is. Repeatability over tubes of the bias condition of the circuit's operation is a big deal in making the right sound.
    There's often a tipping point where it matters - e.g. Blues Jrs, Peavey C30s & Pro Jr's run around 350vdc on the plates here, bias to 70% and they sound noticably terrible, you need late 20's mA wise, 30mA if at all possible...I deal with a lot of harp players and some run 6-15mA per tube in big fixed bias amps, if it drops below 5mA the amps won't hold a constant note, rises above 15mA you get a lot more feedback, these things are tangible & easily noticed by the player.
    From a technical perspective, that makes sense. Getting up out of cold-bias crossover distortion is very noticeable. And gain
    at an operating point can rise a bit as current changes. That's how most of the bias tremolos work. Yeah - operating point matters.

    Setting easy/safe/ball-park/rule of thumb works in a lot of cases, but just as you get the guys who must have a perfect match at 70% idle, you get odd cases where deviating from this can be make or break from a user perspective.
    The other week a guy dropped by for check over on a prospective purchase, I was giving it a general appraisal & suggested we look at the bias, "Oh yeah, what are we looking for? 35mA, a good match?" he asked...well the guy's opening statement, previously, was that this was the best amp he had ever heard (why he wanted to buy it in the first place), when we discovered the amp was cool & mismatched from one tube to the other by a factor of x3 he suggested that this "needed fixing"...but hang on, when he heard it & played it like this, it was the best amp he had ever heard?
    Makes sense to me. In the late 90s I wrote some stuff on tube matching, the best information I had at that point. I speculated that MIS-matching tubes might well sound good in certain conditions. One tube of a pair operating at a different point in its gain curve than the other will amplify one side of the signal more than the other, and clipping will be at different signal amplitudes. That makes for non-symmetrical amplification of the signal, and the human ear *likes* this in modest amounts. Makes sense to me.

    Given that the effect of biasing at idle seems somewhat arbitrary, when we consider what goes on dynamically, it does amaze me that it works so well & is entirely repeatable.
    It is a bit amazing, isn't it?

    Again, thanks for the insight. This is helping me put stuff into perspective as I go.

  31. #31
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    I should probably be less mysterious about this. I'm dreaming up a way to mechanically mount an individual per-tube bias pot on the back of an amp that doesn't require hacking the chassis, just mounting a small (maybe 3" by 3" by 1") box on/under the chassis somewhere. It would let you attach this to an amp easily - if individual per-tube biasing is something you want.

    The idea is to bring out test points for per-tube current measuring from the output tube cathodes and protected access for a screwdriver adjust, while spending some thought about how to make this safe for the amp - for instance, making an accidental probe short not be an amp-changing event.

    This required me to know more than I currently do about the range of voltages and currents in a wider range of amps than I'm personally familiar with.

    I've come up with some ideas about how to do this, and while we discuss it here, I've been drawing things. I like my red-light/green-light bias indicator, so I'm also thinking about making this an option if someone wants it. We got really positive reviews on that from everyone we heard from that tried it.

    It's not particularly earth shaking. As the bard said, a small thing, but mine own.

  32. #32
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    Whatever the bias voltages normally are, I like them slightly colder for bias-vary trem. (Feel free to factor that in to whatever you're up to R.G.)
    Building a better world (one tube amp at a time)

  33. #33
    Better Tone thru Mathematics bob p's Avatar
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    I was going to ask about asymmetric clipping in that mismatched harp amp. Good point.

    Now that R.G. has let the cat out of the bag, so to speak, I'd like to offer some feedback on a practical application for this type of device. R.G.'s heard all this before as we've been emailing one another for the past week, but I'd like to hear other peoples' thoughts too. Honestly speaking, I think that the amp I'm working on now represents a worst case scenario in terms of needing this sort of solution that R.G.'s thinking about building.

    My application is a 6-tube Fender Super Twin Reverb that I recently obtained as a DOA. It's drive section for the output tubes uses cathode followers to drive three sets of tubes that are bussed-up in parallel. Having been built in the era of "uniform" tubes, bias is set by fixed resistors at -61V and is not adjustable. Well, there is a balance pot. I have to admit that the amp doesn't particularly sound very good. I attribute a large portion of this to the horrible bias arrangement, where the bias is non-adjustable and the amp currently has 6 tubes that are poorly matched. Looking to the future, I can see where an adjustable split-bias system would make long-term maintenance much easier in an amp like this one, so I'm giving it serious consideration. I had already designed a 6-way independent bias circuit of my own when I received an email from R.G. about his microcontroller-based bias box with LED indicators.

    One potential problem that we both noticed immediately is that the real estate in this amp is very cramped, and that there's no room inside of the chassis for an add-on board. This amp already has an extra board in it (compared to the garden variety Twin Reverb) that's used for the 5-band EQ and the ghastly-bad preamp distortion circuit. The result is that things are so crowded that there's no room inside for another board unless you re-work the boards that are already there to free-up some space. There's also no room on the back panel because those 6 tubes and the switching jacks take up a lot of space. We considered that the only mounting option for something like this may be to use an add-on box on the belly of the chassis. Here's a picture of the stock amp:

    str-chassis.jpg

    I really like R.G.'s idea for a bolt-on bias box that would give you independent bias. It looks like it would be pretty straightforward to deploy on an amp that uses single pairs of tubes, but a bit more complicated to use on amps that have multiple pairs of output tubes that are wired up in parallel. To make things work you'd have to de-bus the drive circuits.

    I do like the idea of having externally adjustable bias pots, and LED that work as idiot-lights to help simplify the bias procedure. One potential problem in deploying such a box on this particular amp involves where to locate the box so that it's visible/accessible without removing the chassis. As you can see by viewing this amp from the rear, there's not much room to mount anything on the chassis. All of the chassis space is occupied by 6 tubes, a transformer, or the doghouse. Maybe hanging it off of the doghouse might be an option. If it were something that needed to be mounted directly on the chassis, for this amp, that would mean mounting it toward the front of the amp (behind the doghouse if you're looking at the amp from the rear) which would block line of sight to the LEDs and access to the trimpots. From a practical standpoint R.G., I think if the application would require mounting the box somewhere that the LEDs and trimpots aren't easily accessible, and you have to stick your head into the cabinet or remove the chassis to bias the amp, then there's not a lot of reason not to just pull the chassis and bias the amp on the bench using a meter.

    In a nutshell then, I really like the idea for 6-way adjustable bias. I really like your idea to use LEDs as bias indicators. And I like the ease of use that would come with having everything mounted on a back panel. That would make the package very easy to use for end users. But would it matter to users who are more technically inclined? I don't know. But in this particular application, I'm thinking that I might do just as well to have a small board with trimpots located somewhere inside of the chassis, to pull the chassis and to use a meter with mini-clip leads to grab onto both sides of a cathode resistor.

    Just to reiterate a comment that I made earlier: considering the wide range of bias currents/voltages that would be required for different tube types, if you're interested in building a "universal" microcontroller-based bias circuit, I imagine that you'd need to provide some way for the user to program the controller for different tube applications. I was thinking that DIP switches might be handy in that respect.

    I hope this brings out some helpful ideas from other people reading the thread.
    "Stand back, I'm holding a calculator." - chinrest

  34. #34
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    Quote Originally Posted by bob p View Post
    Just to reiterate a comment that I made earlier: considering the wide range of bias currents/voltages that would be required for different tube types, if you're interested in building a "universal" microcontroller-based bias circuit, I imagine that you'd need to provide some way for the user to program the controller for different tube applications. I was thinking that DIP switches might be handy in that respect.
    I think I have a solution to that. I put in a way to tell the uC to the effect that "this tube is the right current, right now; make your green light for all the tubes be here". So the procedure for setting it up on an amp would be to
    (1) using the measurement points so thoughtfully provided ( ) on the unit, set one tube to the right current by meter and the bias adjuster, ignoring the LED guide for the moment.
    (2) hit the "do it like this" adjuster till the LED turns green
    Done. Now all the tubes' lights go green when their current matches within the few percent error budget. Adjust the other tubes. And as long as you don't care to change that set point, it will always look for that current from each tube.

    @bobp
    I changed to a red-gree-blue LED. Blue = too cold, red = too hot, green is juuuuuuuust right.

  35. #35
    Gaz
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    I haven't read every single reply, but I just wanted to mention that a very good practical argument for individual tube bias is not to match hypothetically unmatched 'new' tubes, but rather the ability to replace a single expensive power tube on the road, or maybe use an old spare if that's all you have to get through the next couple of shows.

    Another consideration is that any guitarist who buys a unit like RG is imagining already has some idea of what bias is and what they're getting into. Not every guitarist cares, but even if they have an amp that has user-adjustable bias controls doesn't mean they actually have to use them, and can still take the amp to a tech if they prefer. In theory, any honest tech would charge next to nothing for biasing an otherwise healthy amp using its external bias controls. I mean unless you taking the chassis out of a Fender

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