self biasing amps???

Discussion in 'Amps and Cabs' started by britishampsrock, Nov 5, 2005.

  1. britishampsrock

    britishampsrock Member

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    hey i have an orange twin channel amp for a while and the sound is starting to get pretty harsh. im pretty sure i need to replace at least some of the tubes, and since i've heard that my amp is "completely self biasing" i was wondering if that means all i need to do is find out which tubes are bad, pull them and just plug new ones in and it will be ready. is there any other kind of adjustment that needs to be made?? or is self biasing really as east as it sounds?
     
  2. riffmeister

    riffmeister Gold Supporting Member

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    you should replace all the power tubes at one time with a matched set.
     
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  3. drbob1

    drbob1 Silver Supporting Member

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    There's ongoing debate about matching power tubes-the distortion may have more harmonics if they're not perfectly matched, but you're risking having a SERIOUS mismatch if you're using old and new tubes together. At the very least I'd get a set of new, tested tubes from Lord Valve or another reputable supplier here...
     
  4. LaXu

    LaXu Member

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    Yes it is as easy as it sounds. Tube out, new tube in. Just make sure you get a matched set of power tubes.
     
  5. hasserl

    hasserl Member

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    There is no such thing as self biasing, that is a misnomer that is often applied to cathode biased amps. Cathode biased amps are more tolerant of different tubes, tending to stay within the correct operating range due to the nature of how that type of biasing works. But they are not self biasing, and the truth is you should always have the bias checked and adjusted as needed when you replace the power tubes, regardless if the amp is fixed biased or cathode biased. Not having it checked means the amp may not be operating in the best range, which could have a negative effect on tone or the life span of the tubes.

    With that said, lots of people of the years have never had the bias checked when changing tubes, and as far as they knew everything was just fine and dandy. So if you don't want to have it checked, don't do it. If you do want the tubes biased in the correct operating range then have it done.

    You pay your money and you take your chances.

    Hasse
     
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  6. Luke V

    Luke V Member

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    It means you don't have to take it to a tech every time you change power tubes. Everything will be fine if you replace them. With an amp that has a fixed bias you should take it to a tech when replacing power tubes.
     
  7. baron55

    baron55 Member

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    Hassserl, about 8 years ago I owned a Crate Blue Voodoo 60 watt combo amp. And in the operating manual, it said it had an "Auto biasing circuit" that would automatically set the bias for either 6L6 types or EL34 types. Was this just a cazthode biased circuit then. By the way I did run both types of tubes in that amp with no issues at all.
    I mean it is possible to design an auto bias circuit, but I seems it would be pretty involved.
     
  8. hasserl

    hasserl Member

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    "the truth is you should always have the bias checked and adjusted as needed when you replace the power tubes, regardless if the amp is fixed biased or cathode biased"

    You never have to take it to a tech! You do because it is the right thing to do, not because you have to. There is no law that you do so, no one wil arrest you if you don't. The method of achieving bias does not mean that the bias does not need to be checked.

    With cathode biased amps you should still check the bias and adjust as needed, just as you would with a fixed bias amp. If you don't want to because you are frugal and don't want to spend the money, or don't have the time or just plain don't want to, fine. Don't do it. But don't tell people everything will be fine if they don't, because you really don't know that. Chances are it will be fine, but chances also are that it won't be.
     
  9. hasserl

    hasserl Member

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    I don't know Baron, I'm not familiar with that amp, at all. But I thought they were more of a modern high gain type amp and not likely to be cathode biased, which is what is typically called auto biasing.

    I did a quick search on the net for a schematic but didn't come up with one. So I don't know what Crate has going on there. But if you think about the different bias needs of the two tube types this doesn't seem likely. But anything is possible, all it takes is time and money. How expensive was that Crate? It'd be cool if one of the techs around here that has worked on one of those amps and seen the bias circuit would respond.
     
  10. Blue Strat

    Blue Strat Member

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    Another way of saying it is, yes, they self bias, but not necessarily correctly.

    Not every pair of the same type of power tubes will run within spec in every self biasing amp. Also, many "self biasing" amps (notably EL84 amps) run the power tubes at the point of imminent destruction (150% to 200% of maximum idle power dissipation). Why amp designers do this is somewhat of a mystery.

    It's always best to have the bias checked.
     
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  11. baron55

    baron55 Member

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    Yep they were a high gain amp. I don't really know what they meant by auto biasing. I am totallly familiar with fixed bias and cathode biasing amps.

    I thought it was interesting that they made the "auto bias" claim.
    The manual stated that when changing tube types, you had to let the amp run for 20 minutes before playing.


    It was funny, although I no longer have this amp, I still have the operators manual, and I just happened to find it whn I saw this post on "auto biasing"

    Far as I know there are only cathode and fixed bias amps.
     
  12. Blue Strat

    Blue Strat Member

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    Cathode biased is often referred to as "self biasing".
     
  13. reaiken

    reaiken Member

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    You can make an amp that is truly "auto-biasing" by using a few opamps and transistors in a servo loop to control the DC bias voltage to the grids, which then controls the current through the tubes and keep them at a constant level.

    I have an old article somewhere around here from one of the "golden age" audio engineering magazines which describes an auto-bias circuit like this using tubes! I believe it used a pentode for the error amplifier. The trick is in making the DC current not vary when the output tubes clip and create a DC offset due the asymmetric signal at the cathode.

    I spent a lot of time many years ago coming up with a nice autobias circuit that was truly independent of this offset voltage shift, but decided it was too much sordid-state stuff to be putting into the middle of a perfectly good tube amp. There's a lot to be said for simplicity!

    I don't know if Crate actually implemented a bias servo scheme, or if they just implemented a fancy marketing scheme.

    Randall Aiken
     
  14. electronpirate

    electronpirate Member

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    My point on this was that when tube amplification came into it's own, did most (even professional touring) musicians spend alot of time worried about Bias?

    Hell no.

    Time to get hackles up:

    Get matched tubes, and they should be fine, but don't imagine for a second that all this talk of perfectly biasing your amp DEFINATELY will improve your sound. It will LIKELY make it sound better, as it will go with what the amp manufacturer intended, but NOT certainly.

    I personally back this up by years and years of conversations with club and touring musicians who routinely said 'Have no idea what's in that thing' (the amp). Didn't know, didn't care. Just listened to it and said 'sounds good'. This shows me that worrying all about 'exact' biasing, NOS tubes, and 'replacing bright caps', take a huge back seat to just playing your f&*king instrument.

    Yes, this is a variation of the 'tone in the hands' thing, but I believe in making *reasonable* effort in small things regarding your amp (i.e. get decent tubes, and replace them occasionally), but otherwise, spend less time changing tubes, and more plinking.

    -Ready for the flame
    -EP
     
  15. Blue Strat

    Blue Strat Member

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    Someone else will have to do the flaming.:dude

    The point is, if the power tubes are biased too hot, they won't last as long as they should. This can mean minutes, days or years depending on the severity.

    Yes, it's true that people rarely worried about matching and biasing back in the day, but there were instances where amps blew up for no apparent reason (which, at least part of the time, was due to power tubes biased over spec).

    The chances of this happening are probably only about one in six, but it sucks if you're in the one in six group.

    Check your bias or don't be surprised if something unexpeted happens.
     
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  16. murrayatuptown

    murrayatuptown Member

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    Wow, 12 year-old thread. Regarding Crate amp 'auto-bias:
    A question, but first my observations/interpretations.

    I started looking up Crate amps for someone...looking for a schematic for a TV-50H. I did find one on the Loudtech support page. It only had 2 of the 3 pages...missing the output stage. This led to more intense study of what was presented and what was missing.

    I want to share something maybe someone else understands already.

    If you look at a Crate TV-60 schematic (because it's complete and includes the output stage), https://supportloudtech.netx.net/loud-public/#/asset/1704

    sheet 3/3, coordinates B2, you see that the 6L6GT's, pin 5 (control grid? closest to cathode) are connected to a negative bias supply. I'm rusty with tube theory, so bear with me please. I'll call this fixed bias (hoping I am using the term properly).

    Now if you look at sheet 2/3, coordinates F2, there is a complex cluster of fuses and diodes and Q16, D14 & associated components that tie into the output tubes cathode returns. I think most if it is some kind of fault or transient protection. D14 is a 47 V Zener diode, and Q16 (MPSA56 PNP) is in parallel with the 47V Zener. Now if you look at sheet 2/3, coordinates E3, diode D17 produces a -62VDC bias voltage source. Through R111, approximately -45 VDC is applied to output tubes V4 & V5 control grids through 1.5k resistors.

    Heading toward a question, what I see is a fixed grid bias on the output tubes/valves, and a connection to the cathode return that isn't just a simple ground path, but rather a Zener diode that would not be 'Zenering' at the -45V bias.

    If I go out on a limb, I think what might be present in this circuit is output stage fixed grid bias and a tricky cathode return that has a transistor-paralleled Zener. I think the PNP transistor when turned on shorts out the 47V Zener, and the combination of fixed bias and whatever the PNP/Zener pair do to the cathode return result in some interaction between fixed bias and an indirect cathode return path to ground, dependent in some way on the grid bias supply.

    Some of these amp variations can reportedly utilize 6L6/5881 types as well as EL34. While they share the same pinout, I assume they are slightly different, and may need different bias. Near the fuses associated with the cathode returns, there are 12 ohm resistors, so the cathode return is not floating at a high resistance/impedance above ground. But I think there is some possible floating bias between the fixed bias and the cathode return path. Some condition can cause the 47 V Zener to conduct, but the typical -45 V tube-dependent bias is below this (or above if you think about it being negative). Possibilities are 6L6-family tube aging, or use of alternate EL34's.

    Hey, if you are still awake, my question is...is this how Crate accomplished an 'auto' bias that accommodates both tube aging and tube substitution? This seems like a simple self-balancing voltage loop that just depends on a transistor shorting (perhaps gradually) the Zener, causing a 'shift' in the bias voltage. I think the user manual says to wait 20 minutes for stabilization after changing tubes (I don't remember if that is changing the same type or changing between 6L6 and EL34).

    If my interpretation is close to reality, this seems more agreeable to clipping & overdrive in a guitar amp environment that a real bias servo might be unhappy with.

    If this is close to accurately interpreted, that might lend some credibility to the 'auto-bias' claim vs. quickly concluding bogus marketing. We primates tend to be skeptical and intolerant. Maybe there is really something ingenious there other than creative marketing.

    Thanks for reading and any opinions.

    Murray
    Holland MI
     
    Last edited: Jun 24, 2017
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  17. GT100

    GT100 Member

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    Because it sounds good and lack of understanding in some cases.

    Lloyd
     
  18. teemuk

    teemuk Member

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    Basically, it is a circuit that monitors cathode current and adjusts grid bias voltage accordingy and automatically.

    So, it's a "fixed" bias amp, but in a sense that it develops a DC bias voltage to the grid instead of cathode. (The tube only cares about grid-cathode voltage difference). The grid bias voltage, however, does not remain "fixed" but self-adjusts like in a cathode-biased amplifier where voltage is developed to the cathode and is dependent on cathode current (which explains their self-adjusting feature).

    The difference between fixed and cathode bias is that "fixed" bias -scheme can develop higher power output because of operation at lower cathode DC voltage. It also features less degenerative feedback via cathode.
    This self-adjusting scheme also develops a cathode voltage across resistor, but it's at least decade lower in magnitude than cathode voltage in a typical cathode-biased circuit. This circuit self-adjusts like a cathode-biased amp but does not generate high cathode DC bias, which would limit maximum power output. Degenerate feedback via cathode is also lowered.

    Modern Orange, Bugera, Fender, Marshall, etc. amps have similar circuits but they utilize microprocessors (instead of crude discrete "logic circuits") and PWM to develop the DC control voltage.
     
  19. rawkguitarist

    rawkguitarist Member

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    I have a Carol Ann Tucana 3. It has this Auto BiasMon circuit and it works amazingly. It measures the key parameters like plate voltage and current draw just like a good tech would (or how I do with my Dr. Z's). My amp is set to auto bias with 6V6, EL34 and 6L6. Alan tweaked the bias circuit to focus on this slightly lower power band than the stock KT88 in this amp model.

    Of course the bias is set to an optimum that the amp designer chooses, but of course he knows his amps so that's not a problem for me. But if you like running at 5% more or less plate dissipation (for instance) I'm sure you could ask Alan to change that point for you. IMO, this is a good example of what's out there that covers what this original thread asked about...
     
  20. murrayatuptown

    murrayatuptown Member

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    Cool, thanks for the responses. Glad I'm not the only person thinking about such things.

    Last night I realized there was more there than I realized (helps to have a schematic) in front of you instead of trying to remember.

    Without having seen the amp guts, or any Crate amp for that matter, it was kind of an armchair theory exercise. I concluded it was, in theory, simple and sophisticated, unless it sounded like krap and/or was unreliable. I doubt the latter and the former I'm in no position to speak about.

    I read the TedWeber bias charts for EL34 and 6L6 and verified the 50-60-70% bias for myself (might as well learn something on the way. (Not sure where I was headed or coming from).

    So, my friend doesn't like the sound of this amp. That leaves a lot open to interpretation. His amp buddy suspects a bias issue. In theory, I think the owner's manual points to very different specs for tonal range for each channel compared to other random Crate amps, and that might be something to inspect for previous owner tweaking.

    I suggested that since it doesn't seem popular or valuable, and the documentation Crate provides is missing the 3rd power output board, it doesn't have tremendous resale value and is much closer to complete and cheaper than building an amp from scratch. But he didn't bring that up...my opinion leaking out ...that's what I would do if I were me...um...whatever.

    So, I'm thinking about suggesting (read Aiken's last word on biasing article) disconnection of the bias circuit diodes, PNO & Zener) and repkace with simple resistors + pot divider in (10k pot, 27k, 15k or whatever), change 12 ohm 6L6 cathode resistors to 10 or 1 ohm, put DMM voltage test jacks across cathode resistors, or put in a bias circuit for each tube.

    That's a step closer to saying it still has EL34 compatibility. I would hope smart choice of resistors in that bias string will have more adjustment range than the stuff that's there now. That can be figured out later, along with reviewing the preamp stuff...stage by stage...or change it to someone else's vintage circuit. He's more of a blues guy, and it sounds like Crate's high gain stages catered to later generation rock.

    I have a lot of opinions for never seeing the inside of his amp and being 1200 miles away...

    Thank you
     

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