Tube change by amateurs

Discussion in 'The Workbench' started by LPMarshall hack, Dec 9, 2009.

  1. 5er driver

    5er driver Active Member

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    Sh*t man, I want to bias my own amps so bad so I don't have to take them to a tech for it but all this technical stuff is giving me the heebie jeebies. I can do a cap job though, I recapped my '79 Fender Twin - 13 caps, 7 in the dog house, 6 on the chassis, without electrocuting myself. Simple component soldering. :shock:
     
  2. Wilder Amplification

    Wilder Amplification New Member

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    In its simplest terms (i.e. what you NEED to know) -

    When the amp is sitting idle with no guitar plugged in, the power valves are drawing current from the power supply. This current draw cannot exceed a certain value. This value that it cannot exceed is determined by the plate voltage on the valve (i.e. the DC voltage at pin 3). It is this plate current draw that you're adjusting when you adjust the bias control.

    When you combine voltage and current, you have power. When the valve is drawing "X" amount of current with "Y" amount of voltage on it, it is dissipating "Z" amount of power (watts). By multiplying this current by the plate voltage, this will tell you how much power it's dissipating.

    The max rated plate dissipation for an EL34 is 25 watts. In a Class A/B amplifier, you do not want to exceed 70% of that at idle, which is 17.5 watts. To figure out how much current would put you at the 17.5 watt maximum, you would measure the plate voltage, then perform this equation -

    17.5 / Measured Plate Voltage = Maximum bias current in Amps

    Example - say you measure 475VDC on pin 3 -

    17.5 / 475VDC = 0.0368 Amps, or 36.8mA (move the decimal to the right 3 spots to convert Amps to Milliamps)

    This would be your MAXIMUM plate current at idle for that plate voltage. You can set it lower than that, but you don't want to exceed it at that plate voltage.

    Now the trick is in how you measure your plate current. I prefer to set the meter up to read DC Amps, then hook it up between pin 3 and the HT fuse, which shunts out one side of the output transformer and makes the current flow through the meter instead of through 1/2 the OT.

    In a 50 watt amp, you have one power valve on each side of the OT primary. You have to check the current draw on both of them to figure out which one is drawing more than the other, then measure off of that one when setting the bias.

    On a 100 watt amp, you have TWO valves on each side of the OT primary. Same deal, you have to check the current draw on both pairs to figure out which one is drawing the most current. However, since you're seeing the current draw of BOTH valves when measuring current across the OT, the reading will be DOUBLE the value you got from the above equation.

    Now...when doing this, you have to switch your meter between DC Volts and DC Amps. When in DC Amps mode, the meter is essentially a dead short from one probe to the other. This means that you don't wanna try to do a voltage measurement while in Amps mode otherwise the meter will create a short to ground and you'll short the supply out. You also don't wanna touch the other probe with the meter in Amps mode while the other is connected in the amp or you'll shock the shit out of yourself.

    Now when setting bias, you have to watch the plates in the valves to make sure they're not "redplating". If they are, set the bias back full cold and figure out where you went wrong, then start over.

    Also, once you get it set right at idle, play the amp at your normal playing volume while watching the plates to ensure that they aren't redplating at your normal playing volume.

    Then you can play with the bias control for the tone you want WITHOUT EXCEEDING THE ABOVE DETERMINED MAXIMUM PLATE CURRENT.

    If you're still with me, I will post up the instructions on how to bias using this method. However, if after reading this you feel that it's something you wouldn't be comfortable doing, let me know.
     
  3. 5er driver

    5er driver Active Member

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    Thanks Jon, that's what's giving me the heebie jeebies. But seriously, thanks for the info, the way you have explained it I think I can figure it out. So the bias procedure you described is the shunt method? Is it more accurate than using a commercial bias meter? To tell you the truth, I would feel safer not putting my hands into the chassis to get the necessary measurements. Of course I'll have to reach in anyway with an insulated screw driver to adjust the bias pots (100 watt amp). I suppose I could use a bias meter that reads both plate voltage and bias current with the tubes in the meter's sockets and those sockets the amp's tube sockets. But I'm all for accuracy. I'll be getting a quad of TAD's 6L6GC-STRs in a month or so for my SL-X, when they come in I'll run the calculations by you to see if I'm on the right track. Thanks again for your professional time and technical information.
     
  4. Wilder Amplification

    Wilder Amplification New Member

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    I'm not sure how the bias probes are wired as I've never used them (was always taught the old school methods so I never saw a need for them), but for the guitarist just wanting to set his own bias, it would be about the safest way to do things. The main point here is to ensure that you set the bias control in a way that gives the best tone without allowing the valves to go into over-current/redplating at anytime. Using the above formula to calculate the maximum for your plate voltage will help you to achieve this.
     
  5. tresmarshallz

    tresmarshallz Well-Known Member

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    Get a Weber Bias Rite https://taweber.powweb.com/biasrite/br_page.htm

    I've been using one for years to bias all my tube amps, they are fall down easy to operate. All you have to do is:

    1. plug them into the chassis tube sockets then plug in the tubes
    2. switch the Bias rite toggle to read the plate voltage
    3. switch the Bias rite to read get the bias reading and adjust

    This completely eliminates poking around in the amp with probes and worrying about how to set the meter AC or DC. SOOOO much easier and safer than the old school methods for those of us that aren't at the amp tech level yet.
     
  6. MajorNut1967

    MajorNut1967 New Member

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    The bias rite might make taking measurements easy, but you also have to the skill & experience to know what these readings mean and what to do if there are inconsistent readings or why one tube is red plating and the rest aren’t! Its not that straight forward, you may get away with it for quite some time, but the one time when an issue does arrive and you have the potential to blow up you amp and you don’t know what to do, then what?
     
  7. Iron Mang

    Iron Mang New Member

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    Wow Brah, you have to be so harsh? I mean your point is well taken, but jeez!:confused:
     
  8. 5er driver

    5er driver Active Member

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    Contact the Marshall Forum!
     
  9. tresmarshallz

    tresmarshallz Well-Known Member

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    I don't think he was being harsh, it is true, the bias rite doesn't solve everything :) It's good to point out that a person should strive for as good of an understanding of everything involved as possible.

    The bias rite gets me by 9 times out of 10, if there is a problem that I can't fix then I take it to my amp guy, and it's usually something really simple that I overlooked.

    Bottom line, the bias rite is perfect for the person me to eliminate most of my tech visits and lets me experiment with tube changes a lot more.

    In my opinion, the worst thing that can happen if you figure the bias equation wrong or if you set it too cold or hot (which I've done many times), is that either it sounds bad, or in extreme cases you blow a tube or fuse. Big deal, you live and you learn, and then rock on:dude:. I have never blown up my amp by getting the bias setting wrong!
     
    Last edited: Dec 11, 2009
  10. Wilder Amplification

    Wilder Amplification New Member

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    Just because it hasn't happened to you doesn't mean that the potential for it doesn't exist.

    However, the fact that Marshalls have an HT fuse helps in keeping you from blowing anything up.
     
  11. tresmarshallz

    tresmarshallz Well-Known Member

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    I'm trying to help the guy, I could relate to his level of knowledge and apprehension for doing biasing by relating that for ME the bias rite was the perfect solution. In my humble opinion, the old school method is way to dangerous and confusing if you are novice and you should only attempt it like that if you have seen someone do it firsthand and learned from them.

    I certainly didn't claim that there is no existing potential to screw something up, please stop reading so much into what I type Mr Wilder. I am simply said I have never screwed up any of my 4 amps by setting the bias too hot or too cold...nothing more, nothing less.

    peace out
     
  12. Wilder Amplification

    Wilder Amplification New Member

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    I don't mean to read too much into what you type. I'm experienced enough to know what you meant. However, lots of people here are novices and WILL read too much into what you post because they simply aren't at that knowledge/experience level to know any better. Statements like "I've done this and that has never happened to me" are taken by the novice who doesn't have the knowledge/experience to know any better as "Well since he's never had problems with it that means I won't either". I was just making it clear that while your experience differed, that the potential for something to go wrong still exists.

    I realize some of my posts may come off as "harsh"...I don't mean for them to. When I type stuff, I take into account all the possible ways that my post could be misinterpreted, and word them in a way that is cut/dry to the point with no possibility for them to be interpreted except for exactly how I stated my point. Sometimes that comes off as me being "pompous" and thinking I know more than everyone else...but that is not my intent by any means. I just want to make sure that when I say "This is what's going on...blah blah blah" that it's crystal clear, it makes perfect sense, and that anyone who reads it will interpret it the exact same way and IMHO I feel that others should take into the consideration that anyone could be reading their posts at anytime, and should word it in a way that is commonly comprehensible by all who wish to read it. That pretty much eliminates the possibility of others "reading too much into" what you post.
     
  13. big dooley

    big dooley Well-Known Member VIP Member

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    in fact jon's way of biasing an amp, makes me eagering trying it next time with my JVM... just to see how the testpoints compare with actual readings...
    at this point my 410H with KT77's has a plate voltage of 430Vdc with 80mV at the testpoints, which (according to marshall) results in a 40mA current draw per tube (69% of max dissipation, pretty high)
    but i'm not sure how accurate those measurements are
    the biggest issue i have is the main AC voltage in our rehearsal room which, depending on how many other equipment is being used, differs between 200Vac and 220Vac...
    i can see this will also affect the voltages/currents in my amp
    i should check the bias with all the other equipment on and off, so i know what this actually does to my amp...
    or better yet... overhaul the main powernet, for a more stable main AC voltage
     
  14. Iron Mang

    Iron Mang New Member

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    Sorry guys I was just teasing! MajorNut1967 is my Brother-in-law. I was just razzing him.
     
  15. solarburnDSL50

    solarburnDSL50 Well-Known Member

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    I read that and scratched my head WTF? I have always biased mine after powering up. I didn't know any other way.

    Wow. I'm thinking about what you explained later. Its just too much work now hehe.
     
  16. MajorNut1967

    MajorNut1967 New Member

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    Well I am glad you have never blown up any of your amps. But there are a few amps that if you go a 1mA too hot they are gona blow "especially the Marshall Major!" And if you figure the equation wrong and red plate the tubes and blow the tubes or the OT, its no "Big Deal"? Man I'm glad you don't work on my amps. Because if you ever blew up the tubes or the OT in my 1968 Model 1987 I'd be extremely unhappy! First because it has two Mullard NOS XF1's in it and the OT is the original and its sound is Holy Grail.
     
  17. MajorNut1967

    MajorNut1967 New Member

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    LOL :lol::lol::lol::lol::lol::lol:

    What Wilder meant was to turn the bias pot up to max cold (or max negative voltage) before you turn the amp on and out of stand by. He's super cautious about new tube installs so its the way to lessen the potential damage.
     
  18. solarburnDSL50

    solarburnDSL50 Well-Known Member

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    That would be regrettable X10!:mad::(
     
  19. MartyStrat54

    MartyStrat54 Well-Known Member

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    You gotta love some people's attitudes about things.

    I stepped in a big pile of dog shit...I guess I'll throw my shoes away.

    Oh look. Someone put a small dent in my car door. Time to take it to the junk yard.

    I over biased my amp and the OPT blew along with the tubes. Time to throw my Marshall in the trash.

    Oh well, you live and learn. Party on!

    :wtf:
     
  20. tresmarshallz

    tresmarshallz Well-Known Member

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    "blow the tubes or the OT, its no "Big Deal"? Man I'm glad you don't work on my amps. Because if you ever blew up the tubes or the OT in my 1968 Model 1987 I'd be extremely unhappy!"

    You guys need to relax a bit and stop making silly comments on everything I say. I thought I made it clear that I am a novice who is learning this stuff, which is why I do biasing the easy way with a weber bias rite. I sure as hell wouldn't be working on anyone elses as I am not an amp tech, I work on my own amps and it has saved me a lot of money and I've never screwed one up. Has anyone ever blown an OT by biasing thier tubes too hot?
     

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