Bias Probe - What To Get?

Discussion in 'The Workbench' started by pedecamp, Sep 17, 2013.

  1. pedecamp

    pedecamp Well-Known Member

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    Some have meters, some don't, some have switch boxes, etc. The first one is highly recommended by some of the members here, but I see there's so many different ones to choose. What to get and why?

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  2. Micky

    Micky Well-Known Member VIP Member

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    For others it may depend on the amp, but I always use a multimeter.
     
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  3. chuckharmonjr

    chuckharmonjr Well-Known Member

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    a 1 ohm resistor, a soldering iron (once) and a multi-meter that reads milli-amps
     
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  4. pedecamp

    pedecamp Well-Known Member

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    Chuck youre gonna have to elaborate on that one, I'm not sure what you mean.
     
  5. bulldozer1984

    bulldozer1984 Everybody's Favorite Member VIP Member

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    A 1 ohm resistor is basically all that is inside these probes and if you can solder and work on amps safely then you can do very cheaply..

    Personally i like probes and I have the first probe you posted. Works great. I would not buy those - cheap made in China - pieces of junk.
     
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  6. chuckharmonjr

    chuckharmonjr Well-Known Member

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    Insert a 1/2 watt, 1 ohm resistor in the bias feed in series. In other words, lift the wire from the power tube pin, solder the resistor there then solder the wire to the other side of the resistor. You have now created a millivolt shunt in the line. Set your multi meter to millivolts, read across the resistor and you have what the bias current draw is in milliamps.
     
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  7. pedecamp

    pedecamp Well-Known Member

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    Yeah, whatever I get I want it American made. Probably will get the one you have.
     
  8. johnfv

    johnfv Well-Known Member

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    I have the Amp Head probes (EL84 as well), I like them. Mine are the more basic ones without the plate voltage switch.
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  9. aw9190

    aw9190 Active Member

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    i have the amp head probe as well. Great tool really, i have the one that gives me plate voltage which is very useful.
     
  10. brp

    brp Well-Known Member

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    I need to get one of these too. Hopefully in Canada but I will go to the amp head site ebay if necessary.
    Any of my Canadian bros. know where to get a good one here in Canada?
     
  11. anitoli

    anitoli Well-Known Member Gold Supporting Member

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    Read this:

    Bias FAQ

    Unless your meter shows plate voltage no milliamp reading will tell you proper dissipation.

    V x I = P

    For example, 465V x 35ma (.035) = 16.2W.

    Read the article and you will be on your way to properly biasing your amp.
    Ill be honest i dont like those probe things at all. 4 resistors and a multi meter is all you need.
     
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  12. pedecamp

    pedecamp Well-Known Member

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    My bias probe showed up today. Popped in my tubes and bias probe in the amp, took a plate voltage reading and got 380 if I did it right, so according to my calculations 70% dissipation I need to set bias at 38mA. Checked the bias on each tube, got 61 on the left and 59 on the right, so lowered left to 38, checked right which measured at 36. So if I raise the right to 38 the left will be at 40. Is my goal to make both sides match or just split the difference between the two, set left at 39 and right 37 since I cant make both match?
     
  13. Micky

    Micky Well-Known Member VIP Member

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    Get them as close as possible.
    I think bias specs say plus or minus 5%, so a couple ma isn't gonna matter that much as long as they are as close as possible.
     
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  14. MartyStrat54

    MartyStrat54 Well-Known Member

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    Exactly. They won't be identical readouts. The circuits would have to be perfect and the power tubes identical. Ain't gonna happen.

    Also, a trick is to swap the tubes and sometimes this will yield a closer bias match.
     
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  15. pedecamp

    pedecamp Well-Known Member

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    They're only off by 2mA from each other, so I'm good to go? I'll just split the difference the best I can, set one at 39 and the other 37?
     
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  16. pedecamp

    pedecamp Well-Known Member

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    Thanks David!
     
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  17. NewReligion

    NewReligion Well-Known Member VIP Member

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    Sounds like you have figured out bro. The bias may drift a bit with time too as the tubes get older and burn in. This can all seem a little overwhelming at times with so much information available and techniques. I too use a bias probe and multimeter then usually run my tubes between 60%-70% dissipation.

    I go so far as to record all of my findings right down to which tube in what amp at a specific location. This too aids me in being able to mix and match sets as I see fit (like yubes).

    If you have any additional specific questions you are welcome to PM me and I will address your questions to the best of my knowledge.

    Respectfully, David ♫
     
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  18. pedecamp

    pedecamp Well-Known Member

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    The 2204 sounds good, nice and crunchy and open sounding with the Winged =C=. Maybe tomorrow I'll try the GT-EL34M and the JJ KT77 in there and see what I like best. Thanks for everybody's help figuring out how to work this thing. :thumb:
     
  19. MartyStrat54

    MartyStrat54 Well-Known Member

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    You'll be a pro by tomorrow.
     
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  20. tazzboy

    tazzboy Well-Known Member

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    Get a Ted Weber Bias Rite Probe they are worth the money.
     

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