Do i need to Re-Bias my amp if i replace the tubes with the same type?

Discussion in 'The Workbench' started by thinlizzy96, Mar 24, 2011.

  1. thinlizzy96

    thinlizzy96 New Member

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    Say I was to replace my tubes and I replace them with the same exact type/model, do i need to re-adjust the bias?

    thanks!
     
  2. T-Bird

    T-Bird New Member

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    Hi.

    Ideally no, but You should always check the bias in any case and adjust if necessary when changing power tubes.

    Mesa claims are mostly BS and so are GT claims as well when it comes dow to the need (or the lack of it ;)) for re-biasing after a tube change, if that's what You're wondering.

    Talking about A/B PP fixed bias circuit mainly, obviously. On class A SE it shouldn't be necessary. If the same type of a tube is used.

    Regards
    Sam
     
  3. thinlizzy96

    thinlizzy96 New Member

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    Thank you so much!
    so essentially it isn't completely necessary if i'm using the same type, but occasionally it'd be good to check to make sure?
     
  4. thrawn86

    thrawn86 Well-Known Member

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    I would check them.
     
  5. thinlizzy96

    thinlizzy96 New Member

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    i guess it couldn't hurt!
    thanks!
     
  6. Wilder Amplification

    Wilder Amplification New Member

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    ALWAYS CHECK THE BIAS WHEN REPLACING OUTPUT VALVES!!!

    "Theoretically", if you get valves that have identical matching numbers (i.e. PC and TC numbers) to the ones you're replacing, the bias SHOULD be perfectly adjusted for the new set. However, there is the human factor, and when there's the human factor, you remember really quick that we don't live in a perfect world. Who's to say the matching was done correctly? Who's to say the valves got the correct labels on them? Who's to say they didn't end up in the wrong box with the wrong labels on the box?

    You do not know with 100% certainty unless you have the bias checked. Just like you check all your bearing clearances, degree the cam, check the ring gaps, valve to piston clearance, valve train geometry, etc etc when you build a performance motor so that you know with 100% certainty that everything is right on the money, you ALWAYS ALWAYS ALWAYS check the bias when replacing output valves. Plain and simple.
     
  7. Ron Meersand

    Ron Meersand New Member

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    See this amp:
    [​IMG]

    Had E34L JJ tubes in when It was brought to my shop.
    I put all new E34L in same tubes but different bias point and 3 were labeled #49 while one was labeled #48 in the next photo you will see the difference in the bias point of one.
    [​IMG]

    you will see from the read out that at this set up V8 is doing 15.8 watts @ 34ma and the odd tube out is only making it to 13.5 watts @ 29ma
    BTW V6 & V7 are spot on with V8 This is a very good reason to bias every time you replace your power tubes this type of circuit tends to drift over time.
    I could not have replaced this amps tubes with out testing and adjusting the bias even though I was replacing the same tubes.
    The old tubes were running at some crazy 57ma and of course one of the old tubes was bad and had low Emission under 20% a good tube is 80-100%.
    This amp has switch's on the back to switch tubes this does not work I don't know what this designer was thinking. Any way it's not Marshall but it gives you and idea.
    (I hope this keeps it simple enough for you)
    Best of Luck,
    Ron
     
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  8. thinlizzy96

    thinlizzy96 New Member

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    thanks for all the detailed posts guys!
    yeah i'm definately gonna bias it, or get someone to do it for me considering i have never had to change tubes before
     
  9. Wilder Amplification

    Wilder Amplification New Member

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    I'm gonna add a side note here for those who aren't yet in the know but are still learning.

    The wattage figures that Ron is talking about are what the valve itself is dissipating in a quiescent (i.e. "idle") state. This says nothing about what the load is dissipating which is very little compared to what the valve is dissipating in a quiescent state.

    Furthermore, this also says nothing about what the load will dissipate when the amp is putting out a full clean power signal.

    The valve dissipation will increase when hit with an input signal. Assuming a full power clean signal, valve dissipation will be at its highest when the signal is at 1/2 the peak swing, then dropping down to its lowest at the peak of the swing. This happens because most of the power is being transferred to the load and the voltage across the valve is at its lowest, while being at its highest across the OT primary, yet current is the same through both the OT and the valve (current is the same at all points of a series circuit).

    What does this mean? This means that "plate dissipation" and "output power" are not one and the same like most tend to think. People ask all the time "How can you get 50 watts out of a 25 watt valve?". That's because plate dissipation power and output power to the load are not the same thing. Furthermore, power does not come from the valves. It comes from the "power supply" (hey...maybe THAT'S why they call it that!). The power supply draws current through both the valve and the OT primary, while the valve is just a metering device that meters this current based on the input signal that is fed into it. Think of the input signal as a "control signal". The greater the amplitude of the input signal, the more the valve opens to allow the power supply to draw more current through the OT primary (wow...is that why they call it a "valve"???).
     
  10. thinlizzy96

    thinlizzy96 New Member

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    wow thanks for all the info!
     
  11. dezzy

    dezzy New Member

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    Not sure U could mistake a 6l6 tube for an EL set, most marshall amps carry ELs
     
  12. chuckharmonjr

    chuckharmonjr Well-Known Member

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    Not a question of 6L6's vs. EL34's Dezzy. EL34's can vary widely all over the map. Thats why you always re-bias with a new set.
     
  13. mrrstrat

    mrrstrat New Member

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    Great info on this thread for you!

    When it comes to replacing a grade of tubes with the same and not rebiasing...

    Its YES if:
    (1) Not a lot of time elapsed since tube change (weeks, normal playing)
    (2) You know the tubes are from the same batch and the grading is good
    (3) If you dont bias at the upper end of idle dissapation (see above posts for definition of this)
    (4) You dont mind rebuilding the power section if a tube fails, or giving your local tech something to do for an hour of so and some money

    Its NO if:
    (1) You have significant time between changes (component degradation is NOT at a known constant, neither is what true state of the amp since the last time you retubed)
    (2) You have significant play on the amp since last change (again, the wear uncertainty thing)
    (3) You have amp idle biased 'at the upper end' (like above 70%)
    (4) You value your amplifier and dont think the risk is worth not spending 20 bucks for a rebias
    (5) You cannot fix the amp

    Its a risk/reward thing. I rebias my own gear because I can every time, and I like to observe the component wear to understand the natural progression of wear first hand.
     

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