How do you bias Your slightly un-balanced Power tubes

Discussion in 'Amp Tech Center' started by Platefire, Jul 21, 2018.

  1. Silverface

    Silverface Poster Extraordinaire Ad Free Member

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    I answered already but you either missed it or are stuck on the math.

    Use the calculations ONLY as guidelines. You HAVE to use your ears or the bias process is a waste of time. There is NO tone to "70%, or "60%", balanced or imbalanced. Those numbers exist only to develop general guidelines and "safe ranges" for tube types. They have nothing at all to do with sound.

    To repeat - bias based on an average - and it's arbitrary. Use 70% dissipation as the center point, or 65%, or whatever.

    Then ***play at normal volume and adjust the bias a few ma in each direction. Find the "sweet spot". Then STOP***

    Check to make sure you're in a safe range for the tubes. If not, your tubes might be shot, because it will probably be fine - maybe a bit on the hotter side if you're like most players.

    Then you're done.

    The difference between the two tubes is immaterial, really, because (as I and several others already noted) the driver is rarely balanced anyway. That makes i impossible to precisely balance "matched" tubes.

    Anyway - it sounds like you're hung up on the math. Don't be. Use your ears and only use math as a VERY loose guideline.
     
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  2. Silverface

    Silverface Poster Extraordinaire Ad Free Member

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    PS you could buy a Weber Bias Rite with the plate voltage option and check plate voltage and bias in ma without doing anything but removing the power tubes, inserting them in sockets and inserting the sockets in the amp. No scope or multimeter required and no amp mods.

    You can check and/or adjust the bias with one in 5 minutes. There are other brands as well.
     
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  3. Platefire

    Platefire Friend of Leo's

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    I usually reference the Weber Bias Calculator site when I do bias here:
    http://www.tedweber.com/webervst/tubes1/calcbias.htm

    If you know your plate voltage you can decide rather you want it cool 50%/Normal 60%/Hot 70%

    Like you say, setting it and playing it at several setting tells you where it's sounding best. Platefire
     
  4. gusfinley

    gusfinley Tele-Holic

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    Yes!

    Your "matched tubes" would have to be pair with a matched phase inverter tubes and matches resistors on both sides of the power amp AND the output transformer would have to have matched windings.

    If you a concerned about balanced output, measure the difference between the two power tubes, then swap them with each other and measure the difference in that position. Choose the position with the best match, or the best tone.
     
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  5. ocduff

    ocduff TDPRI Member

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    Add a bias balance along with your bias pot (good for only a 10ma difference between tubes I've found).

    Better yet, install a bias pot for each tube.

    Then you can make Matched tubes unbalanced, make unmatched tubes matched, whichever suits your fancy.

    But really if we are talking about modern production tubes, just set them where it sounds best - just under red plate. :) Take that as a joke or however you want.

    If you do above you won't have to debate anything. Just set them to whatever you want.
     
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  6. Silverface

    Silverface Poster Extraordinaire Ad Free Member

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    Concern about mismatched tubes - unless one is simply bad - is misplaced if the primary point of setting the bias is the sound of the amplifier.

    If, OTOH, your major concern is the life of your tubes - move along, nothing of interest here.

    OK, they're gone. :D

    But because there will be stragglers -

    Reality check #1: A driver circuit is rarely balanced.

    Reality check #2: "Matched power tubes" may or may not sound better than mismatched power tubes.

    Reality check #3: "Matched power tubes" in the guitar amplifier market is an "upselling marketing term". It is also a "technical term". It is NOT a "superior audio quality" term.

    ("Upselling" = the tube companies get you to pay more for matched tubes. Do you NEED matched tubes? Read on...)

    Reality check #4 Setting bias by mathematical calculations based on one or more established standards is a guarantee only that you will make calculations that meet those standards. Read that again.

    Your amp may sound wonderful. Your amp may sound like crap. Your amp may sound somewhere in between the two. You can't predict WHAT your amp will sound like because math doesn't have good or bad tone.

    Reality check 4a: Math has NO tone. And in case there are any left that use an oscilloscope to "set and forget" power tube bias - I can't help you.:eek:

    Audio - "sound" - is subjective. In my subjective view, if you have a mismatched set of power tubes try an initial bias setting that is the tubes' average 70% dissipation based on the plate voltage (without knowing the plate voltage you can't do ANY bias calculations) -

    - then play through the amp at normal usage volume - not turned up to "2" if you normally play with the amp cranked.

    And *listen*. Adjust the bias a few ma above and below that average (you can't just set the negative bias voltage - it doesn't take the tubes into account).

    Find the best-sounding point. Are the tubes redplating? No? Good, you're done.

    But what about making sure the tubes are close to 70% dissipation? What about tube life?

    Errr - if you asked that didn't you actually leave earlier? :lol:

    Seriously - sure, go ahead and check that they're in a "safe" range. You probably should note the figures anyway so you can check for drift if the amp starts to sound bad. But if they're not redplating at usage volume you should be fine.

    And if they ARE redplating at the "sweet spot" you have a decision to make - either dial it back and live with somewhat lesser tone or don't use those tubes.

    And FWIW I find perfectly matched tubes and drivers to generally sound wonderful in audiophile systems but cold/sterile in guitar amps.
     
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  7. Platefire

    Platefire Friend of Leo's

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    Thanks everybody for all the input. Imbalance of a set of power tubes is a given, as long as it's not an outrageous amount. After reading everybody's input I've decided to adjust the hot side of the pair approximately 70% Dissipation and let the cooler side fall where it may. This is my story and I'm sticking to it:D
     
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