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Thread: Matched Tube Question

  1. #1
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    Matched Tube Question

    Ok, this is a novice Question but...when they match tubes do they match them to get the same plate Voltage or the same bias number at the same Voltage. Also could someone explain the ratings on the Mullard RI that say red hard blue soft or whatever. THANKS

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    Tube matching refers to a pair or quad of tubes that closely match current draw and gm,current draw being the most important factor.In hi-fi applications it is desireable to have exact matching,in guitar amps close is good enough,in fact a slight mismatch is prefered by some,as long as they are close enough to cancel hum in the output stage,like in the area of +/- 5ma's.I am not familiar with the Mullard Reissues,but in general hard refers to a bit hotter current draw,soft a bit less current draw.

  3. #3
    Senior Member Enzo's Avatar
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    To add a bit:

    When they go to match tubes, they set up a circuit - for all the world it is a large glorified tube tester - for the tubes to operate in. There is a test socket. They apply a certain B+ voltage to the plates and screens. They have a certain resistance plate load. They set a certain bias voltage. They apply heater voltage for the filament. They leave all those things the same, and they stick tube after tube after tube into that socket and measure the amount of current it naturally draws under those conditions. They also measure the transconductance. They then sort the tubes by those numbers, so your matched set will have all the tubes with the same current and gm.

    What are those certain voltages and loads? Who knows, there is no standard for that. All we know is that under whatever conditions that tube seller used, these four tubes are the same as each other. So the numbers on the tubes have nothing to do with measurements in your amp.

    Then marketing takes over. All that red and blue and 1 to 10 stuff is just labelling to make it easier to sell tubes. I am not suggesting that the tubes are not different at ends of those arbitrary scales, just that the scales themselves are just made up.

  4. #4
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    I do a little of my own tube matching right in the amp, however I only match the current draw because I don't know how to measure transconductance. I set the bias so that my "standard" tube draws it's standard safe current - let's say 22 mA. That tube is marked "22". Now I stick another tube in the amp and measure the current draw and mark that tube. I have a pile of tubes with numbers ranging from 15 to 29. I can pick and choose a pair that are reasonably close and stick 'em in the amp and adjust the bias on one tube and the other will be close enough.

  5. #5
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    Thanks for all the info. So am I right when they say a hard tube has a later break-up or is it visa-versa...Thanks Again

  6. #6
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    Quote Originally Posted by 89custom View Post
    Thanks for all the info. So am I right when they say a hard tube has a later break-up or is it visa-versa...Thanks Again
    Like Enzo says,the tube rating"scales" are arbitrary,but in general a "harder" tube would break up sooner than a "softer" tube.
    Diablo,you would need a tube tester to measure transconductance,but with guitar amps,the current draw is the main concern,the way you do it is just fine.

  7. #7
    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    Hard --- Soft

    Typically a soft tube is one that draws more idle current with a reference bias and plate voltage and hard does the opposite, draws less current.
    Hard = Cold
    Soft = Hot
    More often then not, a really soft power tube sounds like it is almost worn out and will not produce minimum rated power.
    Bruce

    Mission Amps
    Denver, CO. 80022
    www.missionamps.com
    303-955-2412

  8. #8
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    Quote Originally Posted by Bruce / Mission Amps View Post
    Typically a soft tube is one that draws more idle current with a reference bias and plate voltage and hard does the opposite, draws less current.
    Hard = Cold
    Soft = Hot
    More often then not, a really soft power tube sounds like it is almost worn out and will not produce minimum rated power.
    Jeez!How'd I do that.I had it backwards,sorry.

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