At What Point Is A Matched Prair of Tubes No Longer A Matched Pair

Soul Tramp

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I have a matched pair of Mullard (re-issues, not old) EL84 tubes. When I bias one at 20.1mA, the other is at 14.7mA.

How far off can the tubes be from each other and still be considered a "matched pair"? The other set I have for the amp bias at 20.0mA & 20.0mA.
 

roeg

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I have a matched pair of Mullard (re-issues, not old) EL84 tubes. When I bias one at 20.1mA, the other is at 14.7mA.

How far off can the tubes be from each other and still be considered a "matched pair"? The other set I have for the amp bias at 20.0mA & 20.0mA.


Not matched when one draws 5 ma more than the other.but how do they sound?
I have purposely mis-matched mullards in my 2204.one draws 38ma(or is it 40?,can't remember) the other 33ma.Its said that mis-matched tubes tend to emphasise even(2nd) order harmonics.Sweet.
 

Soul Tramp

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I asked this question of the tech at Tube Depot. To be matched they feel the tubes must be within 10% when biased.

You're correct about the 2nd order harmonics, which is great for blues and/or distortion. The downside is that you will begin to hear AC hum that would normally be cancelled by properly matched tubes. The greater the bias variance, the greater the hum.
 

Fuelish

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The downside is that you will begin to hear AC hum that would normally be cancelled by properly matched tubes. The greater the bias variance, the greater the hum.
Ahhhhhhhhh....ya only hear the hum when not playing ;)
 

nauc

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when people arent willing to pay extra for them.. duh :laugh2: jk
 

Splattle101

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I asked this question of the tech at Tube Depot. To be matched they feel the tubes must be within 10% when biased.

You're correct about the 2nd order harmonics, which is great for blues and/or distortion. The downside is that you will begin to hear AC hum that would normally be cancelled by properly matched tubes. The greater the bias variance, the greater the hum.

I know! Run them in triode so you get decent power supply rejection. That way you won't hear the hum!! Now where's that old box of 300B triodes I had here...:naughty::thumbs:

But seriously, 20 mA and 13 mA is not matched in anybody's book, is it?
 

wagdog

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Most tubes I've received from tubedepot are within 5ma of one another, sometimes less. Eurotubes is better, typically under 2ma and sometimes closer.
 

NativH

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A specific mA deviation really doesn't let you know if they are matched or not. I look at percentages since the bias current is based on bias voltage and that voltage can vary pretty widely. I have Marshalls that run at 380V and some over 510V. At 510V the bias current should be around 34.3 mA but at 380V it would be 46 mA. So a 5 mA deviation would be a little less than 15% at 34.3 mA but about 11% at 46 mA.

In general, I like to have my power tubes match up at around a 5% deviation from high to low and max out around 7.5%. I've had great results with tubes with a 15% spread from high to low, but I realize that I will be running one pretty hot unless I set one very cold. I typically set my idle current at 70% based on the midpoint between the high and low tube when I have a pair, or quad, within 5%. If the tubes are one of those 15% pairs, then I set the bias current pretty hot instead of trying to set the hottest tube at 70%. I just figure that pair/quad will sound awesome from a hot bias setting, but have a shorter life than expected.
 

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