Here is the scenario, need a new set of power tubes. I see two matched pairs, One set has a plate current of 33 and the other is 39. Which is a better set and why?

Need more information. Just by the numbers 39 ma should be better than 33 ma, higher emission.

I agree with Norm--more information is needed to answer this question of which is the better buy. Such as what type of gear are they going into, what are the relative prices of the two matched pairs, etc. Also, particular brands of the tubes, when and where they were manufactured, whether they are current-manufacture, NOS or used tubes, etc., may also enter into the decision.

In theory the pair at 39ma should be better than those at 33ma, but only if this so-called "matching" was done by the same person using the same tube tester.

Properly matching tubes requires more than an ordinary tube tester. But on the popular auction site all that "matching" means is that they look the same and have the same date codes. If you want a real matched set, get them from a company that can do it correctly.

+1 to Jim's advice. As an example of such a company, see the following website:
http://www.thetubestore.com/Resources/M ... e-Matching

That's like saying, "here's a pair of brown shoes and here's a pair of black shoes. Which is better?" The answer depends on what else you are wearing today!

Plate current in a tube is the result of its cathode emission, inherent plate resistance, gain, and the voltages applied to all the electrodes. Assuming the voltages are the same, then you've got three variables in play. Different results for different reasons, so there's no way to make a fair comparison without more information. Tube testers give you the result but they don't tell you how they arrived at it so matching based on such results is only approximate.

I agree with the advice given above, if you need a matched set of tubes, get them from somebody who has the resources to do it correctly.

Depends on the circuit. Many of the older P-P circuits aren't very fussy. Close is good enough. If it's a big Mac amp, then pay for burned in matched pairs.

Exactly as he said.

On the well-know auction site the word matched has been used/overused and abused to the point it no longer has any useful meaning.
If all the test conditions are not detailed the values you're reporting are useless and won't tell you anything about the tube's condition. Also, a few mA difference in plate current is insignificant in most situations.
Buy only from a reputable dealer with good customer service history.

I understand the concept about matching tubes and the equipment needed to do so. Out of curiosity, how long do they stay matched or at least close throughout their life?

Since they weren’t nessasarily manufactured any different, one would think that 2 months down the road they could be different. Just curious I guess.

I’m wondering the same thing. I would expect except for laboratory grade equipment, simply replacing tubes in pairs (so you don’t have a brand new tube paired with a very weak old tube) would be good enough.

That is correct, tubes do age at different rates. Normally if you have a piece of equipment that requires this level of accuracy their is also a scheduled interval to retest / match / balance the circuit. There may also be a scheduled retube interval which would assure proper match each time. But for most audio applications this is not required, nor would anyone be able to hear any difference even if a slight mismatch occurs.

Except of course those who use oxygen free speaker wire......

"Except of course those who use oxygen free speaker wire......"

How do those speaker wires breathe?

"Except of course those who use oxygen free speaker wire......"

How do those speaker wires breathe?

The answer is: not very long, and two months is already a very optimistic estimate. Some Special Quality industrial tubes intended for critical applications who (unlike audio) really needed matching went through a pre-aging process (sometimes taking many hundreds hours) and were sold with stabilized characteristics (for a limited time) but NONE of the currently produced tubes are subjected to any pre-aging process at the factory, not even the "premium" gold-plated pins ones.
Matching (whatever that means) is not a critical issue in most (correctly designed) audio equipment, except for those... (see above)