"Matching" of dual triode tubes - how close?

There are actually two questions; balanced triodes in a single tube and balanced tubes, and the need for either in a preamp depends on how and where they are used in the circuit.  

markus, you should send atmasphere an email.  He builds amps and likes the 6SN7 tube.  I'm sure he could tell you whatever you wanted to know about it.  He posts in the forums here pretty regularly.  If you do correspond with him, please post what you learn here.

6SN7s are dual triode small signal (preamp) tubes. My reply and the information from the Upscale Audio webpage applies to 6SN7s used in a preamp.

The 6SN7 is my favorite tube, and I've used them extensively in at least 3 different preamps. My experience is matching within 20% is more than enough, but here's something else to consider: some (maybe many?) preamps only use 1 of the 2 triodes for each channel. This true for my current Modwright LS100, confirmed by Dan Wright, who said you could use tubes until they were below nominal for one triode and then swap them and use the other triode. In that case, it wouldn't matter that the triodes in the same tube matched, only that triode 1 of one tube and triode 2 of the other matched, and vice versa.

So the takeaway is I mostly agree with millercarbon. Buy tubes that are close and don't sweat it. And if you buy tubes from private parties, invest in a tube tester. I'm glad I did.

To be specific, my question is focused on use of 6SN7s in a preamp.

I’ll answer so that maybe someone who really know what they’re talking about will correct me. For most equipment, 20% matching of small signal tubes (preamp tubes like the 6SN7) will work. Tighter matching may be necessary for high quality phono preamps or other devices that amplify very small signals. The lower measuring tube of a pair will wear out sooner, so you may want to match them within reason so that they both get used up at about the same time.

Here is what Kevin Deal of Upscale Audio has to say about it. He has bought, sold, tested and listened to more tubes than your average Audiogoner.

https://www.upscaleaudio.com/pages/grading-matching

Driver / Buffer grade -

Note: There is nothing wrong with these tubes in many applications and they are an opportunity for you to save money!

The tubes that are low in noise, but not lowest in microphony. There are some tubes that will never be low in microphonics... this is by design. Usually these tubes are used in a circuit where they don’t need to be low in microphony, such as the driver stage of an amplifier, or an output buffer in a DAC or CD player. In these circuits, purchasing a low microphony tube will not yield any performance benefits, so take advantage of the lower price, and take your sweety out for a nice meal.

Please note that with some tubes types, almost 90% will be this grade. Count on us to walk you through and get you the right stuff.

Gold - This is a tube that is low in microphony and has excellent noise and triode balance measurements. Typically triode balance for Gold Graded tubes will be between 15% and 20%, but may be higher depending on available stock. Perfect for preamps, phono stages, and the input stage of power amplifiers. It can’t get much better, unless you go to Platinum Grade or Kevin’s Stash.

Platinum - Sometimes you just want the cream of the crop... Whether it’s personal preference or a difficult circuit, our customers demand the absolute best. So for this customer, we offer Platinum grade. The difference between Gold Grade and Platinum Grade is a measurable difference in triode balance. Small signal tubes such as 6922, 12AX7, 12AT7, and others are like two tubes in one glass envelope. That is why they are called "Dual Triode." Triode balance in Platinum Graded tubes is typically 10% or tighter, but may be higher depending on available stocks. They will also measure at the tippy top for transconductance.

Honestly never tried the exact comparisons you're talking about. Always used matched tubes. Well, not always. Occasionally something happens, one goes bad, and instead of swapping out the whole set I run a spare. Over the years this has probably happened a dozen times at least. The spares are never matched, and swapping just one in I've never really noticed a difference. Well maybe its there, just not enough to notice without really trying. And I notice an awful lot of things even without really trying.

So I would put this in the same category as something like bias. If you let your bias wander way off and then dial it in, does that make a difference? I want to say yes. A few times there seemed to be a small barely noticeable improvement in focus or palpable presence. Never easy to quantify such things but I would say its in the same range as cleaning a few (not all, just a few) contacts but less than demagnetizing or even cable dressing if yours are all tangled up on the floor, and certainly way less than putting them up on cable elevators.

So if it was me I'd just buy the tubes, wipe the pins, and call it good.