I've written several posts on this topic, anyone interested in the finepoints of tube matching should read them. There is no such thing as a tube matched for everything so the buyer needs to inform the matcher what parameter needs to be matched. The problem here is that the customer often doesn't know what to ask and the seller probably couldn't match for it anyway. However all is not lost. The maker of your equipment should be able to tell you what kind of matching is needed. Unfortunately most tube matchers only match for one thing, Gm, which is basically useless. I imagine the converstion between the customer and seller goes like this..
customer "Are your tubes matched?"
Seller "yes", how many pairs would you like?
THE END.
Here's another conversation about matching.
customer: Are your tubes matched?
Seller: Yes, of course, we always match tubes. Would you like them the same size or the same color.
customer: Huh?
Seller: Just kidding. What do you want them matched for, what parameter?
customer: I don't know, I've been told to ask for matched tubes. How do I know how you match them? I'm just told to ask if you do?
Seller: Of course we do, doesn't everyone? We have a recently calibrated Hickok tube tester, it was the best ever made. We make sure the needle goes to the same place.
customer: Same place for what?
Seller: I don't know, we have some guy in the back that turns the knobs and looks at the needle. The numbers aren't important as long as they are the same. The meter has some big numbers and with the word transconductance under them. There are lots of numbers, we make sure they are the same.
customer: How close to the same?
Seller: The guy writes the number on the box then he puts similar numbers together. Let's not get too technical, they are matched, I assure you.
customer: Well if you say so, I'll take a pair?
Seller: How about the color?
THE END
Quoting Tom Stoppard in
Rosencrantz and Guildenstern Are Dead. "We only know what we are told, and even that isn't true"
Here's what's true though unfortunately a bit complicated.
The simple and correct answer is: We match power tubes for Gm and Bias (two point matching) which is something I believe I was the first to discover around 1980. Now everyone claims to do it, though most don't do it right because they really don't understand it. Some have even instituted three point matching because that must be better than two. I can imaging matching for a great number of points (parameters) but it gets silly to match for things that normally don't matter like matching screen current (that's what the third point often is).
Traditionally power tubes were stuck in a fixture with a set plate voltage, screen voltage (often the same as the plate to make things simple) and the plate current was measured. Two tubes with the same plate current were then considered "matched". However, if the tube is used at a different plate and screen voltage than the test what would happen to that match. Answer: the tubes would no longer draw the same plate current, in fact it could be very far out of match.
What I discovered is that if both the plate current and transconductance (Gm) were matched in a pair of tubes they would "track" each other over a very wide range of plate and screen voltages. Thus a pair of output tubes used in a Dynaco ST 70 where the plate and screen (ultralinear connection) were at the same 475 volt potential would be matched at a much lower screen voltage of 300 volts in regulated screen amps like Audio Research. Since screen and plate voltages, not to mention bias voltages and cathode currents, are completely at the command of the amplifier designer one had to find a way to make a group of tests work for all.
Perhaps I was lucky to have seen this or perhaps I saw it because I like to measure things and design amplifiers. To my knowledge, most, if any, tube sellers don't design amplifiers. They either buy a Hickok or have someone make them a traditional tester, possibly with a computer to record the data, but my computer does much more than that. It actually sets each tube to a constant plate current before the test is made. Thus the bias and transconductance are measured at the same plate current for every tube.
The problem with the fixed grid bias test is that the tubes will settle at different plate currents. Measuring the Gm at these random plate currents will not produce the same results as measuring them at a constant plate current as Gm goes up with plate current quite sharply. I use 50 mA for most tubes as that is a good bogey value for many amps. Even though I run tubes in the RM-9 at 30 mA the matching holds very well. Proof of concept.
Other than for special applications like differential amplifiers or RM-9 input tubes, matching dual triodes is often a fools errand. What you want in a preamp tube is low noise and low microphonics. Of course we can match for gain (mu) if you know how the sections are used. In some preamps the two tube sections are used left and right. In other preamps they are used one complete tube in two stages left and one right. In any case if there are two stages in cascade (one stage following the other) the gain is the product of the section gains.
If you want to know more about tube matching, testing and selection have a look at five articles at:
http://www.tubeaudiostore.com/tubin1.html
