Power Tube Failure

About the tube testers out there for sale. If the tester measures mutual conductance it is a better one and provides a lot of useful data. If it is not a mutual conductance tester it’ll usually only tell you good, bad , shorted ,not shorted. Mutual conductance testers give you a couple of readings in the chart for new values and minimum values. So you’ll know how close to new it’s available electron supply is , how new it’s performing . 

@inagroove 

@brownred6 

"I’ve been told by tube experts most tube testers can not match tubes. They do not supply actual high DC voltage needed to the plate of the tube and thus can not measure actual plate current."

The above statement is absolutely true. I've been told by a very reputable tech that is working on a piece of equipment I have who said exactly the same thing and pointed at a tube tester under his workbench and said he may have only used it five times in the last several decades. I believe he prefers to test his tubes in circuit.

@faustuss @brownred6 @jea48 

Thanks for all of the insight...

OK, it seems that a basic good/bad tester will get me to first-base, providing % Conductance and Leakage, but a tester with "Mutual Conductance" is needed to access the quality of dual-triode tubes (I have a dozen12AX7s hanging around).

A. Did I get that correct?

B. If yes, I wonder why do so many NOS sellers stress the need for "Matched Pair" tubes (L/R) if Mutual Conductance is more relevant than matched pairs?

C. Does the need for Mutual Conductance testing go away if a Amp/Preamp is self-biasing?

Thanks in advance for any insight and clarity.

Best,

I have a modern MaxiPreamp II which is indeed very good for assessing & matching small-signal (preamp, input, driver) tube types. There is nothing to calibrate and no tubes in it to replace. You can easily match your tubes based on its transconductance (gm) and gain readings. Generally try to get all triodes / elements within 10%. Weak / worn tubes are easily spotted by low gm readings. I would note that modern Russian tubes, when new, often measure much stronger than even "tests strong" or "tests NOS" vintage tubes of the same type. Once I accidentally severe;y abused a pair of Siemens E88CC over a ~20 hour period (running them at 12V heater instead of 6.3V lol), and the gm readings went down 25% from before (still good range, but they were super strong before this) - yep that tracks lol. 

Also have their MaxiMatcher (specifically for power tubes), and it’s super for matching or re-matching sets, and spotting tubes that are getting worn down. It can identify shorted tubes (never put these in your amp). It applies close to "real world" DC plate voltages up to 400V. That said, these modern Russian or Chinese tubes always have an element of risk. Earlier I mentioned receiving a bad octet of Mullard KT88 from Viva Tubes - put them in the tester today and they actually measure great! Strong gm, mA in range, readings pretty steady, no shorts. Unfortunately it’s just one of those edge cases where they measure good but I can’t use ’em. I do think Upscale does a better job screening new tubes with their burn-in process. Still have yet to get any power tube from them that was a problem. 

I love vintage gear, but didn’t want to mess with vintage testers. I wanted something that was consistent and just worked without fuss. I have enough tubes here to justify the spend. I don’t really see the point in an emissions-only tester - you cannot use those to match tubes.