How accurate is a "life test"?

[JJGross]

I recently got a Sencore TC28 tube/transistor tester & I've been going through every tube I have the check them out. So far, all of the tubes I've got here are passing every test but a few have failed the 'life test'. One in the '?' area and 2 others dropped below that while holding the test button.

Any thoughts on what that really means? Even the 'no life' tubes passed everything else. Should I ditch them?

Cheers,
- JJ

[Alex R]

I use a B&K 700 which also has a 'life test'. Don't know if it's the same as your Sencore but I think mine works by dropping the heater voltage. I test many a tube and every one passes the life test, including those that fail other tests. So I join you in asking the question...

[JJGross]

*snip* I think mine works by dropping the heater voltage. *snip*

That makes sense because when I ran that test I could see the filaments go just slightly dimmer. Let go of the LIFE button and they'd brighten right up again. Not a huge difference, but enough to see in a lit room.

[Old Tele man]

...originally, "life tests" were intended to show the amount of "usable-life" remaining before the tube transconductance had fallen off to 70% of it's brand-new value (see RDH 4, page 245)...ie: 6L6GC gm = 6000 micromhos, so "end-of-usable-life" would be when gm fell to 4200 micromhos...thus, if the tester measured 5000 micromhos, it would indicate a "life" of 56% or 0.56 or something similar.

[Enzo]

All of which is for the most part meaningless. If the tube had been working OK for you before you got the tube tester, wouldn't you have continued to use it? Just because a tube tester has some feature soesn't mean you should change your life around.

Tube teasters come from an era when TV repairmen came to your house and fixed the set right in your living room. Most of the time it was indeed just a bad tube. The guy had a tool kit and another largish case called tube caddy, full of tubes. He probably had the tubes you needed with him, though once in a while he had to go back to the shop to fetch one.

When you took your TV in, some less scrupulous shops tacked on extra charges for "chasssis cleaning" and other stuff. Just like mortgage lenders today tack on extra charges. The TV man could pull a good tube out and throw it on the tester, and if the "life test" showed it was "getting pretty old," he could sell you a new tube "just in case." Mom knew no better, so she went for it.

I knew a local organ repairman who would go to the home and fix organs. Old Lowery organs were just full of tubes. Each note of the scale had its own oscillator and divider circuits - 12 notes of the scale times four or five tubes. They were oscillators, not amplifiers, so they worked forever. This guy whipped out his tube tester and showed the customer how many of them were "weak" and would inevitably sell them a new "set" of 12AX7s for several hundred dollars. Never mind they worked perfectly and would have continued to do so.

The Sencore company in particular is geared towards doing consumer repairs and they often market their gear in part as some way to extract extra dollars from repairs. They make a leakage detector - the kind of thing that finds out how much leakage current there is on the chassis of someting. Certainly important in medical gear, and of course an old two-wire guuitar amp would have driven it nuts.

They would market this thing - a legit piece of gear I admit - suggesting that on top of each repair, you quick check the repaired unit for leakage curent and charge the customer an extra $10 for the "important safety check." For this $10, you plug the customer's thing into the mains outlet on this gadget and touch a probe to the chasis. A big fancy meter swing over to indicate any leakage. 10 seconds tops. Granted, a hot chassis would be a danger to the customer, but can you imagine charging for each reading you take with a voltmeter while repairing?

Say, there's an idea!

In any case, Sencore does make nice gear, but they also are not above making it easy for a service dealer to make an extra buck. If your tube tests OK, forget what the life test says. No tester will tell yo when the tube has lost its tone anyway.

[JJGross]

So far all the tubes I've tested that fail the 'life test' are unknown and came to me through a recent trashpicking score. Since all my other tubes I've had before all past this test, I really didn't have a decent comparison.

Thanks for the info. AFAIC, if they pass everything else by a decent margin (shorts, emissions, grid leakage), light up and sound OK once I actually plug them into something, they're keepers. I was just wondering how accurate this "Life Test" is in determining actual life - or if it's some arbitrary test designed to sell tubes to people who didn't know any better. I know there was a lot of that back in the old days. My Dad was a part-time TV/Radio repairman in the 60s & had one of those service kits with the giant tube caddy with a tester built into it. Full of tubes too in their original boxes. I wish I had that today, it'd be worth a fortune.

Thanks for the info.

Cheers,
- JJ