Tube Testing: What do the numbers mean?

[Alexo]

Hey all,

Does anyone have a link to or a nice explanation of what the results from tube testers mean? If you have a 12AX7 that tests "140" what does that refer to? 140 out of what?

Thanks!

[tarzanalog]

Depends on your tube tester. It should have a scale on the meter that shows the minimum good value or even a good range, a bad range, and a "?" range like on my EICO testers.

this... it's an "Idiot's Guide to Tube Testers" (...no offense.)

http://tone-lizard.com/Tube_Testers.html

[goes_to_11]

Hi all. I'd like to bump this one because I've been wondering the same thing. One common rave by tube sellers is 'tests *BETTER* than new!!!' or something along those lines. What is the value of new? Full scale on whatever tester they happen to be using?

I know that it is possible for a tube to exceed the average of new but I've never come across any reference to what that value could be. Any comments?

Thanks.

[jckid66]

Hi,
I have a sencor mighty might tube tester.. according to my meter any thing from 70 to 130 is concidered good. but I try to match tubes as close as possible... basicly if i have say 4 6L6 tubes and they measure 115,120,80,90 from there i will match up the 115 annd 120 as an acceptable pair and the 89 and 90 as also a pair.. but it has been my expeiience that mis-matched tubes will lead to bad sound quality and premature death of the amp.... but thats just my $0.02



Bill

[Geek]

Hi,

To complicate matters....

No tube tester of different brands will read the same. Among the lower quality testers (emission only types), there can even be a difference between manufacturing runs within the same brand. The reason for this is in the way the tubes are being tested.

Cheers!

[geoff_bell]

and they'll likely need calibrated regularly to ensure true readings.

[VintageJon]

I use a Hickok 800 to determine the Transconductance of pre-amp tubes.

Every amplifier tube has a transconductance Minimum figure in the spec.
For 12AX7's it's 1000.
This is given in micromhos and abreviated Gm.

Figures given for emmission type testers are nearly meaningless,I'm sorry to say...

-Jon

[tarzanalog]

No tube tester of different brands will read the same. Among the lower quality testers (emission only types), there can even be a difference between manufacturing runs within the same brand.

A calibrated tester is a MUST. If a seller (on Ebay, for instance) claims readings for their tubes it's useless if they are not using a calibrated tester.

Figures given for emmission type testers are nearly meaningless,I'm sorry to say...

While that's true, emission testers are handy for testing tubes quickly as good or bad.

My EICO is a "dynamic conductance" tester. It's also calibrated and I can basically rate tubes for strength pretty closely. Are those "matched" tubes?!? NO. Many tube vendors sell tubes that are paired up based on strength and they call them "matched". While it's best to have tubes that have relatively the same amount of life left in them and the same output potential, it's even MORE important for them to draw the same amount of current in an AMP.

To me, that's what a matched set of tubes is... a pair, quad, sextet, whatever that draws the same current and is seen by the amp as essentially identical. To match for current draw I have a bias probe that I can use to see what tubes are drawing for current in an actual amp. Something like a Maxi-matcher can give good results, but even that doesn't run tubes at the actuall operating voltages that they would see in an amp.

When you're looking at tube ratings strength is one factor and current draw is another.

My EICO BTW has a 140 point scale on its meter where 80 is minimum good and 100 (think of it as a percentage) is considered minimum spec for NOS. Many of the newer production tubes I test right out of the box test between 80 and 90 on my tester. Many old NOS tubes that I find can test off the charts, but most fall in the 90-110 range.

[VintageJon]

I agree with Tarzanalog's post.

In output tubes current is the most critical for "match", (though technically they must "match" for transconductance also.) Believe +/- 20% to be the Industry Standard, but I go with +/- 10%.

Emission testers are useful for finding grossly defective examples. If it fails an emission tester then it is useless. For 12AX7's the Hickok or my old Weston is the thing.

Life Test is especially useful as I've found OUS Mullard 12AX7's that pass everything BUT Life Test, and I won't sell them. Interestingly enough, replacing tubes that fail "Life Test" makes a difference in Tone and general dynamics of the amp!

Here I "match" output tubes by cathode current. This is done "real-time" in a chassis I have converted from a Twin Reverb wreckage. The Octal sockets are all equipped with 1 Ohm cathode resistors and I can do up to 4 at a time. There is a dedicated EL84/6BQ5 socket that I can make Class A Single-Ended measurements in.

I used to subtract screen current but found it didn't matter much for "match" in most examples.
Screen current is pretty critical when trying to bias EL84's. I always figure it when setting-up a Single-Ended or A/B example.

May The Tone Be With You,
Jon

[veets]

And ultimately, the best way to test a tube is to plug it in and listen to how it sounds. And while pre-testing with a tube tester is an excellent idea, especially for sorting, it doesn't always show a "bad" tube. I've had a couple of preamp tubes that test just fine, yet sound hissy and scratchy when I try to use them.