Info about 12AT7 tube

[dumbassbob]

I have a little 1960 Good- Bad tube tester..When a put aNOS 12ax7 in it reads at the #30 or below...But with a put a NOS 12AT7 in it goes way over like to 35 or more If I use a new tube like a JJ it reads #30..Do 12AT7 read higher than 12ax7.....

[J M Fahey]

You seem to have a very crude tube tester, which typically connects grid to plate, then applies , say, 24V AC or 60 or 120VAC, (yes, they do not even bother to rectify it, even less filter it) and measures ëmission current" from that diode with a series mA meter..... labelled good/bad .

VERY crude.

Since 12AT7 typically passes more current than 12AX7 under same conditions (and 12AU7 much more than both) I'm not surprised at your results ... nor would read much from them, besides the tube still having a functioning filament and some emission.

Now if you compare a known good 12AX7 with a suspect one (apples to apples), that might mean something.

[dumbassbob]

12AT7 tubes

Yes this tester is not much,but I was wondering why when I put it at the recomended Load of 30, a New tube will read @30 or below,but a NOS RCA,GE will read @35-38 Do NOS12AT7 have a high current...

[g1]

A new AX or AT ?

[dumbassbob]

A new AX or AT ?

A 12AT7...... do old ones have a higher current or voltage than a NEW 12AT7.... simple question..All my Good NOS 12AX7 Read a little lower,But ALL my NOS 12AT7 read higher

[Enzo]

They are reacting to the tester, not real life circuits. Don;t try to read too much into it.

Do old ones have more emission or less emission than newer ones? Maybe...at those conditions. In an audio circuit? Maybe not the same.

[The Dude]

I've no idea what tester you have and/or how exactly it works. That said, it's not uncommon for different brands/batches/types of tubes to read slightly different on a tube tester. Tube testers don't tell you much and are only really good for determining if a tube is definitely bad. They won't tell you for sure if it's even good, as most testers don't load the tube to actual conditions. I wouldn't put a lot of stock in those readings. Also, your implication is that tube gain is linear from lower plate voltage to actual plate voltage, which may or may not be so, depending on a multitude of variables. This is why they match tubes. Even single tubes of the same brand and type will operate differently.

Edit: You beat me to it Enzo! I guess I'll have to learn to type faster.

[dumbassbob]

i FOUND SOME GOOD INFO ABOUT THE 12aT7 AT GUITARBLUEPRINTING The 12At7 has a current output of 10.o milliamps..... a 12AX7 is 1.2 milliamps and the 12at7 has a trasconductance of 5000 vs 1600 for the 12ax7

[J M Fahey]

Well, you are quoting part of what the datasheets say, without applying that to any particular problem .

As quoted, those are just random b its of information.

It does not mean that one is "better" than the other in a generic, blanket statement way.

So far all you can tell is that there are 3 quite different tubes (12AX/T/U7) optimized for different purposes.

Choose one or the other depending on design needs.

[Enzo]

Bob, you are confusing ratings. What it says is the maximum current a 12AT7 should have to handle is 10ma, and for the 12AX7 1.2ma. When I buy a stepladder, it says maximum weight 250 pounds. That doesn't mean that everyone climbing the ladder will weigh 250 pounds. Just as there is nothing about a 12AT7 that will MAKE it carry 10ma.

Notice that the 5500 transconductance was at 250v on the plate, but only 4000 with 100v on the plate. So the circuits the tubes find themselves in are a MAJOR factor.