OK.... So I have a bunch of NOS JAN GE 12AX7A tubes ....
Quite a bunch of them measure way over 100%... Almost to where the characteristics are no longer a 12AX7A...
For example..Using the standard test at 250V plate and -2V grid....
Data Sheet Boogie tube should be 1.3mA with gm of 1.6 mA/V ....
Some of these tubes measure at 3.8mA with gm of 2.4 mA/V ....
Test Equipment is rock solid and results consistent with 3 separate testers at same operating voltages...
I have plenty of other 12AX7A tubes that measure up on target...
The question is then...When is too strong not beneficial ???
I know it depends on the application...such as a guitar amp may sound more lush with such a valve... Feedback in ciruits will lock in the gain.. plate resistance is probably different along with open loop frequency response..
But when is too strong something that should be returned when purchasing NOS 12AX7A tubes ???
Interested in your views on this...
Thanks..
Quite a bunch of them measure way over 100%... Almost to where the characteristics are no longer a 12AX7A...
For example..Using the standard test at 250V plate and -2V grid....
Data Sheet Boogie tube should be 1.3mA with gm of 1.6 mA/V ....
Some of these tubes measure at 3.8mA with gm of 2.4 mA/V ....
Test Equipment is rock solid and results consistent with 3 separate testers at same operating voltages...
I have plenty of other 12AX7A tubes that measure up on target...
The question is then...When is too strong not beneficial ???
I know it depends on the application...such as a guitar amp may sound more lush with such a valve... Feedback in ciruits will lock in the gain.. plate resistance is probably different along with open loop frequency response..
But when is too strong something that should be returned when purchasing NOS 12AX7A tubes ???
Interested in your views on this...
Thanks..
Describe your testers and/or the circuit the 12ax7 are running in. Are these testers, amps? How are you setting up the voltages and loads and K bypassing?
Use cathode resistor bias, and a resistor as anode load (from a decent supply rail voltage), and this variation in valve characteristics will not matter very much.
On the other hand, if you use LED bias and a CCS load you could end up with a rather too low anode voltage and so limited anode swing.
Vg (or, equivalently Vk), Va and Ia: you can set two of these (or two linear combinations via resistors) and have to accept whatever the third is set to by the valve itself.
On the other hand, if you use LED bias and a CCS load you could end up with a rather too low anode voltage and so limited anode swing.
Vg (or, equivalently Vk), Va and Ia: you can set two of these (or two linear combinations via resistors) and have to accept whatever the third is set to by the valve itself.
Hi cerrem,
I normally discard anything too far from the average for a group. Same for transistors as being away from the average is a bad thing no matter what the direction from normal it takes.
With readings like you're getting, you should examine the internal structures. Are those ones a different tube marked with the wrong number? If it does look like the rest of your tubes, then maybe it is simply out of spec. Grid spacing from the cathode would be my concern here.
Too good is no good!
-Chris
I normally discard anything too far from the average for a group. Same for transistors as being away from the average is a bad thing no matter what the direction from normal it takes.
With readings like you're getting, you should examine the internal structures. Are those ones a different tube marked with the wrong number? If it does look like the rest of your tubes, then maybe it is simply out of spec. Grid spacing from the cathode would be my concern here.
Too good is no good!
-Chris
These are my tube testers that I use...I have rebuilt, modified and calibrated all these testers better than factory spec..
http://i286.photobucket.com/albums/ll101/cerrem/amps001.jpg
http://i286.photobucket.com/albums/ll101/cerrem/WESTON 686.jpg~original
http://i286.photobucket.com/albums/ll101/cerrem/IMG_3841.jpg~original
https://www.youtube.com/watch?v=zPy3dZUAFgk
Chris
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Some nice machinery. Have you had a chance to burn any of those in for 10-20 hrs. to see if they settle down any? I guess if you put them in a circuit designed to run a bogey tube then you'd have to check their drivability in and out, in the circuit, to see if they can do what's expected without tweaks for accomodation.
I test lots of tubes on my Avo, and when I find one that measures suspiciously high in gm/Ia, I immediately check for presence of gas.
The other possibility is grid emission, but most testers don't have a test function for it. You would have to create your own test circuit.
The other possibility is grid emission, but most testers don't have a test function for it. You would have to create your own test circuit.
I've tested 100 pc 'master pack' from '70's Siemens each tube through AVO tester and it followed the Bell Curve. The tubes I value most are the ones between the two shoulders of that curve. Too strong or too week simply dont bias the same, simple as that.. as a DIY type you can make adjustments, but the tube is out of spec.
LH/S
LH/S
Hi Chris,
I have a TV/2U like yours that is unrestored so far. Yours is in far better condition than mine, but I can say that all the meters and sockets appear to be intact. It's bugging me to do it, always in the back of my mind. Can you share any information or pictures of yours in various areas? I'll need the references when I dive into mine.
Please PM me so I don't clutter up this thread.
-Chris
I have a TV/2U like yours that is unrestored so far. Yours is in far better condition than mine, but I can say that all the meters and sockets appear to be intact. It's bugging me to do it, always in the back of my mind. Can you share any information or pictures of yours in various areas? I'll need the references when I dive into mine.
Please PM me so I don't clutter up this thread.
-Chris
My Website goes into detail about the problems with the TV2 and the remedies...
I can send you photos of the rebuild ....
Go to this page and scroll down to the TV2...
Merren Audio - TUBE TESTERS****The behind the scenes truth**** * * * * * * * * * * * * * * * * * * * * * * ** under Construction **
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Hi Chris,
Many thanks. I'm there. That's the information I was hoping for, many thanks!
I know you don't do requests, but I think you may have run across a few other testers and was curious about your thoughts on something like the Stark 9-66. Its my workhorse unit and it is made in Canada under license from Hickok. It seems reliable and I do get repeatable numbers. I use a variac and run the line adjust to the top (0 resistance) as power tubes tend to drop the voltage under test. If they aren't very good, I may build something using that chassis, meter and transformer, adding another if needed. A big job of course. How does someone acquire a "standard tube", which I understand is a graded 6L6 in the metal envelope? The goal is rapid testing with more accurate numbers.
Excellent web site, waiting for the AVO critique. Those are extremely expensive and I know someone who wants to sell their own unit. Last I heard he wants $1000 for it. I'm not in the market.
-Chris
Many thanks. I'm there. That's the information I was hoping for, many thanks!
I know you don't do requests, but I think you may have run across a few other testers and was curious about your thoughts on something like the Stark 9-66. Its my workhorse unit and it is made in Canada under license from Hickok. It seems reliable and I do get repeatable numbers. I use a variac and run the line adjust to the top (0 resistance) as power tubes tend to drop the voltage under test. If they aren't very good, I may build something using that chassis, meter and transformer, adding another if needed. A big job of course. How does someone acquire a "standard tube", which I understand is a graded 6L6 in the metal envelope? The goal is rapid testing with more accurate numbers.
Excellent web site, waiting for the AVO critique. Those are extremely expensive and I know someone who wants to sell their own unit. Last I heard he wants $1000 for it. I'm not in the market.
-Chris
… which makes an awesome justification for a capacitor bypassed CCS in the cathode-to-ground bias location instead of a resistor. The CCS delivers “1.8 ma” (say, or whatever the design spec is), self adjusting the raised cathode voltage appropriately. The cap-bypass then delivers variable current AC for the signal. Doesn't matter whether one's running resistor anode multiplier, high value choke load or even an interstage transformer.
LOL!
GoatGuy
CCS in the cathode could, at least in principle, go wrong with a valve which has low current - the opposite of the OP's problem. A low current valve forced to run at a particular anode current will have low Vgk and so potentially suffer from grid current. A resistor would cope, because the anode voltage would change too and reach a better balance.
By all means use modern circuit techniques, but then you really do need valves which do what the datasheet says - which are probably rarer today then they were 50 years ago. Older circuits were more flexible and so better suited to the reality of valve manufacture.
By all means use modern circuit techniques, but then you really do need valves which do what the datasheet says - which are probably rarer today then they were 50 years ago. Older circuits were more flexible and so better suited to the reality of valve manufacture.
When I test tubes, I use these instruments :
If I need to measure tubes (that is to say with more accuracy), I use a small bench test :
And yes, like you I find tubes "under" and "over" specs about the plate current, regarding the datasheets.
But I remember that RCA states in their manuals that the performance of a tube can be accepted within a margin of -30% to +30% about the plate current. Outside that range, the tube should be compulsorily discarded and considered defective.
This is the rule that I apply when using my Metrix 310CTR tube tester. This instrument is able to test the tube under its true nominal datasheet conditions, which is taken as the reference, unlike the Hickok and many other tube testers with proprietary tube charts.
Of course, in that margin, some tubes will work better than other, or even not work properly, depending on the criticity of circuits where they are installed.
Too high plate current may also reveal leaks, gas, grid current - even in the -30 / +30 margin, so complementary checks must be performed if the tube application is critical.
It is also possible that many new tubes today don't benefit of a satisfactory "ageing", "stabilization" or "sorting" process like it was the rule in the Golden Years of tube manufacturing...
A+!
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If I need to measure tubes (that is to say with more accuracy), I use a small bench test :
An externally hosted image should be here but it was not working when we last tested it.
And yes, like you I find tubes "under" and "over" specs about the plate current, regarding the datasheets.
But I remember that RCA states in their manuals that the performance of a tube can be accepted within a margin of -30% to +30% about the plate current. Outside that range, the tube should be compulsorily discarded and considered defective.
This is the rule that I apply when using my Metrix 310CTR tube tester. This instrument is able to test the tube under its true nominal datasheet conditions, which is taken as the reference, unlike the Hickok and many other tube testers with proprietary tube charts.
Of course, in that margin, some tubes will work better than other, or even not work properly, depending on the criticity of circuits where they are installed.
Too high plate current may also reveal leaks, gas, grid current - even in the -30 / +30 margin, so complementary checks must be performed if the tube application is critical.
It is also possible that many new tubes today don't benefit of a satisfactory "ageing", "stabilization" or "sorting" process like it was the rule in the Golden Years of tube manufacturing...
A+!
The hot ones sound like good candidates for guitar amplifier use. Heck, if they're cheap id be interested in a few...
I would run them in a few dozen hours and test again, could just need to settle in and burn off impurities in the vacuum...
I would run them in a few dozen hours and test again, could just need to settle in and burn off impurities in the vacuum...
Hi Lingwendil,
Actually, they aren't good for anything. Just target practice with a couple .22's. Now, that sounds like fun!
-Chris
Actually, they aren't good for anything. Just target practice with a couple .22's. Now, that sounds like fun!
-Chris
.22? Meh. I'm a 8x57 Mauser, 7.62x39, or 5.45x39 guy these days 
still, i wonder how the tubes would do after a break-in period before being tested again.
still, i wonder how the tubes would do after a break-in period before being tested again.
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