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Author Topic: How to test tubes?  (Read 11740 times)
babybear2011
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« on: May 19, 2011, 01:26:14 PM »
Does anyone have an idea how to test for microphonic or bad tubes?? I really don't fancy forking out cash for a big old AVO Valve Tester type of machine. Tubes are costing me a fortune lately. I use 6L6's and 12AX7's. Thanks!!
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« Reply #1 on: May 19, 2011, 06:21:04 PM »
Quote from: babybear2011 on May 19, 2011, 01:26:14 PM
Does anyone have an idea how to test for microphonic or bad tubes?? I really don't fancy forking out cash for a big old AVO Valve Tester type of machine. Tubes are costing me a fortune lately. I use 6L6's and 12AX7's. Thanks!!
I think it might be better to explain why you think your tubes are bad.

Checking for microphics..one way is to take a non-conductive object like a chopstick and tap on the tube. You will hear noise and changes in the noise. And then things like squeeling and other nasty sound is a good sign.
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Chris

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« Reply #2 on: May 19, 2011, 06:38:09 PM »
I know exactly what you mean. The amp actually 'amplifies' the sound of flicking the tube with a chopstick. Does this mean the tube has to be discarded in the trash? Seems like a waste of workmanship, especially if the tube is vintage like a GE or RCA for example, it can't be 'reconditioned'??
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plasticvonaband
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« Reply #3 on: May 19, 2011, 09:41:31 PM »
Nah, once they're gone, they're gone. For the amount of work it would take to theoretically recondition a tube, a manufacturer could easily build a new one. Here's some basic info for ya

http://www.vacuumtubes.net/How_Vacuum_Tubes_Work.htm
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« Reply #4 on: May 27, 2011, 10:36:27 AM »
Quote from: babybear2011 on May 19, 2011, 01:26:14 PM
Does anyone have an idea how to test for microphonic or bad tubes?? I really don't fancy forking out cash for a big old AVO Valve Tester type of machine. Tubes are costing me a fortune lately. I use 6L6's and 12AX7's. Thanks!!

The best tube tester is your amp... trust me on this.
...obviously a dedicated tube tester is also very handy tool and you don't need to get an AVO.... those are really expansive.
You could get a cheap emission type tube tester but a device like is not very accurate... it's more like an indicator GOOD/BAD and even than the result is not 100% accurate.
You can spend a little more (still a lot cheaper than AVO) for a vintage transconductance type tube tester. Those will deliver even better results.
Learn about vintage tube testers here...
http://www.tone-lizard.com/Tube_Testers.html

... but if you want to mach tubes or search for balanced preap tubes you will have to get a modern type device.
..a device like this http://www.maximatcher.com/
but there are many other available as well... this is only one example.

BTW it just happens that I have AVO CT160 on my bench at this moment... I borrowed it from a friend... and let me tell you this thing is just to complicated. I don't like it... for that kind of money, I would definitively buy some modern type tube tester.
I also have a very old cheap Precision tube tester... very easy nice and simple tester... but like I said not very accurate.

* AVO.jpg (210.14 KB, 821x767 - viewed 710 times.)

* Precision.jpg (420.24 KB, 1600x1200 - viewed 678 times.)
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SoundPerf
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« Reply #5 on: May 27, 2011, 04:18:47 PM »
@mcinku....I sure do like looking at cool older test equipment. Nice! Cool
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Chris

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« Reply #6 on: May 27, 2011, 08:59:40 PM »
Quote from: SoundPerf on May 27, 2011, 04:18:47 PM
@mcinku....I sure do like looking at cool older test equipment. Nice! Cool

I know what you mean... I have a nice modern type tester too but just had to own a piece of history as well. It smells soooo vintage.  Grin
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« Reply #7 on: May 28, 2011, 03:29:58 AM »
Keep your eyes open for a vintage Hickok tube tester.  Pretty decent, the later ones with solid state guts are more affordable, think that the model 6000 is that model, but not absolutely positive.  I use it mainly to test preamp tubes. 

When it comes to power tubes, tube testers are pretty much useless.  The voltage used in tube testers is not the same as in an amp, usually less.  Even Maxi-matchers aren't all that wonderfully accurate, because most amps don't run their b+ at exactly 400v, it's usually above or below that figure.  With that said, if someone gave me a Maxi-matcher I'd be happy to give it a home right next to my Hickok...

If I can, I like to get amps with a bias adjustment and bias balance adjustment, most of my push pull amps use two, not 4 power tubes.  I have three boutique amps with that feature, and I like that very much.
« Last Edit: May 28, 2011, 03:42:22 AM by 212Mavguy » Logged
mcinku
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« Reply #8 on: May 28, 2011, 06:49:39 AM »
My Longhin tube tester goes to 460V plate voltage.... it's adjustable from 30 to 460V...max plate current up to 200mA, bias from 0 to -50V, adjustable heaters from 1.5 to 50V
I think I can measure any tube out there at working condition. ...but unfortunately device like that doesn't come cheap.
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« Reply #9 on: May 28, 2011, 07:00:00 AM »
I got a bit intrigued by you discussion of tube testers - the new ones are quite expensive (almost the price of a DIY amp  Shocked) and the old ones on ebay are quite expensive too.

It must be possible to build one yourself - either a complete unit or  just a test setup with a sine generator for which you use your standard multimeter to measure output.

Any experience with that Huh???
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« Reply #10 on: May 28, 2011, 10:56:35 AM »
I didn't read everything but it seems a lot information about DIY tube testers can be found here...
http://www.vintage-radio.net/forum/showthread.php?t=48853
or here
http://www.audiokarma.org/forums/showthread.php?t=275663
... look around you can find a lot of info about this topic.
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« Reply #11 on: May 28, 2011, 03:30:59 PM »
I’ve tried friends Amplitrex tube tester and it is absolutely fantastic device.
Check it out here http://www.amplitrex.com/index.html

Unfortunately it costs $2.700
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« Reply #12 on: May 29, 2011, 12:31:17 PM »
My amptech friend has one of those as well... that has to be the best tube tester out there.
...but costs way to much for an average DIY guy.
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212Mavguy
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« Reply #13 on: May 29, 2011, 01:32:06 PM »
Here's the stupidest reply yet... 

Why not build up a single ended amp power section with some sort of rheostat/pot  or other device(s) to vary screen and b+ voltage and use it to measure plate curent draw from batches of a given power tube tube type?  Sounds like a possible ampgarage project...and would not cost over 2 grand.  My tube dealer friend in SLC has a rather impressive tester that can vary the plate and screen voltage all over the place.  But it's big and bulky, however, it can process quite a few tubes at the same time.
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mcinku
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« Reply #14 on: May 29, 2011, 06:18:07 PM »
Sure why not... but it won't be that cheap to build such a device. And what happens if you want to measure something else apart from octals. You will need some adaptors as well.
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