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rockon1
02-24-2006, 11:03 PM
OK so I found a "budget" tester on Ebay. Its an EICO 635 model.I noticed that of all the 12AX7's I own it seems the EH are ones I have the most weak or "bad" ones and I didnt even know it. They still work OK in the amp but have less gain than others that tested good. One measured 15/30 on a scale where 50 is minimal and still worked! Any of you guys have this experience with EH's or is it a fluke or something? ALSO my12AT7's seem to test as strong as the 12AX7's or is that just a"relative"reading?They arec tested on the same settings as the 12AX7's...
VintageJon
02-24-2006, 11:56 PM
I suspect your Eico 635 is an "emmission tester". This is to say that it tests for gross shorts and if the filament will heat the cathode and the plate attracts the resulting charge carriers.
This is better than having No Tester.

The 12AT7 has much more Gm, transconductance, than a 12AX7. Gm is a measurement of the change in plate current for a change in grid voltage.
This is Current Gain.
Conversely a 12AX7 has more Voltage Gain with less Gm.
Same settings for 12AT7 and 12AX7 are a sure sign it's a Emission Tester.
As I've said it's better than not having one.

I have perhaps the last working, and fully restored, ETA drug-store emission tester for Public Use. You wanna test your tubes? Step back 40 years and come on in! ETA Drug Store Tester is not a bad unit at all.

Also have Weston and Hickock Transconductance testers that we spec tubes on when we receive them. Transconductance testers will give you the Gm reading for a tube and do a much finer job of testing for "leakage".
MUCH more expensive equipment. They run around $300 and up...
These are not for Public Use as they are delicate instruments.

Don't get me started on Plot-Against-Moose-And-Squirrel EH rebranded Sovtek... No surprise that they test weak...

May The Tone Be With You,
Jonm
rockon1
02-25-2006, 08:11 AM
Thanks, a bit of that went over my head but I get the basic idea. Only part I didnt really understand was the 12AT7 VS 12AX7 and why the 12AT7 tested higher when its a lower gain tube....At any rate I realize it is "budget" tester.Still it seems to do the basics and let me know if the tube is weak,strong or balanced. Just kind of strange that out of about 50 12AX7 tubes I tested(Ei,JJ,Sovtek,JAN Philips,Tung Sol,Chinese) 3 bad ones were EH,1 Tung Sol and 2 (older but unused) Sovtek WXT+.
r9player
02-25-2006, 08:35 AM
Looked it up Eico 635 is just an emissions tester so there are some inherent lack that you can't test transconductance.
I actually have an Eico 666. I'd recommend you upgrade?
brad347
02-25-2006, 08:43 AM
I have an old emissions tester, too. Am I correct in assuming you can't match tubes on an emissions tester?
rockon1
02-25-2006, 08:45 AM
Looked it up Eico 635 is just an emissions tester so there are some inherent lack that you can't test transconductance.
I actually have an Eico 666. I'd recommend you upgrade?

Well I just got this one. I dont know how deep I want to go with this stuff.
samwheat
02-25-2006, 08:53 AM
I bought a hickok 533 on ebay .... 400 bucks and it was rebuilt/calibrated 2 yrs ago by the late Chris Haedt

FWIK, it will cost around 200 or so to get one rebuilt/calibrated by roger kennedy at alltubetesters.com

the maxi-matcher tests power tubes at amp plate voltages ..... mine doesn't

maxi is coming out with a new model to test preamps tubes in addition to power tubes
VintageJon
02-25-2006, 08:41 PM
Rockon1- THe 12AT7 tests higher than a 12AX7 because it has more CURRENT GAIN than the 12AX7. (The meter is current driven...)

Guitar players usually refer to VOLTAGE GAIN and this is not the Engineering-Way. I have a whole different test for voltage gain, but there's no need to post it as it involves a Twin Reverb chassis I have converter to tube tester and Surf Bug build/test platform.

May The Tone Be With You,
Jon
rockon1
02-25-2006, 08:50 PM
Rockon1- THe 12AT7 tests higher than a 12AX7 because it has more CURRENT GAIN than the 12AX7. (The meter is current driven...)

Guitar players usually refer to VOLTAGE GAIN and this is not the Engineering-Way. I have a whole different test for voltage gain, but there's no need to post it as it involves a Twin Reverb chassis I have converter to tube tester and Surf Bug build/test platform.

May The Tone Be With You,
Jon

Got it! Thanks Bob
VintageJon
02-25-2006, 09:17 PM
Groovy, carry on...

-Jon
tedm
02-26-2006, 01:49 AM
so you have to test twice and verify the 1/2 you test is the 1/2 being used in your amp application.

I use and swear by a calibrated TV 7 D/U.


Thanks, a bit of that went over my head but I get the basic idea. Only part I didnt really understand was the 12AT7 VS 12AX7 and why the 12AT7 tested higher when its a lower gain tube....At any rate I realize it is "budget" tester.Still it seems to do the basics and let me know if the tube is weak,strong or balanced. Just kind of strange that out of about 50 12AX7 tubes I tested(Ei,JJ,Sovtek,JAN Philips,Tung Sol,Chinese) 3 bad ones were EH,1 Tung Sol and 2 (older but unused) Sovtek WXT+.
monkmiles
03-02-2006, 08:55 AM
Can someone in the know about tube testers recommend a tube tester to buy? Preferrably an 'affordble' option if one exists? Thanks!
donnyjaguar
03-02-2006, 09:13 AM
Guitar players usually refer to VOLTAGE GAIN and this is not the Engineering-Way.
Well if the control grid doesn't draw any current how can you measure current gain? :)

I think you'd need to put a current source on the plate to determine actual voltage gain (mu) of a triode. Or you could just hold your thumb out and guess that it'll be half the mu factor once you add a plate resistor. :)

DJ
Lord Valve
03-02-2006, 09:19 AM
This thread is hilarious.

LV
r9player
03-02-2006, 09:25 AM
iono I have a very cheap Eico 666 tells me if tubes are good no leaks and good transconductance, but I can't "match" tubes very well with it.
Fun though :)
brad347
03-02-2006, 09:59 AM
This thread is hilarious.

LV

why? We can't all know everything! Educate us!
rockon1
03-02-2006, 02:53 PM
Heres my budget one in action. Nice shape though.http://tinypic.com/ouuofm.jpg
VintageJon
03-02-2006, 10:58 PM
Gm is determined by the change in Plate Current with a change in Grid Voltage.

You know which 1/2 of dual triode you're testing by the settings on your tube tester, if it's a good one. This is why I love the Hickok- the setting switchs are labeled Grid,Plate,Screen,Surpressor,Cathode and the # on the position corresponds with the pin on the tube...

The Weston is my second fav with it's "1st triode/2nd triode" switch. In a 12AX7 testing has confirmed that 1st triode means pins 1,2,3, while 2nd triode means pins 6,7,8. It's intuitive. I guess that's why the Navy bought them. (This one was from the UT College Of Engineering here in Austin...)

-Jon
donnyjaguar
03-03-2006, 09:18 AM
Gm is determined by the change in Plate Current with a change in Grid Voltage.
Yeah, I know. I'm just giving you a hard time.

I had a tube tester at one point but sold it almost as quickly as I bought it. Hell, it ran on vacuum tubes and you know how unstable those things are. :jo

I guess it was useful to confirm suspicions about lack of mu, shorting and leakage through the filament. If you can't tell that a filament is blown, tube is contaminated or over-heated by visual inspection there isn't much hope. :D

I usually design amplifiers on voltage gain through the stages. If this is the wrong way, so be it. But I gotta say it works for me. If I'm guilty of anything its designing power amplifiers with too much gain and then applying more negative feedback to quell it.

DJ