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A couple of questions about testing 12AX7 tubes.

I have been testing a bunch of RCA and some Fisher 12AX7 tubes on a Hancock tester at the local Antique Radio Museum. Two things happened while testing the tubes that I am not sure about.
The first thing that happened is that 3 of the tubes (out of the 15 that I tested) glow bright right after plugging them into the Hancock tube tester, then the immediately go down to the normal tube glow. Does this indicate a bad tube? One of the RCA long plates and two of the Fishers did this.
The second question is that while testing the triodes of 2 tubes the meter went all the way to the right and was pinned on the meter, which goes up to 3000. On one of the tubes both triodes pinned, on the other tube one triode pinned and the other read around 2800. Is this good or bad? Do I just have excellent tubes or is there something wrong with them?
Thanks for your help.
Joe
jbcello
4 responses Add your response
tomcy6's avatar
tomcy6
2,588 posts
 
A common mistake made with Hickok testers is that the English setting on the roll chart is used for micromhos testing. It should be used for good/bad testing only. I'm guessing that this setting on your Hickok is 58, but it depends on the model of tester you are using.

The setting used for micromhos is the red dot for the scale you are using. For the 3000 max scale this red dot should be around 73.

I think that this mistake accounts for many of the super tubes you see advertised. There just aren't that many super tubes around.

I've read that tube matching is not necessary for most applications. Exceptions are phono stages and fully balanced/differential preamps. Tubes age at different rates so a matched pair will probably not stay that way as hours are put on them.
jbcello OP
25 posts
 
Thanks for both of your answers. Yes, I do agree that the only way to know you have a good pair of tubes is to listen to them. The reason I test the tubes is to try get pairs of them that closely match for my phono section in my Magus preamp. I assume that the better they are matched the more accurate the sound will be coming from both channels. Although I must admit my knowledge about tubes is very limited.
thanks,
Joe
jallen
502 posts
 
Rating tubes is way overrated. The only way to test them accurately is in circuit with a distortion analyzer, and listen to them. Who cares what the numbers show if they buzz or have high distortion. they can check fine and still buzz or be high distortion. Tube checkers area yes or no to a tube being dead, or near dead. Tube buyers are only getting some of the information from tested tubes. RAM and Upscale Audio are a few of the places who do very thorough testing and I believe Jim McShane also does thorough testing. Jallen
viiu
34 posts
 
Regarding the first question, the glow indicates they are likely rebranded Philips or another brand which flash on startup. This is normal.

My guess on the second one is that you have excellent tubes. If the reading was wavering back and forth, I'd be concerned but if it's constant I think you're OK.
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