Unmarked tubes ????12a?7

[rafe]

I come across tubes from time to time that have little or no markings left on them I am mostly interested in the ones that appear to be 12a*7's or 6l6's ...Is there any way to visibly ID them.....or test them I have a decent tube tester but it seems that it wouldent be much help..........any insight on this will be greatly appreciated What do "YOU" do

[shooter]

from someone, somewhere; put them in the freezer for ~~~ 30min, have a magnifying glass handy, pull it out look were the "labels" usually are.

[rafe]

from someone, somewhere; put them in the freezer for ~~~ 30min, have a magnifying glass handy, pull it out look were the "labels" usually are.

It's worth a try, thanks

[Platefire]

Let us know if it works, I have the same issue with some of my tubes

[bmccowan]

I have had some luck with using an LED flashlight shined at an angle to the glass surface. I have no idea why this works, nor do I remember where I learned this. Might be fun to try that along with the freezer thing.
Also, I do think its possible to visually identify the structure of tubes and get pretty close. Power tubes are pretty easy, unless they are smoke glass tubes, and preamp tubes you can usually first identify that its in the 12__7 class dual triode and go from there. Comparing to known tubes is the key, of course.

[stratomaster]

You can always put them into an amp against a known reference. Run a 100mV signal into the amp and measure the AC voltage at the grid and plate.  Divide the two numbers to get the amplification factor.  If you have a few known good tubes then you can compare the amplification to get a better idea of what type of tube it is.

[rafe]

I have 5 in the house that I checked one is marked 4 are still unknown but I'm pretty sure they are from Super reverb or tried in it. So 4 are 12a  x,u,t,y  7's or 5751..... maybe Ill try them in my 60 ga-5 and see how the perform and compare them ......I have lots more out in the shop most are marked but lots are not.......

[Platefire]

I tried the LED Flashlight. It made it some easier to make out where some of the inscription still remained but where all was completely gone, it didn't help. I didn't try the freezer trick