How to distinguish tubes?

[rich]

I've got a box of 12Ax7 tubes and 12Au7 tubes all mixed with the lettering rubbed off. Is there any way to visually distinguish the tubes?

 

[PRR]

No. While AU and AX from the same factory may look a little different, if you have a mixed lot from several factories you can't do it.

Set up a socket with heater power, grid strapped to cathode, and 48V on the plate. Measure plate-cathode current. AU will read much higher than AX.

 

[rich]

[quote author="PRR"]

Set up a socket with heater power, grid strapped to cathode, and 48V on the plate. Measure plate-cathode current. AU will read much higher than AX.[/quote]

Sorry for my naivety but does strap the grid to cathode mean connect the grid pin directly to the cathode pin?

 

[rich]

[quote author="PRR"]No. While AU and AX from the same factory may look a little different, if you have a mixed lot from several factories you can't do it.

Set up a socket with heater power, grid strapped to cathode, and 48V on the plate. Measure plate-cathode current. AU will read much higher than AX.[/quote]

Getting ready to go through these tubes and wanted to make sure I undestand this correctly.

Am I only hookin up 1/2 the tube? So I've got 12 volts dc to pins 4 and 5 right? connect or strap pin 2 and 3 together? and pin three goes to ground?
Then I measure current with my meter between pin2 and 48 volts?

 

[rich]

Oops I meant measure current between pin 1 and 48V?

 

[PRR]

Yes.

 

[rafafredd]

Note that the meter should go in series.

 

[rich]

[quote author="rafafredd"]Note that the meter should go in series.[/quote]

I've got the positive lead of the meter directly on the 48V and the negative lead on pin 1, correct?