ID this Tube?

[norby3]

Would appreciate it if anybody could narrow this tube down to the exact letter. I know it's a 12a...7

http://i569.photobucket.com/albums/s...y2/photo-7.jpg
http://i569.photobucket.com/albums/s...y2/photo-8.jpg

also the pic I want to show up isn't--it says "ca" and "ahy" on the tube and it's Sylvania.

Thanks

[greekie]

It looks like their 12AX7, grey plate. It's a good sounding tube.

[Jazz P Bass]

One way to find out is to plug it in a working circuit & compare the parameters to another known good 12A(X,T,U)7.

[norby3]

It's quite a bit smaller than a 12AX7, also I just compared it tone and volume wise and it has less gain in the same circuit as a known 12AX7.

[Jazz P Bass]

Without actually measuring the gain change you will never know what tube it is.
Back of envelope: If a 12AX7 is 100, then a 12AT7 would be an 80 & a 12AU7 would be a 40.

[g1]

I know it's a 12a...7

It's quite a bit smaller than a 12AX7,

I don't see how these 2 statements could BOTH be true. How do you know it's 12a?7, can you see that printed on the tube? Aren't all 12A?7 (AX,AT,AU,AY) variants the same size?

[Tom Phillips]

It's quite a bit smaller than a 12AX7...

Smaller in what way?
The photos you posted don't look abnormal.
The outlines section of the RCA receiving tube manual list the size specifications for the 12A*7 as follows"
Diameter: 7/8" Max.
Height from bottom of glass envelop to top of seal nipple: 1 15/16" Max

Interesting that there is no minimum size listed in the spec.

Tom

[dai h.]

kinda looks like "12AU7A USA" (the "U" is a bit difficult to make out)

[Steve Conner]

Sometimes breathing on the glass will show up the remains of the original print.

If it helps, the guts look very like some Philips-ECG 5814s that I bought a while back: similar to 12AU7s. The grid structure is turned through 90 degrees compared to every other 12A*7 I ever saw.

[Jazz P Bass]

A 12AU7.
That would explain the obvious gain drop that was observed.
A 12AT7 would not be so dramatic a change.