Amperex 12AU7

[Blair]

Hey guys.

I was at a local parts store this past weekend, and the guy behind the counter knows me pretty well. I brought in my poseidon amp, and he asked what the tube compliment was. Then he came out of the back room with 4 NOS Amperex 12AU7s. He gave them to me and told me to take them home to see if they worked. I did so. First thing I noticed was that they "flash" when first energized. They say "Made in Britain" on them. I can only find orange globe information about these tubes from Asia and Holland. Does anyone know anything about them?

They sound very nice, and what I'm finding is that they run between 25-50 bucks for NOS tubes of this variety.

I read about a half moon shape cut out of the plate being the difference between the Asian tubes and the ones from Holland. Mine have the holes like the Asian plates, but flash like it says the Holland tubes do.

http://www.vacuumtubes.com/12au7.html

Any ideas about these? and for free why should I care right Yellow_Light_Colorz_PDT_02

Blair

[EWBrown]

I've seen the "filament flash" in other Hollandaise and some German tubes, basically because the cold filament has lower resistance so it momentarily draws higher current. Depending on the filament's physical characteristics, it can briefly "flash" or just warm up gradually.
Just think of the common light bulb. 99% of the time it blows out, is upon applying voltage to the cold filament.

FWIW, in out favorite older tubes, IE, 12AX7, 12AU7, etc, their filaments were designed for parallel connection only (the usual case in 95% of applications), so they were more voltage and less current-rated.

The -A suffix (12AU7A, 12AX7A), was designed for either parallel or series connection, as in the older series filament TVs and radios. Their filaments were more accurately current-matched. The tubes destined for TV service were often rated as reaching operational temperature in 11 seconds.

HTH

/ed B in NH

[Blair]

Thanks Ed.

Do you know if these tubes are any good? I can get a few more for $5 a piece if I want them. I'm just trying to justify buying more tubes.

Blair

[sorenj07]

from what I've read online, the 12AU7 is a fundamentally less linear tube than the 12BH7 or 6CG7, to give a few ~20x gain examples. then again, plenty of good amps use them, so basically buy more if you like the way they sound, or want to resell them on ebay or something. it's your call.

[Blair]

Are either of those tube interchangeable with the 12au7? They look like they could be.

Blair

[erichayes]

The 12BH7 is a drop-in replacement, but has twice the heater current. As I recall, you weren't hurting for heater current capacity. It'll work fine with no tweaks, but will work better if you massage the plate and cathode resistor values a little.

The 6CG7 (6SN7's kid brother) and 6GU7 (12BH7's first cousin) need to have the connection between pins 4 and 5 broken, and the heater lead going to pin 9 removed and attached to whichever pin--4 or 5--doesn't have a heater lead going to it. Then ground pin 9.