Tube ID?

[vinylaudio]

I'm trying to identify this Sylvania tube that came out of a Harman Kardon A300. It was in the V3 position, but there were a mix of 12AX/U/T7 tubes installed. The markings on the tube are wiped off, so I can't tell which variant it is. It has "mf" and "AES" next to the "Sylvania" logo. I've done the looking at an angle thing and there is no residue from the original markings that I can make out. Any ideas?

 

[Tom Bavis]

Testing is the only way to tell a 12AX7 from a 12AU7. Plates are identical, grid wires are much more closely spaced in the 12AX7 to give it its higher gain. V3 should be 12AX7.

 

[vinylaudio]

Thanks. V1 was a 12AT7, so that will give you an idea what I am working with. :/

 

[gadget73]

its not too uncommon for people to interchange the 12A*7 family. They all plug in for one another and usually function to some degree.

 

[vinylaudio]

In this case no apparent thought was given, other than what will produce sound. I threw a pair of Gold Lion 12AX7's in V3/4 and used the one Sylvania 12AU7 that was already there in to v5. Mercifully, the 7408's were there and all work well. At least I didn't pay a lot for it. It sounds really good so far. It has oddball 12a...7 tubes in V1/2 until I get around to messing with the phono stage. I replaced the power supply electrolytics to be safe. Most of the coupling caps had already been replaced and all are within tolerance. I replaced the diodes to be safe, as I read they may be suspect. Im beginning to see why people really like these amplifiers.