Finding a Bad Tube?

[Last Nerve]

Last nite I notived some "ghost rattling" happening with my amp.

I'm thinking it must be a microphonic preamp tube?
I just put in some NOS Amperex 12AX7s.
Have a NOS Mullard in the AT7 position as well.

I know I could pull them out, one by one, and figure out the problem tube.
But isn't there a way to "test" them while in?
Something about a soft smack from a pencil or something?

[1radicalron]

With the amp "on" - moderate vol, gently tap each tube with a pencil or small screwdriver. You should hear a Rattle when you get to the bad tube.

[Blue Strat]

Yes, tapping *may* help, but the definitive method is replace the tubes one by one with known good tubes. Never fails.

[dbx]

I'm having a related issue...but slightly different...if the preamp tube is not seated fully in the socket, could that account for a certain bassiness, and weird oscillations at higher eq settings?

[Jahn]

when i had this prob in my old twin reverb, i did the tap method. bingo, it was two of the 6L6GCs. replaced those crap groove tubes with some nice sylvanias and all was well.

[SatelliteAmps]

I prefer tapping with something non-metallic if possible. A small wooden dowel if possible.

[Last Nerve]

Thanks all.
I'll find the culprit this weekend.

[Blue Strat]

I'm having a related issue...but slightly different...if the preamp tube is not seated fully in the socket, could that account for a certain bassiness, and weird oscillations at higher eq settings?

If a tube isn't making proper contact with the socket, almost anything can happen. Clean the pins and socket (check the FAQ on my website under Resources).

[dbx]

If a tube isn't making proper contact with the socket, almost anything can happen. Clean the pins and socket (check the FAQ on my website under Resources).

OK, I found the preamp tube at an angle, I'll clean and re-seat..it's a generic Chinese 12AX7 tube...but looks clean...