12AX7B

Tubes Asylum

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What on earth is this tube, and can I safely substitute an ordinary 12AX7 for it? I can find lots of info about 12AX7A but not -B. Mine is an unmarked Chinese tube that came with my phono preamp. I'm going to put a regular one in its place and see what happens -- I'm 95% sure it's fine because it's the same tube, albeit with some special property. There's always the chance that this special property is keeping the preamp from destroying itself, but I doubt it. I just looked at it and there's an extra metal part attached to the plates. Dunno if that's significant. Finally: the 12AX7B is used as a regulator tube. Is there any point to swapping it out other than for mechanical reasons? The stock Chinese tube looks very cheaply-made. All I can find with a search is that there's such thing as an Allegro 12AX7B.
And this preamp amplifies noise and radio interference to an ungodly level, enough to be heard over quiet parts. My (line stage) preamp also did this at first, but it's settled down in the days since I got it. Problems typically settle down after a bit of time staying in one place. It's not burn-in but it's interesting. I've noticed it with most things... when I moved my turntable, it would jump and mistrack with the slightest vibration. I didn't touch it and now you can stomp your foot without missing a beat. So I'm hoping that this tomfoolery with interference goes away as the unit settles in -- but if it doesn't, are there any tricks around it or will I just have to get used to it? I'm already using one helpful trick, which is to use two preamps, connecting the main outputs of one to an input on the other. You can use the two volume pots in tandem, both for finer control and to minimize noise by cranking up the first one and using the second as the volume control (the noise is independent of the volume, so you can turn up the volume without amplifying the noise).
Just a tip for anyone who's got this problem. Also, you can combine the sonic properties of two tube preamps this way, or you can stick a solid-state in there for it to pass through if you need it uncolored. It probably degrades the signal a bit, but the benefits are well worth the cost IMO.

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Topic - 12AX7B - Mark Tinordi 19:56:53 04/12/07 (6)

About the noise issue, are you using a tube shield on it? (nt) - twystd 14:38:31 04/13/07 (0)

You should try the Chinese 12AX7C - Gofast 04:30:15 04/13/07 (4)

Re: The Shuguang 12AX7C---AWESOME! - IdlerWheel 12:44:37 04/16/07 (0)

Re: You should try the Chinese 12AX7C - MJL 11:49:41 04/15/07 (0)

Where do you get the 12ax7c's?(nt) - msa 08:07:31 04/13/07 (1)

Ebay. - Gofast 19:36:52 04/13/07 (0)

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