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    swapping 12AU7s for 12AX7s

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    Tom

    Posts : 166
    Join date : 2011-04-04

    swapping 12AU7s for 12AX7s

    Post by Tom on Mon May 09, 2011 10:35 am

    Still looking for a hum in an SP-8.

    Humms only with power up and line stage tubes 12AU7s working.
    Pulling the phono stage 12AX7s does not affect the hum.
    Pulling the linestage 12AU7s stops the hum.

    A friend who does guitar amps suggested pulling the 12AU7s
    and putting the 12AX7s in their place to see if the hum is still there.
    Is this doable? I know they're pretty much the same tube except for the mu.

    Trying to keep all the smoke inside the components. Razz

    Thanks,
    Tom

    tubes4hifi
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    Join date : 2008-11-30

    Re: swapping 12AU7s for 12AX7s

    Post by tubes4hifi on Mon May 09, 2011 3:30 pm

    12AX7 tubes can NOT be used as a substitute for 12AU7s, they have 5X as much gain.
    It's not likely the 12AU7s are bad, but it sure wouldn't hurt to have a couple spares.
    More likely your hum is due to wiring or a ground connection.

    Tom

    Posts : 166
    Join date : 2011-04-04

    Re: swapping 12AU7s for 12AX7s

    Post by Tom on Tue May 10, 2011 6:02 am

    Thanks!

    Just to clarify, this is a diagnostic swap, not a permanent one.
    Our idea was 12au7 out, 12ax7 in.
    Power up with no inputs.
    Is the hum present? yes/no
    Is the hum louder? yes/no

    If the hum's gone, it was the 12au7.
    If it's louder, the line stage is amplyfing it and that helps find it.

    We're trying to think this through in our primitive fashion
    using leaves and branches to represent components around the campfire (tubes).

    So, is this scenario something you can try,
    or will the extra gain from the AX7s smoke something.

    Still having fun,
    Tom

    tubes4hifi
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    Re: swapping 12AU7s for 12AX7s

    Post by tubes4hifi on Tue May 10, 2011 5:47 pm

    it shouldn't smoke anything as long as you have nothing connected to it, oh, how are you gonna hear any improvement?
    Do it at your own risk, with volume at minimum, plugged into a very cheap amp.

    j4570

    Posts : 141
    Join date : 2010-08-30

    Re: swapping 12AU7s for 12AX7s

    Post by j4570 on Tue May 10, 2011 7:29 pm

    Why not buy two inexpensive tubes? I don't mean cheap, but maybe reisssues, or some of the ones Roy or Bob sells.

    Why risk toasting something else? This seems like adding gasoline to the fire.

    The best thing I ever did was buy a couple of old tube service boxes full of various tubes. Yes, I got a bunch of junk too, but I have a spare or two of most common tubes. You have to watch what you buy, particularly from Ebay, as most have been picked over, and they are selling nothing but junk tubes in them. The one I got that was the best was from a private seller, had some sweet tubes in there.

    If you were near Tampa, I have a couple of taiwanese 12AU7's I think that I would let you have cheap just to diagnose. Probably not worth the trouble of shipping. If you are going to pay shipping, may as well order new, good tubes.

    Jason

    Tom

    Posts : 166
    Join date : 2011-04-04

    Re: swapping 12AU7s for 12AX7s

    Post by Tom on Tue May 10, 2011 9:22 pm

    Roy (I think?) - yes, thanks, that's what we were thinking. Just trying out logic here before proceeding.

    Jason - my daughter's at school in Tampa, we're just across on the east coast. I'd take you up on the offer (thanks!) but I can get a pair of Pentas for about $25 local. Just don't want to use them once for that price. With 3 to put through college, I'm poor, not just cheap. It's Different! Honest!!

    Rolling Eyes

    tubes4hifi
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    Re: swapping 12AU7s for 12AX7s

    Post by tubes4hifi on Tue May 10, 2011 10:37 pm

    kind of the opposite philosophy, buy cheap spares, I've done that alot in the past, but now . . .
    If you're gonna wind up using the SP8 preamp for your prime system, buy a couple of BETTER than cheap tubes.
    I've been stocking up on NOS RCA 12AU7 cleartops lately!!

    Tom

    Posts : 166
    Join date : 2011-04-04

    Re: swapping 12AU7s for 12AX7s

    Post by Tom on Wed May 11, 2011 8:51 am

    yes, it's got cheap nothing special tubes in it right now,
    out in the garage and playing through an old Technics integrated and baby Ad speakers.
    I figure after ALL the bugs are out, I'll buy real NOS tubes and bring her inside to the real system. Smile

    Speaking of NOS RCA cleartop 12AU7s - this guy seems to have a bunch at a reasonable price. If they are really what he says they are. Unlabeled RCAs for $19 ea? Any opinions?

    http://www.audiotubes.com/12au7.htm



    tubes4hifi
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    Re: swapping 12AU7s for 12AX7s

    Post by tubes4hifi on Thu May 12, 2011 11:51 am

    I've bought tubes from him before, a good, reliable dealer, the no-name cleartops are a bargain at $19 each or $39 pairs

    Tom

    Posts : 166
    Join date : 2011-04-04

    Re: swapping 12AU7s for 12AX7s

    Post by Tom on Fri May 13, 2011 12:13 pm

    Thanks for the input.

    Now that the hum is gone,
    I ordered some of those for upgrades and a Alps pot from Mouser
    to replace the cheap radio shack one.

    Can it actually sound better? Can't wait to find out.


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