tube question 12au7 vs 12ax7

brenner182 · #1 12-29-2010, 07:23 PM

i have a ampeg svp-bsp that has a 12au7. Will it hurt to put a 12ax7 in it? i dont know how old the tube is in it now so i wanted to swap it out.

Any ideas?

BurningSkies · #2 12-29-2010, 07:52 PM

The 12AU7 is a lower gain version of the 12AX7, and has the same pin structure. You should have no problems.

brenner182 · #3 12-29-2010, 07:58 PM

and advantage/disadvantage to either?

BurningSkies · #4 12-29-2010, 08:04 PM

Quote:

Originally Posted by brenner182

and advantage/disadvantage to either?

I don't know enough about the SVP, its circuit or where/how the 12AU7 functions in it to give you an educated guess. Lots of times people use the AU to clean up amps that have too much tube grind or breakup and keep things clean.

levis76 · #5 12-29-2010, 08:10 PM

Swapping in a 12AX7 will give you more gain in the preamp, which could result in some overdrive/distortion. This may sound good or bad, I don't know, I've never used an SVP-BSP. If any of the preamp tubes are noisy or hissy or your bass isn't shielded/grounded well, you will hear that more too. Try it, if you don't like it at least you have a spair 12AX7. You can never have too many tubes man.

BassmanPaul · #6 12-29-2010, 10:36 PM

If you like how the pre sounds why mess with that??? I doubt if the existing tube has failed, they almost last forever, but if you want to replace it do so as designed. The 'U7 has more plate dissipation than the 'X7 and exchanging it could lead to earlier failure.

johnk_10 · #7 12-29-2010, 10:39 PM

my SVP-pro preamp didn't work at all (no sound) when i swapped a 12AX7 for the AU. i believe that the circuit has some sort of detection in it that requires that you use a 12AU7 in the correct location.

jimbilly · #8 12-29-2010, 11:22 PM

Quote:

Originally Posted by johnk_10

my SVP-pro preamp didn't work at all (no sound) when i swapped a 12AX7 for the AU. i believe that the circuit has some sort of detection in it that requires that you use a 12AU7 in the correct location.

You just had a dead 12ax7, a circuit like that would be WAY too complicated to be worth putting in.
There's other mid-gain preamp tubes that are a little under the radar of most tube buyers and you can get some great deals on vintage tubes like the 12AV7, 12AZ7, 12AY7.

johnk_10 · #9 12-30-2010, 12:18 AM

Quote:

Originally Posted by jimbilly

You just had a dead 12ax7, a circuit like that would be WAY too complicated to be worth putting in.
There's other mid-gain preamp tubes that are a little under the radar of most tube buyers and you can get some great deals on vintage tubes like the 12AV7, 12AZ7, 12AY7.

no, it definitely wasn't a dead tube. i tried 4 different AX7's that were all known to be good.
the led array that lights up the logo would come on (click out of its warm up circuit) with any 12AX7 that i tried in it.

billoetjen · #10 12-30-2010, 08:49 AM

I hope this thread stays alive for awhile because I've always wondered about the same question. Usually the "information" out there about it is either too obscure for me to comprehend or self-contradictory and unreliable.

brenner182 · #11 12-30-2010, 08:56 AM

What I've noticed so far was the gain was 75% more and it has a lot nore bass to it

beans-on-toast · #12 12-30-2010, 09:48 AM

Maybe I'm confusing which model you have, but the SVP-BSP preamp that I've seen had a single tube in it. This model was designed to use a 12AX7, not a 12AU7. If your pre has a 12AU7 in it, someone probably wanted less gain and substituted it in place of the 12AX7.

In general, I prefer the sound and performance of new old stock tubes. The important thing to keep in mind is that tubes from different manufacturers have slight design differences and specs. A tube that sounds great in one amp, may not sound as good in another. I'm not saying that a Telefunken or RCA 12AX7 isn't going to sound good in a particular amp. Chances are, it will sound great.

It depends on the design of the amp. It is a matter of having an amp that is optimized for the tube that you are using. Sometimes certain makes and models will prove to be a pleasant surprise. Getting a recommendation from others is a good place to start. Trying tubes from different manufacturers in your amp and listening is the best way to decide which is best.

Classickbass · #13 12-30-2010, 09:57 AM

Quote:

Originally Posted by levis76

Swapping in a 12AX7 will give you more gain in the preamp, which could result in some overdrive/distortion. This may sound good or bad, I don't know, I've never used an SVP-BSP. If any of the preamp tubes are noisy or hissy or your bass isn't shielded/grounded well, you will hear that more too. Try it, if you don't like it at least you have a spair 12AX7. You can never have too many tubes man.

+1

I went from the X7 to a U7 in my preamp driver stage and got hiss. I am now shielding my basses.

MAMMOTHvolume · #14 12-30-2010, 11:00 AM

12au7 can provide higher current and has less plate resistance so will work alot better in tone stacks etc.

When swapping for a 12ax7, you may notice a loss of highs and the tone stack may not be as responsive etc.

billoetjen · #15 12-30-2010, 11:05 AM

NOS makes huge difference. My faves? RCA Black Plates from the 50's for bass.

brenner182 · #16 12-30-2010, 12:49 PM

i did hear that my highs we not as bright as they normally are