tubes...

If they didn't have a 12AU7, then they probably didn't even pull it out of the socket, so it is probably still in the right place.

My guess is that all the new tubes you put in were of worse quality than the original ones that were in there. You did get the old ones back from the repairman didn't you? If not, he's probably gonna re-sell them, or at least re-use them. If you got them back, put them back in, swapping out 1 at a time and hunt for the noisy one(s).

So basically, the only thing that has changed is the new tubes. And crackling and such that you mention is usually attributed to a noisy tube, not necessarily an older tube.

Chris

 

dude, if you switch out the old tubes (you better have your old ones or you got screwed), and if you find a noisy one, bring it back and say they sold you a defective tube.

 

a 12au7 is interchangable wiht a 12ax7, and 12at7, just some have more gain than others. I dont really think that its important to have a 12au7 in there, but certain people like its lower gain ratio. What kind of tubes were in there before, what did you put into it?

 

No, the head is supposed to have one 12AU7 in there.

As others have said, NEVER toss the old tubes when you retube just for situations like this.

Go through the new tubes one by one looking for the one(s) that are noisy.

By the way, preamp tubes last a long, long time...20 years is not unreasonable. Unless the amp sounded bad when you got it, you probably didn't need to retube it.

 

Chris,

If the preamp calls for a 12AU7 I would stick with that number. It is true that the AX7 and AU7 are interchangeable as far as their basing but they are very different in their gain characteristics and the amount of current they draw. I would switch tubes one by one until you find the noisy one. Brand new tubes can be microphonic right out of the box and can cause problems.

I suggest to all tube users to buy your tubes from reputable sources that actually test for microphonics and current match output tubes. Two such sources are Jim McShane in Illinois and Lord Valve (NBS electronics) in Denver. You can do a net search for these guys and either one of them will treat you right and stand behind there products! They both cater to the musical instrument crowd.

tapp

 

I had an SVT4 pro and it had 3 Sovtek 12AX7WA tubes in it.
I screwed around with diffeent ones and always went back to those.
I now have an Eden preamp with 1 tube. Guess what it is? a Sov. 12AX7WA. I fooled around with switching it out with a Mullard, Telefunken,Jan Phillips, NOS GE, NOS RCA, Sov. WB, and a NOS Sylvania.
The one that sounded best was the original WA. Next was the Jan Phillips, Sylvania, then the NOS stuff.
The new preamps seem to be built around the Sov. WA sound.
I'll bet my shoes that your tubes are just fine, and don't need replacing at all.
The 12AU7s usually are driver or gain stage tubes.
I never heard of changing that one at all.
If you use all 12AU7's, or 12AT7's, you will get a much lower gain, but they don't have any character to their sound. Lifeless.

 

Yes, there is a specific spot for the 12AU7. It's not a power triode, it's just a small signal dual triode like the 12AX7, just with different characteristics. Consult the manual to see exactly which socket it goes in.

Chris