ok so i have a ampeg SVT all tube head from around 1992. the tubes in it now and almost done and i need to put some new ones in it now. i want to try and get it to sound like JPJ's bass from zeppelin do you guys have any tubes that you think will help me out with it. right now i have for driver tubes its 6 6550's and they are Svetlana sv6550C's then for pre amp tubes i have 2 tungsol 12AU7's and one tungsol 12AX7's do you think that i should also get tungsol driver tubes and that will help get the sound i want or is there another kind of tube i should try? thank you for your help
I'm not sure exactly what type of valves you'll need - instinct says put ECC83's in the preamp. (They tend to sound more 'British', but have less output than the 6550's. Besides I'm not even sure if they'll fit in the holders!)
But if you do change types the bias voltage will need altered, and that's a time consuming job which could lead to a fried amp if done wrong. Trust that to a reputable amp tech.
Does the manual not say anything about it (tube compatability- you need to make a few adjustments for some tube types)? If you don't have the manual, then the ampeg site should have them.

I'm surprised your tubes have managed to last 16 years, if this is their first change.
I can't give you specific guidelines for the tone you want, but start googling kt88s v. 6550 to find out which will better capture exactly what you want. If I'm not mistaken those are completely interchangable power tubes (but have different tonal characteristics), so you can use either in your SVT.

Can't help you much with the preamp.
Kt88s need a tad hotter bias as they operate on a much higher plate voltage, but you can interchange them.