Well I've decided to replace the tubes in my Engl, theres a bit too much hiss etc coming through from the preamp so thats an indication they are on the way out. The power tubes still have plenty of volume so they seem ok but I'll be replacing them anyway. Just wondering what do I replace the 12ax7 in the poweramp section with? I'm planning on going with 3x Electro Harmonix 12ax7's for the preamp and 2x 6L6 Svetlanas in the Poweramp, but it also has a 12ax7 in there too which I don't really understand. Something tells me not to put another EH 12ax7 in there, but maybe something with less gain... 12at7? Also whats the deal with changing from 5881's to 6L6GC for the poweramp, I thought they were the same thing, just name difference? I know that it would need a rebias anyway. Anything else I should look out for? BTW I'd like to keep the balance between good cleans and heaviosity so bare that in mind. Thanks!

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The last 12ax7 is a phase inverter, it splits the signal to the power tubes. If you're going for svetlana's buy the SED winged C type 6L6 if you can get them. I have heard mixed things about the new Sovtek produced ones, they're probably fine though (some say they sound alike, some say they're worse). Doubt there will be much of a difference, i think the latest Sovtek produced power tubes are decent (Tung-sol and EH).

I wouldn't go with EH preamp tubes in the Thunder really, but it's up to you.
Originally Engl uses either chinese or JJ tubes, depending on how old it is. They're both decent, the chinese are a little brighter and punchier, the JJ's are darker and have a "heavier" sound with more mids (not necessarily more brutal but heavier, like slow heavy headbanging music, heavy rock / metal etc). A good idea would be mixing them, like JJ/chinese/chinese/JJ left-to-right (V1/V2/V3/V4). Some say you need a balanced phase inverter, it's probably just a bullshit sales gimmick but the price difference is small and you're at least not doing anything wrong running a balanced tube so I'd get a balanced anyway for that spot.

For power tubes, go for SED winged C's, JJ's or newer sovteks, like Svetlana, EH, Tung-sol etc.

6L6/5881 doesn't make much of a difference today, some say the 5881's are brighter, some say smoother. I know my current tung-sol 5881's are very "biting" in my 5150, I'm replacing them with something softer sounding (JJ 6L6 probably).
They are swappable, the difference is mostly in the name, and the difference between 5881's and 6L6s today is probably no larger than the one between different makes of 6L6s. 5881 used to be the military version of the 6L6B i think (2nd gen 6L6, rated at 25 watts per tube), and it probably sounded different back then and was more reliable.

For the record todays 6L6GC's (all 6L6's produced today are 6L6GC's, the GC is for "Guitar Combo" and it was the latest version made of the 6L6, capable of handling vibration and rough handling similar to 5881's) are rated at 30 watts per tube while 5881's are rated at 25W before distortion, so your amp might stay clean longer with 6L6GC's. You still get 50 watts output power but it might stay clean longer.

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