6L6 power tube confusion

[avoelp]

I've just bought a 1991 Simul Satellite on eBay and when it arrived I was quite astonished to see that it has 2 blue EL34s in the inner sockets and 1 blue and 1 green 6L6 STR420 in the outer sockets. I guess this is not how it should be, is it?

I want to use this amp together with my 1994 Mk-IV which has 4 yellow 6L6 STR420. Which power tubes should I use for the Simul Satellite to match the sound of the Mk-IV as closely as possible? As far as I could make out there are only STR425s and STR440s available nowerdays, but no STR420s - so which should I use?

I've also noticed that my 1978 60W Mk-II is equipped with 2 6L6 STR415 (Made in USA), and thus I was probably naive to think that, in addition to Mesa's color coding, there is just one type of tubes called 6L6 - so what are the main differences between the STR415, 420, 425 and 440?

Andreas

 

[MusicManJP6]

The different STRs are for where/when they were made IIRC. As for your Simul Satellite - the inner two and outer two pairs of tubes should be matched (draw the same current/voltage). The inners are probably fine but you'll want to use a matched pair (not just the same color, but also tested to match!) for the outer sockets.

 

[MrMarkIII]

The higher numbers (440 vs. 415) are newer. (But not necessarily better).
415s are the original Boogie tube and are prized. They can go for $90/pair.
420s are a bit more recent, but still 15-20 years old. While it's true that power tubes wear out eventually, don't pay attention to when they were made, but how they sound. Pre-amp tubes almost never wear out, despite what the tube pushers say. Especially the old ones, which were made better, etc.
EL-34s are for the OUTER SOCKETS ONLY in a Simul-Class amp, so you should replace them with matching 6L6s. You should run ONLY 6L6s in the inner sockets. Matched pairs should be installed like this in the four sockets: Y-X-X-Y. The "Y"s don't need to match the "X"s, and whether the EL-34 color codes match the 6L6 color codes is irrelevant. So, you can have two yellow EL-34s in the outer sockets, and two green 6L6s in the inner sockets, etc.
The different color codes (blue, green) meant those two tubes don't match. But it shouldn't damage anything to run them as a pair. Keep them for spares.
STR-425s are a bit lower power than the STR-440s and have a slightly different, more "vintage" flavor.
The STR-440s are probably closest to the 420s.