KT88 vs 6550 vs ? for warm midrange - using Cronos Mag lll

I recently managed to get hold of some Svetlana (St Petersburg) KT-88s from valvesnmore.com. They are at least as good as the Sophia Electric tubes they replaced. But I too, have read good things about NOS winged C 6550 tubes.

I'm using Quad II/forty monoblocks, but I am not sure if they are fixed or auto-bias (looking at a circuit diagram I believe it is fixed bias). Quad have told me the 6550 tubes will work, but "since the amp is designed for KT-88 tubes it may not be optimal." Apart from slightly reduced power output (35W vs 40W) the real issue is bias. Am I right to think that the fixed bias that can cope with any number of KT-88s that might be used is also likely to cope just as well with 6550s?

You need to get hold of the plate curves for the two tube types. That’s a graph displaying plate current vs plate voltage with separate curves for each bias setting displayed on that background. See how they compare.Then make some measurements of voltages in the amp itself. See if you’re in the ballpark for the 6550. The ambiguous response you got from Quad(?) was not super helpful. Or just try it.

Work on the signal tubes first.

The old production 12au7's you picked are on the bright/lean side per my experience.

The GE 12ax7's should be OK if they are drivers.

DeKay 

I guess to more directly address your question - exactly who said EL34 would have less lifespan here, unless you’re purposely biasing to run hot? That’s the first ever I’ve heard this. The bias on these amps is infintely adjustible per tube, and an EL34 can handle much more plate voltage than these amps throw at it. In fact the original Cronus and Atlas shipped with EL34 (I had an Atlas like this - it is a Cronus without the 12AU7 "preamp" tube and volume control).

As for tube sonics - Tung-Sol KT120’s give a perception of more powerful bass. Its midrange can be a little drier than with Gold Lion KT88, so probably not the right move for what you want. 6550 should not be a problem in these amps - very close to KT88. I’ve even used Tung-Sol 7581A (very close to a 6L6GC, not that close to 6550 / KT88) in Rogue amps - you’re probably approaching its plate voltage limits, but it worked in the ST100 and gave a bit of a more "classic" tube sound versus KT88 / KT100. A nice tube in the ST100, at least. Been years since I ran my old Atlas with British EL34, but that was pretty decent. The Mullards do what they can on these amps - they will never be "warm"-warm. You put those same Mullards in an Eico EF-87 and you get WARM-warm. 

12AU7 is probably used as a driver tube, which means it adds no gain and probably has little effect on SQ.  However, in my experience, there is no such thing as a good sounding 12AU7.  I replace them with 12FQ7 or 6FQ7 (depending on the available filament voltage), which sound noticeably better. If you're swapping power tubes willy nilly, then you need to be aware of how the amplifier can run one tube vs another.  If the plate voltage, grid bias, etc, are inappropriate for one tube vs another, then that is going to affect how you judge the tube. Ideally you would want to reconfigure the amplifier to run the power tube under conditions that optimize its performance, before you judge the tube. That usually can be done merely by changing a few resistors after taking measurements of voltages.