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

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 

lewm:

Yes, two 12au7’s are used to drive the power tubes and the  two 12ax7’s are phase splitters (I think?).

The 3rd 12au7 would be the one to roll to something warmer/fuller sounding like Mullard or Amperex (just avoid later Amperex 12au7’s with a dimpled disc getter).

So, one or two 12au7 tubes to roll (Amperex,/Mullard/RCA?) as an experiment as opposed to quads of power tubes.

DeKay

No, leave the GE 12ax7's as they should be good and long lasting.

The single (center?) 12au7 is the one I would experiment with as it should have the most affect on sound  and then perhaps experiment with the 2 12au7 drivers if not satisfied.

I do not have a problem with 12au7 tubes and have been using them in gain positions for 20+ years in my preamp and in vintage power and integrated amps.

I've yet to listen to an amp using 6SN7's that I've cared for and the same goes for Audio Research and BAT amps using the "super" tube - so each to their own.

Look for a single well testing Mullard, Amperex or RCA 12au7 with balanced sections (it need not be a super duper expensive version or be NOS - just avoid the later dimpled disc getter Amperex as I previously noted).

DeKay

Also, have you tried a lower bias setting on the power tubes you are using?

Doing so (within reason) may move you further towards the sound you are seeking.

Try it for free before shopping for alternative signal/power tubes.

DeKay

PS:

I’m a big ST-70 fan.

Wow, I just looked at a video showing how to bias the Cronus III and did not realize that it was so easy.

I assumed that you needed to use a multimeter and probes.

DeKay