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

RCA clear top 12AU7 and GE short plate 12AX7s are pretty pedestrian tubes and not known to be warm in fact a bit the opposite. So before changing power tubes. you should research small signal tubes which tend toward the warm or rich and start at that point. I have always found that the tubes feeding the power tubes have more impact on the sound. 

Rogue amps and "classic" vintage tube amps sound are on 2 opposite ends of the spectrum. Rogue's formula is a punchy, fun sound with great bass and lots of tube Watts per dollar. They're going to be very reticent towards rendering a plump, lush midrange - you can only push it so far. If you have a Cronus / Atlas with EL34 (British Mullard) and good NOS small tubes (not later 1970 / 80s Philips ECG / GE), then I'd say that's about as much as you can push Rogue towards the classic tube sound. 

Be a bit wary of small signal tube rolling. It's fun but a lot of the "wow" is from the inital dopamine hit from a initial change. No need to go on a frenzy buying up all the vintage makes (I regret the time & money I spent doing so). You're not gonna make a Rogue sound like a vintage tube amp, in the long run. 

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.

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

Okay - I'm looking at changing the 12ax7's - suggestions ? 

will leave the power tubes alone for now .

I agree the way the Cronos is set up it's an uphill battle to try and get the EL34 sound 

All modern-ish Rogue tube amps use a 12AU7 tube as driver per push-pull pair of KT tubes. The Apollo and Zeus used a pair of 12AU7 to drive fully three push-pull pairs of KT. I agree with the asssement that 12AU7 is not a great tube. I definitely prefer push-pull amps that use 6SN7 drivers, or really anything else. 

But due to the fact it’s perhaps a bit "under-powered" in this driver role, it does make a noticeable difference to tube roll these slots. Similar-type subs are a good choice for most effect. 12BH7 is a sub I’ve used many times - it’s much more dynamic, detailed, and punchy than a 12AU7, though to be fair the Rogue amps were already strong there. The 12AU7 will give a more "homogenous" sound - maybe preferable to some ears and systems. I didn’t try other type subs. Note a 12BH7 eats up more heater curent - 0.6A per sub versus 0.3A - but when each KT tube is sucking down 1.5A - 1.8A, the extra A’s from a couple 12BH7 are unlikely to cause issue. Rogue (Mark) used to bless this sub, though they might have clammed up on this more recently. I used it for years with no issue.

The extra center 12AU7 on Cronus is an input / gain slot. Good to roll there. 12BH7’s tends to be noise and microphonic so it’s best to keep them at the downstream driver slots.

If you do try 12BH7 - Sylvanias and Tung-Sols are very clean, detailed, and punchy (Tung-Sols tending a bit more toward bright); RCA’s are warmer and richer. 

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

Phase splitter is usually done by driver tube and is better done by a 12AU7 than by a 12AX7, but anything is possible. A tube that has cathode and plate resistors of equal value, classically 47K ohms, is the phase splitter. But there are other ways to do it too.

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.