Call Rogue Audio and speak with either Mark or Nick. Nobody knows tube upgrades for their amps better than they do, and they will happily help you out.
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I have a Cronus Magnum and experimented with a bunch of different NOS tubes, I finally settled on the following :
Preamp - Cifte 12AU7
Section Drivers - Mullard CV4003
Phase Splitters - Raytheon 5751 w/Windmill getter
This setup gave me the best all around sound (nice midrange with some sparkle on top).
I tried a Mullard 4003 in the preamp position, but liked the Cifte much better. I also tried Telefunken 12AX7 smooth plates in the splitter positions, but liked the Rayhteon's better.
I have the Cifte in the pre-amp position on my RCM as well. Very nice tube, 3D soundstage is correct. The Cifte is flanked by two La Radio Technique Green Box 12au7's. I have 2 Mullard 12ax7's on the way to replace the new production Tung Sol's in there now. Anxious to hear them.
Soon I'll be getting a nice Mullard 12au7 to switch out with the Cifte to see which I like better.
Got everything from Kevin at Upscale, very happy with his selection and customer service.
I agree with adam18 regarding a n.o.s. 5814. On my side I had a GE 5-stars, triple mica from de 1960's. Make sure that both triodes of the tube you'll use are perfectly balanced.
In the time I had my Cronus, I exchange some email with Rogue's owner Mark O'Brien and he always had good suggestions for tubes in this amplifier.
I bought a cryod Mullard 12au7 nos tube from Kevin at Upscale Audio. I noticed the difference as soon as I inserted the tube as it was quite dramatic. I received it earlier today. The Mullard seems to have done the trick. There is now the desired richness in the midrange and I am quite pleased with the results. Thank you all for the help.