Rogue Audio Dark Upgrade.

@samzx12 The CBS 7729 was their low noise 12AX7 for instruments. From Brent Jesse’s page: "New Old Stock in original boxes. Low noise industrial version of the 12AX7, with heavily gold plated pins. These were made by CBS and then screened for precision laboratory or avionics (Raytheon labeled) use and sometimes sold under the Leeds & Northrop label, with their lab grade number on the tube and box. This is a nice, but very scarce, version of the 12AX7 only made by CBS. Perfect for tube microphone preamplifiers, and incredible in any other hi-fi audio application."

I left out the price cause it’s rare, thus expensive. : ) The plate seems to a standard ladder (long plate). I’m not aware of any long plate triple micas TBH, could be wrong. The CBS 7729 is a fine tube. It appears to pare well with its 12AU7 brother, the CBS 7730, but that could be my imagination, as its the driver tube in the Stereo100 that affects the sound the most.

I am totally not recommending this (lol), but I’ve run a 12BH7 sub for 12AU7 in my Apollos for several years. I also have a friend that does the same in his Zeus. It’s a much better driver tube than 12AU7 in my experience. The driver tubes in an amp are often overlooked but can make a big difference. Sometimes it feels like 1 or 2 small tubes are struggling to drive all the KT’s (especially in Apollos). If you want detail & dynamics - this is the sub.

Mark from Rogue used to "OK" me that sub for a couple of components (Apollos and Ares - very hard to find quiet 12BH7 for the latter, though), but Nick has not been friendly to it lately. The issue is 0.6 Amp heater draw per 12BH7 vs. 0.3 A per 12AU7. In context of the 1.8 A per KT120, it’s not much extra total draw (is my reasoning).

Lots of good 12AX7 and 5751, and fun rolling them in the Rogue tube amps. Lots of different tonalities / balances that will work with different systems & ears. Roughly from brightest to warmest, here’s what I liked:

Mazda silver plate 12AX7
Telefunken Ribbed Plate 12AX7
Tung-Sol 12AX7
GE 5751 TM BP "silver clips"
Amperex 12AX7
Mullard 12AX7, CV4004

Despite being on the bright side, the Mazdas are actually my favorite. They have beautiful mids along with more detail and dynamics than the other tubes. It was like a full component upgrade to my non-Dark Apollos. With Apollo Darks and now slightly brighter speakers, it’s not the best match anymore - which kills me because I love that tube. I also recall really liking the GE 5751 "silver clips" version which was only produced for a couple years in the 50s (something like 52 - 54). Both Teles are decent, and easy to find. The Mullards end up being a little warm and thick for my liking. 

I’ve also briefly tried a GE 12AY7, that worked pretty well and lowered the gain (Apollos have a LOT of gain).

The original Apollos shipped with Electro Harmonix small tubes and Electro Harmonix KT90 - the result was a bright and lean sound! KT120, and any of the aforementioned small tubes, are much better. 

I have run Rogue Monoblocks for over 20 years starting with the 120s that were upgraded to 150s. That jump was massive. Better amp in all areas. They also ran much cooler as the 120s were mini space heaters.

I then sold the 150s and bought a pair of 180 darks. They have a bit more "darkness" and top end detail, but the difference between the 150s and 180 darks was very subtle. The most interesting thing is to my ear, they need to warm up at least an hour before they really get going. After that hour...they are a steal at the price point.

Also want to add that Rogue has splendid customer service. Usually Mark (the owner) responds personally. Just my 2 cents 

Thanks guys for some very informative information plus tube suggestions.  I have several 12au7 to try but will need to search for 12AX7 or 5751 that don't break the bank.  Or wait a little while until prices return to normal.  Hopefully.  This is definitely where I'll start before any other upgrades.  

@charcoalchuck agreed that Rogue is a steal at the price point.  I've had some really nice tube amps and the ST-100 rates extremely high.  Not sure its at Audio Research Ref level but pretty close IMO. Referring to older Refs and not the newest series.