One of the favorite pastimes of many original Leben CS-600 integrated amplifier owners has been trying different combinations of vacuum tubes in their CS-600 to tailor its sound to what best matches their personal taste.

The Leben CS-600 tube complement is four twin-triode 6CS7 vacuum tubes used in the first-stage of amplification, a single slow turn-on 6CJ3 vacuum tube that is "a single heater-cathode type diode intended for service as the damping diode in the horizontal-deflection circuit of color television receivers, " which is used in the Leben CS-600 to provide a slow ramp-up of high-voltage (B-voltage) to the output tubes in order to extend their life, and of course, the four output vacuum tubes themselves (either an EL34 or 6L6GC/5881 as it shipped from Leben). 

NOTE: I have received quite a lot of messages from owners of the original Leben CS-600 expressing concerns about the current lack of availability of the NOS twin-triode 6CS7 vacuum tubes from vendors, which means that when the 6CS7 vacuum tubes in their CS-600 finally expire that their Leben CS-600 integrated amplifier will no longer be usable. I understand your concern as I am a Leben CS-600 owner myself. As a result of these concerns I have sent an inquiry to the Leben Hi-Fi Stereo Company about the development of a service bulletin that provides the necessary details to allow a service technician to update the original CS-600 so that it can utilize the 12AU7A and 12BH7A as used in the new CS-600X's first stage of amplification. So, if you are the owner of a Leben CS-600 don't panic just yet about the lack of availability of 6CS7 tubes, as it is likely that help is on the way to keep your CS-600 playing music for years to come.

IMPORTANT UPDATE: In response to my earlier inquiry, the fine people at Leben Hi-Fi Stereo Company are in the process of developing a service bulletin that will provide instructions for a service technician to modify the CS-600 to utilize the same 12AU7A and 12BH7A vacuum tubes as used in the new CS-600X's first stage of amplification. The modification is a fairly straight-forward one in that it requires only changing out four resistors in the CS-600, although the tube sockets will need to be rewired to accomodate the 12AU7A and 12BH7A vacuum tubes, so most owners will want to engage a qualified service technician to do the work. So all of you owners of the original CS-600 that have been writing me about your concerns over the decreasing availability of the 6CS7 tubes can take a deep breath and relax, the Leben Hi-Fi Stereo Company is going to provide the information you need so you can enjoy your Leben CS-600 far into the future! I'll write a dedicated blog post on this topic as I find out more. Happy Holidays!

This pastime of "tube rolling" in the Leben CS-600 integrated amplifier was greatly enhanced by its ability to accept a variety of output tubes by switching between different cathode resistor and plate voltage settings, as can been seen in the table below (there's more information on this topic in my March 2010 post HERE).

The new Leben CS-600X maintains this ability to utilize a variety of output tubes by switching between different cathode resistor and plate voltage settings, which you can see in the lower left corner of the photo of the interior of the Leben CS-600X below.

Given the twin-triode 6CS7 vacuum tubes are no longer in production and are largely unavailable from suppliers, the Leben CS-600's circuit was modified to accept pairs of 12AU7A and 12BH7A vacuum tubes in their place for the first-stage of amplification, which adds two new vacuum tube types to experiment with - what fun!

The Leben CS-600X that I have in for review has the following tube complement: 

The power tubes are the Electro-Harmonix EL34EH.

The slow turn-on tube in the CS-600X sent for review is a classy looking NOS Raytheon 6CJ3.

The 12BH7 used in the new CS600X is the JJ 12BH7-A.

The 12AU7 tube used in the new Leben CS600X is the JJ ECC802-S / 12AU7.

This tube complement sounds very nice indeed, but who can resist the temptation of tube rolling with premium or NOS vacuum tubes? Not me!

Long time readers know that my favorite output tube in my CS-600 is the beautiful blue glass Sophia Electric EL-34ST (review HERE), which have never left my CS-600 after their review way back in 2017.

If you read my last post about the Leben CS-600X (HERE), you know that Sue at Sophia Electric has sent me a quad of their premium Sophia Electric EL34-ST, and a pair of their premium Sophia Electric 12AU7 tubes, to try in the new Leben CS600X (alas, they don't make a 12BH7).

That is really exciting! Thank you so much, Sue! 

Sue sent me a follow up message a few days ago that they would like to send me a quad of their blue-glass ST-style bulb KT88-ST ($175 USD per tube) to try in the CS-600X as well in the coming weeks, so tube rolling fun is afoot! 

Now that only leaves the stock 12BH7A as a tube to roll in the CS-600X. Just for fun - and for the sake of completeness - I ordered a pair of NOS Sylvania black plate 12BH7A vacuum tubes from Roy & Dale at Radio Electric Supply for a complete set of tube rolling fun!

Ok, so now we're all set to try the premium Sophia Electric EL34-ST, KT88-ST, and 12AU7A vacuum tubes, as well as the NOS Sylvania black plate 12BH7A's, in the new Leben CS-600X.

First up, I'll insert the well run-in quad of premium Sophia Electric EL-34ST vacuum tubes from my Leben CS-600 into the Leben CS-600X in place of its stock complement of Electro-Harmonix EL34EH's, and give them a listen, while the new Sophia Electric EL-34ST vacuum tubes get some run-in time in my CS-600.

Next, I'll insert the premium pair of Sophia Electric 12AU7's in place of the stock pair of  JJ ECC802-S / 12AU7 in the CS-600X, and give them a listen.

Then, I'll insert the NOS Sylvania black plate 12BH7A's from Roy & Dale in place of the stock pair of JJ 12BH7-A in the CS-600X, and give them a listen.

Finally, I'll insert the quad of Sophia Electric blue-glass KT88-ST vacuum tubes into the CS-600X and give them a listen.

It is likely that the the quad of Sophia Electric blue-glass KT88-ST vacuum tubes will be a longer-term listening evaluation, as it took a good long while for the Sophia Electric EL-34ST vacuum tubes to fully run-in and display their full splendor, so you'll likely see multiple updates on them over time.

As always, thanks for stopping by, and may the tone be with you!