If your output tubes are still strong: Try some Amperex 7308s, like these:(http://www.upscaleaudio.com/Amperex-7308_p_100.html), in the driver positions first. If that provides what you consider an improvement(it's almost guaranteed to); buy some for the preamp inputs(they have to be graded very low-noise for that slot though). Mention what flavor of presentation you prefer(ie: warmer, tubey, Brit sound or more detailed and transparent) and further recommendations can be offered. Any of the better NOS tubes will open up your sound stage, provide better ambience recovery, and focus(depending on the rest of your system and room of course).
I think you'll find that the consensus on those Sovtek 6922's is that they are sturdy and reliable tubes available in production quantities (the reasons ARC uses them) that happen to sound 'harmonically bleached' and about as far from the sought-after sound of NOS tubes as you can get.
I don't have the CA50, but for what it's worth, my LS7 preamp and PH3 phono stage were both transformed when I replaced the stock Sovteks with a mix of Amperex Bugle Boy 6DJ8's and Amperex orange-label A-frame 6DJ8's. I've never sprung for the pricey 7308's or CCA's ($$$!), which I'll bet would be even better. But once I listened to the NOS tubes for a while and then put the Sovteks back in "just to see" - it was almost unlistenable - seriously. Output tubes might be a different story, but tube-rolling the input and driver tubes is a total breeze, and if you don't like what you hear, you can simply put the stock tubes back - no harm done.
The driver tubes have to be matched and the voltage adjusted when they are changed. The procedure may be in the manual. I dont think you can just swap out the tubes.
If this is true then the design is wrong and arrogant !
I mean, one of the joys of owning tube equipment is to be able swap tubes every once in a while isn't it ?
p.s. is there anyone out there who actually owns the CA50 and tried tube rolling with it ?
7308/6922/6H23/E88CC/CV2492/E188CC are all just 6DJ8 variants, and direct replacements. No voltage adjustment should be necessary. Any paired tubes in any circuit should be balanced/matched, triode to triode and to one another in the case of his 6922's. Any responsible tube seller would have seen to that(like Kevin at Upscale), if the application was mentioned.
I had a CA50 and the ONLY thing that I really disliked about it was the fact that the voltage needed to be changed/adjusted whenever your swapped out the driver tubes. I would call AR back and ask to speak to someone more seasoned, they will be able confirm the procedure. Yes, you can just drop in new tubes w/o adjusting the voltage but it will not be optimized plus it will shorten the tube's life. The CA50 is not a friendly tube roller's amp. Also, AR uses the Sovtek because the circuit is tough on tubes and the Sovtek have a reputation for being rugged. I tried using NOS tubes but the amp ate them within 6 months (I never adjusted the voltage) and I did nit want to send the amp back to AR as they requested.
Testpilot, which NOS tubes did you try ?
I used 4 different tube sets.
Favorite - Amperex 7308 white label. Caveat, the amp eats up these tubes. I went through two sets in 8 months...no more.
Ediswan CV5358 - I liked these tube a lot. These tubes gave the amp some added jump. The highs are a tad rolled off, but that was okay since I was using the amp with CD only.
Valvo 6922 - nothing special in this amp. Did everything okay and they were durable.
Amperex Bugle Boy 6DJ8 - I settled on these tube because the amp and tubes got along. A touch of the magic of the Amperex 7308 and gave the amp some needed dynamics with a nice bottom end.
If you really like to roll tubes, these is not the best amp, but it is a keeper. I wish it wasn't so big, otherwise, I would have kept it for a second system.
I have owned my CA50 for the last 3 or so years and I am very happy with the amp. I currently use it full range driving fairly high efficiency speakers and a pair of add-on ribbon tweeters and the sound is very neutral.
You perhaps should have checked here or any of the other audio boards before getting upset as it is fairly common knowledge the CA 50 is not a good candidate for tube rolling.
I tried both JJ 6922's, Amperex Orange Globe and Bugle Boy 6DJ8's and it ate all in short order. The JJ's (which are nice warm sounding tubes in most amps) actually shorted and damaged the Amp while the Amperex's merely became noisy. This gets expensive in short order. Audio Research and Audible Illusions really stress the 6DJ8/6922's in their products and the Sovetek, Electro Harmonix and some versions of the NOS 6h23 (the 6h23EB I believe but you should check)are all that will last long term in the amp. See Jim McShanes website for his take on the subject. The EH69222's that ended up replacing the original Soveteks are not bad with a smoother less strident sound. I have heard that 7308's may work and I have some Valvo's and Tunsgrams I may try when I get brave (and perhaps foolish).
On the other hand the CA50 seems to tolerate different output tubes with just the normal re-biasing. Currently I am using the Tungsol 6550 Re-Issues with good results.
I wouldn't say I'm upset. I did a lot of reading before I purchased the amp and save for one discussion on diyAudio regarding tube rolling there was nothing else to suggest the amp is not a good candidate for tube rolling.
Either way, this is a very good sounding amp !
Sorry, I did not mean to be so definitive :<}, I was more referring to the numerous discussions regarding Audible Illusions and Audio Research's problems with Various 6DJ8/6922 tubes when used in their equipment.I very much agree that when used within the restrictions imposed by it's power rating the CA50 truly gives you the Audio Research Sound.
No sweat man !
By the way, do you still own the CA50 ? I have couple of operational questions to ask...
Yes I still have it (and it is up and running) what questions can I answer for you.
Is it normal for all the input lights to flash when you turn the amp on ?
Also, is there a sound from the speakers when you turn it off ?
When you turn the CA50 off you do hear a brief moment of the program material just before final shutdown. It's almost as if the muting circuit powers down slightly before the amp. As to all the input the lights flashing I believe that they do. I know some of the LED's flash but I cannot remember if all do (I am actually on vacation at the moment and cannot check to be sure). I will confirm this when I get back home in about 10 days.
Sorry for the long delay but on my CA50 only the mute switch led blinks on start up.
Is it connected to a power conditioner of any kind ?
Let me know,
Yes,it's a mid-level Monster power conditioner(nothing to get too excited about)that was a gift from a friend.