Tubes typically slowly degrade over time as they are used. They rarely out and out fail. They generally last a long, long time unless the design runs them very hard.

Swapping out tubes with replacement tubes can change the sound. Whether it sounds better or worse often depends on what you like from your system. Sometimes a piece of equipment unquestionably sounds better with a tube swap but, in my experience, is relatively rare.

I like Kevin Deal at Upscale Audio. He has always provides excellent advice and service. There are other good tube people as well.

I used to own that particular amp and did do some tube rolling. The soft start circuitry it uses helps the tubes last longer and I would probably start thinking about new output tubes after 2000 hours of use. The small tubes should last a lot longer than that...a lot longer...like years and years.

The best move I made was to change the output tubes to Ei Fat Bottle 6CA7 from the stock Chinese tubes. I also swapped out the stock preamp and driver tubes with several other NOS tubes, including the 5751 which to my surprise didn't do much for the amp. Apparently, the Cayin doesn't have a very high gain circuit as this tube almost always proves to be an improvement over the usual 12AX7. I ended up using a Sovtek LPS for the 12AX7 and was mucho impressed with this tube as was I by the Tung-Sol ECC803 being currently made. My final choice for the 12AU7 was some old Tung-Sol black bottles.

In total, I tried many tube combinations and found that the amp responded most to getting rid of the crappy output tubes which I found way too brittle and harsh.

It's really a fine amp and built superbly, with a large and open presentation that isn't fully appreciated with the awful stock tube combination.

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I've had a Cayin A50T amp for over 3 years, with on and off periods of listening (I travel a lot for work), and am curious about a few things:

1. Do tubes degrade over time, or simply fail at some point? Essentially, how do I know when to replace them? What are typical intervals for changing tubes? I cannot tell if the quality has degraded a bit, or if I have become accustomed to the sound. I do bias them.

2. Can I make a meaningful improvement to sound quality by upgrading the tubes? Or should I trust that Cayin put the best matching tubes into the amp based on their design?

3. Any good sites for buying replacement tubes?

Huge thanks!

From my experience...tubes are not tubes. Some do sound better than others. Tubes first sound better, then the same, and if they begin to sound worse, I have never noticed. When they fail, the fail noticeably. Each tube is an individual so one can go through half a dozen in a 4 tube set before the last original fails.

No, the manufacturer does not always choose the best sounding tubes, though maybe the most longest lived...or the most reliably similar.

Agree about Upscale Audio..