Break in time for Tung-Sol 6550 Tubes

What is the break-in time for a NOS Tung-Sol 6550? I wanted to see if a NOS grey-plate, Tung-Sol 6550 would be an improvement to replace the original Sovtek 6550 I was using in my ARC Ref 6. When I first put it in the sound was bright and edgy—not the least bit musical. Someone in this forum mentioned that these tubes take 200 hours to break in. So—I actually took the time to keep it in my system for what is now about 100 hours, and I did notice a big difference. The midrange was improved, with a sweeter, richer sound. It is now at the point where this tube is acceptable—but there is still a trade off when compared to the Sovtek. The Tung-Sol has a deeper, tighter bass, more extended highs and a decent mid-range. Overall, there is more detail—a cleaner sound. However, the Sovtek has a much fuller sound with a rich, sweet midrange—but not as open. I am still not completely sold on the Tung-Sol, despite the big improvement when it had more break-in time on it. My question now is--will the additional 100 hours of break-in time make that much more of a difference in creating a warmer, richer sound?

After a couple of long sessions, they will sound as good as they can.
Just a little settle time and break-in.. If they sound "BAD" they shouldn't.
The first 24-48 hours keep an eye on them.. 

I fire new valves up watch for an hour or so, shut down, reseat the valves and leave it on at least 4 hours usually 8-12. I don't listen to much either.. Just to make sure there is no red plating.. 

At 24 hours I start listening...


Overall, there is more detail—a cleaner sound. However, the Sovtek has a much fuller sound with a rich, sweet midrange—but not as open.
So one is better at resolving the info while the other isn't but you prefer it. I doubt it will change much after 100 hours. I find the beauty at first sight rule applies to tubes. They bed in with time but their primary characteristics remain.
No better, after 100hrs they should have reached their normal operation plateau by now. Your ARC was designed with a specific kind of tube in mind so by substituting this tube with a NOS one, you will have somehow different sound characteristics due to metals and construction.
Normally with swaps like this you have to change something else in the chain to get what you want. Try a new Sensor made Tungsol 6550 or any other Russian 6550 of same period as your original Sovteks and see how it goes. Though NOS tubes are better sometimes will not match modern equipment perfectly.


I run the NJ Nos Tung Sol 6550 in my REF5se, it was flat out better from hour one, but the 6 is a different beast. i run my 5se on HRS isolation, that made a similar improvement. Run it and see. Please let us know what you discover. I have close to 6k happy listening hours on my pre.


Isn't that tube used as a voltage regulator? If so it's outside of the audio circuitry. ARC designed that gear using the parameters of the new tube, not the old original Tung Sol. Your Ref may not be running any where near up to par. Stick the original tube back in.