I’m curious what folks think the standard life expectancy (in hours) are of the stock tubes provided in the BHKpre and BHK300’s. When do folks think the SQ starts to deteriorate and when do you replace them?
Relatively speaking, they are fairly low current tubes so their lifetime is actually pretty decent. Always have a backup pair if you can, but I usually tell folks about 2,500 hours. Swap them out with your backup pair and if there’s no difference in sound, go back to the old ones.
Thanks James… Interesting that 2500 hours is exactly where I found the drop off point to be myself… Glad to confirm I’m in the typical range.
Here’s an excellent primer on tubes:
I’ve actually started to notice reduced SQ about 2000 hours, but by 2500, it’s definitely time to change the tubes out. The sweet spot seems to be 500-1500 hours.
It really depends on the tubes. I have several duplicate sets because I like their general sound. No two sets of tubes are exactly the same, there are too many variables, though of course really good matched batches can be very, very close to each other.
One of my favorite pairs for my BHK pre from 1958. One set with 1200 hours and the other with 200. Very, very close to each other. Still on stock tubes ~400 hours on BHK 250.
10,000 hours is not unheard of for input tubes. Power tubes are worked far harder and don’t last nearly as long. The only way to know is to swap them for another set and listen.
Yeah, it really does vary quite a bit for people. Absolutely safe to say they’ll sound good 500-1500, but you should be able to expect a lot more than that.
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Just a question in general, have you guys considered running even lower current through them to make them last even longer? Its more of a general amp design question than a specific BHK question.
I don’t know about Bascom, but I know Darren spent quite a bit of time designing the input stage in the 1200s to help increase the tube’s life.
From memory, at some point Bascom said that sound quality of the BHK pre tubes should stay at its best for at least 1000 hours and suggested using one’s ears to determine. Also, on tube rotation, I remember him saying it is best to stick to 12AU7. Given the 12AU7 is low gain, it tends to last longer than the others.