How long for new power tubes to stabilize?

Just curious. I have a Prima Luna Dialog Premium HP integrated. I replaced the stock Chinese EL34s with some JJ KT88s… these were well broken in… I had them from an old pair of VTL monoblocks I sold… 6 weeks ago, I installed 8 KT150s from Kevin Deal at Upscale…

The KT150s sounded great at the start… still sound great… then two weeks ago (4 weeks old), they started to gain upper frequency … not bright… but like the lid came off and the thing was passing super high frequencies. I have Focal speakers with the beryllium tweeters and their response goes up to Jupiter. Then, I have been listening extendedly… working form home and running the amp about 12 hours per day with music for three days. The extended issue is gone. World Trade Federation???

Seriously, how long for these big power tubes to “stabilize”? I never experienced this with my VTL monsters… all new power tubes always sounded great from the start and pretty much stayed there… but then I was using Svetlana 6550 and winged SEDs… Always stable tubes.

BTW, I think you will see way more of these KT150s in bigger tube amps… it really is a good tube.

Bruce in Philly

It varies with type of tube and manufacturer, but ten’s of hours , even as many as 100 hours or more, is within the generally accepted settling time.

Upscale Audio pushes all their power tubes through a 72-hour burn-in period before shipping to customers.

Clearly, 30 hours on top of that still made a big difference for the tubes you’re using.
Nevertheless, 100+ hours fits well within expectation of settling time for larger tubes.

Thanx for the reply…

So now the KT150s seem to have stabilized. The top end issue is gone. I just never experienced this tube burn-in thing with my VTLs and the 16 6550s… yes that is correct 16 power tubes, 8 each amp.

Anywho… for anyone who cares about non PS Audio amps… My experience so far with power tubes:

Chinese EL34s… nice, classic tube sound… just a great tube… these came stock with the PrimaLuna amp. Nothing is really missing with this tube, PL did a nice job selecting it. But…
JJ Slovakia KT88s … more dynamic, better bass, more power. Something odd in the top end… not good. Upscale said they don’t sell JJ brand because their “quality control is awful”. Is this also why the top end is a little odd? I dunno.
Tung Sol KT150s … all the KT88 has, more power still, very transparent… less “tooby” than the EL34s, but just a great tube. Except for the few days of that wierd extended high frequency thing… this tube is it for me.

BTW, the PrimaLuna amps have unique auto-biasing circuits that allow you to try a whole host of different power tubes that most amps won’t support. Most amps support only one tube-type. So these PL amps are really fun.

BTW2: I ordered a pair of NOS Mullard 12AU7 gain tubes thinking I wanted to roll off that super high stuff… well now they are still sitting in the box as the system sounds really good right now. Darn… lesson is you gotta give this stuff some time as you say.

Bruce in Philly

Bruce in Philly

I like KT88 tubes in amps originally equipped with EL34 power tubes, but one does lose a certain midrange magic.

All tubes take a bit of time to settle, but I do not keep track or think about it much. I just listen. If I hear changes, it is mildly amusing. But if I do not this is fine too.