question about the 300B tube

[Larpy]

I built my Kaiju back in January and installed the DC heater upgrade a month ago.  The amp is now virtually silent, even with my 97dB sensitivity speakers.  The Kaiju replaced a VTL ST-150 (4 KT88s in push pull per channel) when I decided to try an SET amp and SET-friendly speakers and, at this point, it's hard to imagine going back to the VTL amp and the Aerial speakers I used for over 20 years.  The improved imaging and spatial cues I now hear on my system are a revelation.

I've experimented with 5670 tubes and, for now, prefer RCA Command tubes.  Though many here expressed their disappointment with the WE 396A, I just had to hear for myself:  sure enough, it's meh.

To my question:  how long can I expect new production 300B tubes to last in the Kaiju?  I've read elsewhere that the original WE 300B lasted on average nearly 40,000 hours(!) Can that be true?  I replaced the KT88s in my VTL every 3 years after 3000 hours or so of use.  Is the 300B really that much more durable? And, if so, why is that?  And is such extra durability the reason why new production 300B tubes are so much more expensive than beam pentodes like the KT88/6550?

[Paul Birkeland]

Yes, I would expect many of the new production 2A3s and 300Bs to last about 5-10 years for most folks in most situations.  I can remember a very regular customer sending me his Paraglows to repair, and his TJ 2A3 tubes were nearly dead.  I had to laugh because he wasn't very happy that they were dead and only ten years old

Having said that, if you go and buy the cheapest Chinese 300Bs out there, you won't get that kind of life.  I would also not expect that kind of life from JJ tubes either (sorry 2A3-40 lovers). 

For the KT-88/6550 tubes, I don't know what contributes to the quick failures.  I used to have a pair of really generic single ended amps sitting around that could use an EL34, KT88, 6550, etc. that I ran 24/7/365.  Good old Sovtek EL34s would run for about two years in those amps before they would fizzle out.  Fancy pants Golden Lion KT-88s would die in less than a year (they would go gassy).  Bottom of the barrel Chinese tubes would go for three or four months before the cathodes were exhausted.

[Doc B.]

Be conscious of the fact that different circuits will put different amounts of wear on the tube elements. Saying X tube last 40,000 hours without saying what its running in is misleading. We run the 300Bs pretty gently, and they have a pretty good lifetime. I noticed that the 300Bs in my amps were getting pretty worn when we moved to the new shop last year, and PB reminded me some had been in use since about 2003.