New tubes are a niche market, whereas years ago they were required by the most demanding applications.
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I heard a story about an RCA tube making machine (whatever they are called) that was new in the 20s and made great tubes but in the 30s the fittings got looser and the 40s looser yet etc. etc., each year tube quality went down due to worse tolerances. They kept the machine for several decades so tube quality at the end of the machines life was relatively poor.
One interesting explanation...
A lot of NOS tubes sound bad. There are good ones and bad ones. I prefer the Winged C 6550 to the GE 6550 for many reasons. First, the WC sound better, less shrill treble, better bass, and more reliable. I do like the Amperex BB 12AX7's. But I hear from friends, they have replaced theirs with the GL 12AX7's and like them as well or better. I think new tubes can be very good and just be open minded before spending 300.00 for a rare NOS that may not be any better.
Tpreaves is asbsolutely right.
Back then the Military had a great need and relied heavily on ruggedness as well as quality. Can't remember where I read it, but the article stated that they spend much less time creating the vacuum in modern tubes. Back then, an hour on the vacuum pump, now 20 minutes. I never had a nos tube that was dated before 1940's and they were great. If quality was waning then, hard to imagine how good the tubes from the 20's and 30's sounded.
Before now, I never gave a hoot nor hang as to "why" NOS tubes sounded better. All of the post's have come up with some interesting points. Tpreaves stated that not all NOS tubes sound better. Not all NOS tubes were manufactured for "audiophiles". Many of you act like we're some kind of new breed. Hitler was an audiophile.
Since I couldn't find anything scientific, I came up with theories. The very first "high end" for the public was the "jukeboxes". "Why do you think they sucked in so many nickels dimes and quarters"? It was because they sounded so good. This was due to "high end" parts, including the best sounding tubes. Since there was a large market and demand for "audiophile" tubes, many were manufactured. Those jukeboxes were all over the country in the 40's, 50's, and 60's; and they used tubes, until they went to SS in the 70's and began to sound like crap.
I was told that new tubes aren't allowed to be made with the same rare metals that NOS tubes use. They are heavily regulated and folks high up on the food chain don't want them readily available to anyone who might want to use them for something else.
All that leaves the newer ones with are better engineering, closer tolerances, innovation and cleverness.
Thats funny, I asked this same exact question to a local high end audio shop nearby. "Deets Sound Room" in Carmichael, CA if you know the shop. I was giving pretty much the same answer as everybody here. But I bet if you ask that same question today to tube makers, will they give you the same answer or will they give you some marketing spill??
I was personally thinking myself tubes made today would be better than the NOS due to manufacture process today could produce a better quality tube. You would think its being built better today, but I guess thats not what I'm seeing to all the answers. No pride in tube making today I guess.
As far as the sound is concerned, I'll attribute the decline to significantly decrease competition and demand (like many of you have already indicated).
For the build quality, durability, and lifespan, I think it's the shift to a "disposable" society. Cars, lower-end receivers, washers and dryers, telephones, televisions, mass-produced furniture, and so much more just don't last as long as they used to. That's not to say that they're not functionally better in many ways; they just don't last as long. I've had a cheap gas stove from the 50's, and a cheap gas stove from the mid 90's, and while the newer stove is safer (avoiding pilot light issues), the older stove might still be functional in another 60 years while the new one might crap out tomorrow. It should be no surprise that vacuum tubes (no longer built in the UK, Germany, or North America) also have shorter lifespans.
Rodman makes a good point. There are a couple companies out there that make excellent new productions; however, EAT for instance is limited to the KT88, 300B, ECC88, and the ECC803S, and that's not all that uncommon. Heck, the only current production 5751 is the Sovtek (as far as I've found). There are some very good options out there, but not for everyone (certainly not for the rollers).
Personally I prefer the NOS tubes. They also seem to last a lot longer. The exception is EAT. The problem I have with EAT KT88's is I bought a quad of used original KT88's that have serial #'s that start with 'D'. 1 is bad after 25 hrs and 2 won't hold a steady bias after 100 hrs. The pair I have that start with 'E' seem rock solid. So as of now I only have 3 good ones. That being said the sound from the EAT's is something else. Just got some Gec KT88's that sound real close to the EAT's and being vintage tubes I hope the last a lot longer.
I wouldn't buy used power tubes, of any brand/vintage, unless I personally knew they were not abused(over or under biased). Either of those conditions can shorten a tube's life dramatically. Besides that; there is no warranty. I recently purchased a premium matched octet of NOS GE 6550A's. One tube began to increase it's own bias(10-20mA/min), and blew a HI-FI Tuning fuse(ouch), after the third week. The tube was returned, and replaced within three days. There's no guarantee that NOS tubes will last longer, SO- it's nice to have coverage.
Rodman - How does under biasing a tube shorten the life? And does a very slight under bias cause problems? It was recommended by Andreas Hoffman from Octave corp to slightly under bias the tubes (biasing is done with leds and not a meter). I'm fairly new to tubes (about a year) and I do know they have to be treated with care. Constantly driving them hard will shorten the life too. And if man made it - it will break (not a matter of if but when).
As far as vintage lasting longer I have TS 6550 solid grey plates where the getter flashing is pink and almost gone but the quad still sounds great and strong. Those I got from E-Pray for a great price and when compared to my NOS quad they are very close (NOS sounding a bit better). Same with the solid black plates (the best TS sounding tubes BTW).
My Octave came stock with SED KT88's and I bought SED 6550's and EL34's from Octave. All good tubes but the EL34 has a mid range magic I love - just not enough low end. Then I got some GL KT88 re-issues' and they are better (like a cross between the SED KT88 and 6550's but the mid range wasn't there). Next up was TS 6550 bp's Ei KT90 type 2 and EAT KT88 diamonds (early version - just had to hear if the rave was true for myself). Anyhow the TS 6550 bp's have the mid range and low end. Very musical and tubey sounding. KT90's nave the mids and the low but a somewhat steely high end (wrong input tube) BUT power like I have never heard. The EAT's are a different animal all together. Much more detailed and linear yet retaining the tubey sound with the mid range which is very important to me. The highs just sparkle. Last I just received NOS KR Audio branded Tesla KT88's and Gec KT88's. The Gec's are very close to the EAT's and the Tesla just trails the Gec's.
For drivers the Octave came with Rca 12at7wa. BUT I really like the Brimar 6060 yellow T and Gec CV6091's with the high transconductance tubes (EAT - GEC and Ei outputs) (slight edge going to the Gec's).
The stock input tube was a Sovtek 12ax7. I tried the GL 12ax7 reissue Ecc83 and ts 12ax7 reissue. Not much difference there. Rca 5751 black plate really smoothed out those KT90's. I'll be trying a Brimar CV4004 soon.
Any advice? I'll probably try a Telefunken input at some time. For current production output I'll want to try either the Shuguang Treasure Z or Psvane. Problem with vintage is they have been out of production and the supply is dwindling forcing prices up LOL.
Mr X- Actually; it depends on semantics. Read the first paragraph of this article: (http://www.svvintageamps.com/tubebias.php) Most often; people refer to, "over biased" as being too hot, rather than the other way around. I simply covered both bases(sorry for the confusion*). Of course; running a tube slightly cooler than it's design parameter, will extend it's service life(as with virtually any other electronic component). Happy listening! *Having reread my earlier post: I did say, "either of those conditions" which was a brain-fart on my part!
Rodman - Thanks I was starting to worry about the slight under bias. I know tubes won't last as long as transistors and want to get the most life out of my tubes. As I've stated overall I prefer NOS (EAT is the only exception and maybe the Shuguang Treasure Z or Psvane). Most of the NOS and A(lmost)NOS I have are very close to EAT Kt88 prices. Thanks again for clarifying the under bias - I'll sleep better tonight LOL.