2. amps-preamps
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first tube rolling: expert advice needed - take 2

Annoyingly my first thread that went under this header has disappeared. Apologies to everyone who had contributed and put a lot of thinking into this.

Here is the plot again: I had bought a second hand Unison Research Unico SE hybrid integrated amp with two Electro Harmonix 12AU7A in the pre-amp section. Very fine in musical terms, the amp caused an unacceptable (in my solid-state biassed opinion) amount of hiss and slight traces of hum coming from the speakers in silent parts. I had assumed this was caused by (perhaps defective) tubes. Not knowing where or what substitutes to start searching for, I have had the benefit of Audiogoner's advice and finally acquired a pair of NOS tubes at Machmat in the Netherlands, namely:
Tesla ECC802S unused NOS (virtually the same as Telefunken's ECC802S, I have been told, picture: see my "Guests" system).

Here is the good news: Am I glad I tried this! I am getting a cleaner and more saturated sound, as if creases had been ironed out; bass appears more finely detailed (tried this with different speakers with reproducible results)
And here is the bad news: The hiss is still there, perhaps even more disturbing than before because the music is better. Guess I'll have to have the amp inspected and probed (hope I haven't been "had"). Anyway, if it doesn't work out with this machine, I'll be looking at a full valve integrated from Unison (I do like this sound).

One tip I gladly pass on (not just to the Europeans): Mattijs de Vries of Machmat sure is someone to rely on!
http://www.machmat.com/index.htm

Finally one question to the experts: Is there something like a burn-in procedure I should observe or time I must allow for with unused NOS?

Thanks in advance and regards
Karel
karelfd
rodman99999
6,035 posts
 
Hello Karelfd- It certainly sounds like "tube rush" the way you describe it. The least little noise put out by a tube in your pre-amp will be magnified exponentially. Contact Kevin here: (http://www.upscaleaudio.com/view_category.asp?cat=44) He very closely screens every tube that passes through his operation, and has tubes that are designated for pre-amp use. Very knowledgable on tube equipment and possible problems. OH- nothing manufactured today even comes close to the Telefunken ECC802S(regardless of what you may be told otherwise). No one can afford to put that much effort into precise/quality manufacturing techniques nowdays. Now if EAT or KR were to manufacture a copy.....
karelfd OP
476 posts
 
Hi everyone. Thanks for your input! Unfortunately, I can't put some of your advice to the test as I sold the amp meanwhile (see my posting of 04.04 above).
Still, very grateful for any further thoughts you may have (I now own a fully tubed integrated :^) fortunately without problems).
Slates, your observation is quite worrying! However, I don't think my tubes were bad since I'm using the very same ECC802S Teslas in my new Unison S2K - without hiss - that I had tried in my Unison Unico - with hiss. The guy I brought them from claims they were thoroughly tested.
Then again, since the Unico is part of my friend's system now, perhaps, just for the sake of it, I can convince him to experiment a bit (although he's happy with its performance, I must add).
slates
35 posts
 
Hi Karelfd,
In my situation my pre-amp takes 8x 6922 tubes, 4 of those 8 tubes are in the input stage the next 2 are for the feedback and the last 2 are from my understanding the interconnect on the back of the unit, now the 4 input tubes are detrimental , if those tubes are not tested close to perfect for all aspects, you will definitely hear the difference. I also found that if the pins don't make good contact in the socket, it will cause the tube issues, and I can only make an assumption that this could have been your problem, the tube is a strange beast once you understand it, most of your problems will vanish.
My findings are from countless hours of tube rolling and experimenting, this
is the only way to be sure, because all systems are different from one another.

Thanks

Thanks
prallamby
1 posts
 
For the record i've used both Telefunken ecc802s and the Tesla version of the same name.
The Telefunken is in an entirely different class, its the best of the best standing head and shoudlers above all others including Mazda Silver plagte 12au7WA 6189 which are also excellent. I've rolled through 40-50 different 12 AU7 ECC82 in my OTO SE, mullards, brimar, rca, raytheon, CBS, sylvania, telefunken smooth plate, rough plate, Ei, Sovtek, Tesla, Cifte. TJ Ful lMusic, The american CBS raytheon are nice too but the Tele 802S is simply in a class of its own.
Is it worth the money? Yes!
orpheus10
8,668 posts
 
The 12AU7 is my favorite tube; it brings out the best in the sound of jazz. The RCA cleartop is a good choice for NOS tubes.
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