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
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.
dave_rosenblum
11 posts
 
Hi Karel,
This post is for your friend's benefit. He should take it to a good stereo repair shop. I've got an SE that I bought used over 4 years ago and I've never heard any hiss.
I just turned it (back) on and only hear a little hiss when I crank the volume way up and put my ear right in front of the speaker. I've never noticed any hiss while listening regardless of volume. I assume you weren't driving some super sensitive horn speakers. My speakers are A.Z. Addagio's, not especially low sensitivity.
I don't have any audiophile power cleaning in place since my PS Audio Duet pooped out on me a few months ago.
Dave

PS: Glad you restarted this. I'm going to do some tube rolling.
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