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
fihigh
4 posts
 
Karelfd,I have experienced the same problem that you have.I tried different tubes,speaker cables,interconnects,power cords,Spikes,cones everything you could imagine.It was only after placing a tubed preamp with a separate power supply in my system did i achieve silence.Of course nothing goes well for the audiophile, although silent I didn't care for the sound of the new preamp.
Good Luck
adcom00
12 posts
 
I just can't agree with you guys.I have a tube pre(BAT VK-30) and SS monoblocks(PSE's) and I have absolute silence in the silent sections.I also have Tesla's in my pre.
slates
35 posts
 
I had some serious hiss as well a while back, I later found that my 6922 pre-amp tubes had very little gain in them. I bought them from a reputable tube dealer, and I paid large for matched sections, matched pairs and high gain. I now own a tube tester and it makes life alot easier. Out of 8 EH6922 Gold Pin only 5 were showing good
EM and Gm tests , 3 of the tubes were showing "REPLACE" in Em and below the Gm acceptance grade.I decided to test my findings and bought a bunch of Tesla E88CC for a great price.
I bought 24 tubes and totally did the nines on testing them,I found 8 really high scoring tubes, and decided to test them in my pre-amp, and in conclusion I was right, the hiss disappeared.I did receive a full refund for my
EH6922 Gold pin, and never bought tubes from that outfit
again. My pre-amp and amp take a total of 22 tubes, it just made sense to have my own tube tester, to think I was going to spend my money on an AC line conditioner, it would have improved nothing, if you have bad tubes.

Thanks
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).
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