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

Showing 2 responses by slates

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
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
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