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
karelfd OP
476 posts
 
I guess the question on the Unico was actually addressed to me. I have been swapping cables, experimenting with isolation bases and moving the unit to different places in the apartment but never succeeded to get rid of the noise. Hooking up different speakers however made a huge difference. With Lua Con Fuoco Mk3 (4 Ohm impedance, SPL rated around 89db) the hiss reached the annoying level I described. With Chario Constellation Cygnus (4 Ohm, SPL 87db) the level was at a point that I could probably live with: audible in 1m, ear at the height of the drivers; to me that is, a friend told me he barely noticed anything in the ambient of a living room at daytime. I had been intending sending the Unison to the distributor for examination whether there was a defect after all, but I didn't get that far since the guy I mentioned bought the amp and the speakers from me.

Meanwhile, I got myself a Unison S2K tube integrated rated at 2x15W (an enormous difference in terms of output from the hybrid Unico SE with its 2x130W) that I'm driving the Lua with, and guess what: dead silent. I have also hooked up a couple of Zu Druid Mk4 (rated at 101db), again: silence. The S2K sports 2 ECC802S (I use NOS Tesla with EAT Cool Dampers) and 2 KT88 (presently the original Sovtek - with my fingers itching to try EAT KT88 Diamond Valves, ridiculously expensive but reportedly as good as NOS Genalex).

I'm not stating that the problem with the Unico was reducible to speaker sensitivity (there may be other tech specs that complete the picture) and I cannot be sure whether there wasn't an inherent defect on top of it (the lottery with buying used gear). But in my case the speakers made all the difference.
jorgeparrapuppy
257 posts
 
Karelfd: I'll have to second Mechans' take on the
SS pre and tube amps set up. I've got that set up as well,
using mono tube amps and get absolutely fantastic
results.

good luck!
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
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