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I'm Freaked Out by this Tube comparison

I was comparing two amplifiers - ARC VT100 Mk II & SF Power 2. They both have Sovtek 6922's in the input stage that were manufacturer selected and matched of about the same vintage. Both have 8 new EH 6550's, matched & measured.

On cymbals, brushes etc, the Power 2 had a more distant and smaller presentation that lacked detail. Chesky guitar recordings sounded thinner, had less ring, less body, less resonance. Overall, less involving sound.

I attributed the sonics to the design of the amplifiers. I had a third set of Sovtek 6922's that I put in the Power 2. No noticeable difference.

THEN ------ I decided to take the 6922's out of the VT100 & put them in the Power 2. I couldn't believe my ears -- now the Power 2 took on much, though not all, of the character of the VT100. The cymbals, etc. became more detailed, more present, more resonant. So did the Chesky guitars, and now with more body and resonance. More decay. In some respects, the Power 2 sounds better than the VT100.

Am I crazy??? I can't believe that one set of the Sovteks could be that much better than the other. Or is Audio Research's claimed intensive selection process responsible?

There is no biasing for the 6922's in the Power 2. SF told me to replace tubes, you just plug and play. The output tubes (6550s) were accurately biased in both amps.

What gives here?
kevziek
clueless
763 posts
 
Interesting story Kevziek. I do not think your nuts but stick around tubes long enough and you will be (Just kidding).

I am not a 6922 user at this time so I am a little out of the loop but I've heard there are a handful (up to 8 or 9?)different Sovtek 6922s that are not always easy to identify. Lots of mislabeling and lazy matching going on too. Maybe they are not as close to same vintage as you were told? Maybe you had a mismatch or a poor tube. I guess that's why places like vintagetubeservice.com/page4.html and tubeman.com , vacuumtubevalley, spend so much time testing pairs and matching. It's almost impossible to know based on what you said. There is a reported very basic difference between old and new Sovtek 6922s but I guess that doesn't surprise either of us.

It's hard to believe that "identical" tubes can sound different but it may be a little like speaker drivers. I have a little more experience measuring that stuff. They might all come out of the same factory but are all a little different when you measure them and the mfg's specs can almost never be depended on and then they take on a life of their own when you make them part of a specific circuit and enclosure. If you want a nice sounding pair you have to hand match and it's not hard to make a mistake.

Thanks for the story

I remain,
j_k
234 posts
 
ARC provides a select group of 6922s that are tested for the specs to match the amp and for low noise and microphonics. I have heard a big difference in 6922s, based on where they come from.

I decided not to use ARC 6922s once in my VT-100. Never again. I will use other equivalents in NOS tubes, but if it is todays MFG tubes, go to ARC. I heard from a reputable source that ARC is rejecting 50%+ 6922s in the inspection process. I am not going to try my odds for $200 price difference for 8 tubes when a bad 6922 tube installed in a VT-100 can take out resistors and output tubes.

I am not affiliated to anyone -- just my experience and 2 cents. Kevziek -- you are not crazy, I have had the same experience.
sean
6,229 posts
 
I don't doubt your experience in the least. Welcome to the variables of tubes. I was trying to discuss this same thing in another thread, so you might want to take a look at it also. Sean
>

http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1015363401
zaikesman
3,017 posts
 
I don't use 6922's, but I have heard differences like are described here - just not between tubes of the same new manufacture and vintage (some can be noisier, more microphonic, etc., but the basic character is similar). Where I have heard these differences is between new tubes of unlike manufacture or vintage, so I would have to assume that the tubes described above are not really identical, despite outward appearances. I do know this: When I have bought "tested" tubes direct from conrad-johnson (who also claims a high rejection ratio and carefully controlled burn-in), I have found the results less than satisfying, and have had to go back and exchange certain tubes multiple times before giving up on them (and their higher prices) and just doing all the selecting myself by ear. Also, I would think that Sonic Frontiers, who sells tubes through their Parts Connection catalog, would have to have some sort of testing and QA program in place as well, and I for one would be surprised if any of these manufacturers' testing programs were that much better or worse than their peers'. Rather than placing faith in one company's pre-selected tubes other another's, I think it just boils down to the particular tubes in question in any given instance - one just has to listen for one's self, and if you're lucky enough to find a batch that sound better, then that's what you go with that time out. Next time, you may find you have to start the proccess all over again. Now, explain to me again, Why don't transistor users have ever-growing stocks of rejected and stand-by input and output devices cluttering up their closets, or have to pay return shipping charges on the losers, or run all over town searching for fresh ammunition for comparisions, or constantly wonder whether the sound has subtley degraded over time? Oh yeah, I almost forgot - They don't understand what *real music* should sound like. At least they have the time to listen!
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