Tube Characterization: 6DJ8, 6922, 7308, CCa, etc


Does anyone agree with me that there should be detailed descriptions of the sonic characteristics of each of the popular versions of each of the above tubes. I've read Joe's Tube Lore and a number of manufacturers web-sites which are great general direction guides but they really don't tell us what we need to know in specific and defined terms. Perhaps starting with an overall sonic characteristic like liquid, or warm, or dry or transparent then their response at the frequency extremes (since this is always an issue with tubes), then perhaps individual characteristis with say female voice, piano etc. and then imaging. We would all purchase a set of cheap base line tubes that are known for consistancy and have clearly defined caharacteristics so that comparisons could be made to this benchmark. Then use benchmark recordings. Even better if we had the same equipment best yet if we benchmarked every component in the chain but not necessary because we would be dealing in relatve values.

Of course there is the question of synergy with existing equipment and the fact that we all don't hear exactly the same and so on and so forth, but again, it would all be relative. "Tube "A" has has better defined bass than the benchmark by a factor of 3 on a scale of 1-10 IMO" for instance. Of course this wouldn't be an exact science but it would give us real direction and be more useful than "this is a really great tube or this is a really, really great tube" or slavishly depending on the opinion of the tube specialty store who may be as honest as the day is long but does have to move what he has in stock. If we can bring this evaluation process closer to science we could spend less time playing this silly expensive guessing games and spend more time exploring the kind of sound we like and buying the kind of sound we want (not to mention, listening to more music) Thoughts?
anacrusis
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I have over 100 hundred pairs of this series of tubes. In the past year I've listened to plenty but not all. One problem you'll find is that most take days to have their signature sound settle.

Regardless, I'd be willing to participate in this experment and am "all for it".
I agree with Albert too. Having said that, I will now proceed to step on some toes so please excuse me for a moment :) :

6DJ8/6922s are not a good tube for audio despite what you might hear here. The reason is they have a tendancy to microphonics (they were built more for instrumentation and not audio), despite their otherwise excellent characteristics. You *really* have to hunt to find one that is low microphonics. The 12AT7 is a lot easier to use and so is the 6SN7 for many of the purposes that you might use a 6DJ8 for.

The 6SN7 is the octal equivilent of the 6CG7, but is usually smoother sounding because again there are less microphonics (my guess is due to the more robust structure).

12AX7s can be very nice but do not expect a wide bandwidth circuit! So from 12AX7s you will get an older-fashioned tube sound as mid and treble artifacts will be audible due to phase shift caused by the upper frequency cutoff. Its not a bad sound- just not very fast. Incidently this is something that 6DJ8s do better (they have plenty of bandwidth) but now you have an 'iridescence' caused by their microphonics.

12AU7s can be very nice as they have good bandwidth, but they are more prone to microphonics again, however with less of the 'bell ringing' issues of the 6DJ8 family (which includes the 6H30).

IF it seems that microphonics is a major problem, you are right. Tubes designed for audio do seem to have less in that department then those designed for instrumentation. There are other more subtle effects caused by means of construction, some of which are lost to time: Telefunken 12AX7s and Mullard tubes are sought due to the excellent contruction techniques employed during the hey day of tubes.

So, while I have offered some general pointers on some tubes, the individual tubes of the specific types will vary enough for considerable overlap! This is why Albert is correct in his assertion. If you are dealing with the best of the best in all tube types you will find them all very close. It is the generel average tubes where the foibles that seperate them become evident- for example the average 6DJ8 will sound pretty bad compared to the average 6SN7.

My advice if you are tube rolling is to be careful about documenting what you are doing, and make sure you can return tubes that you buy from NOS/antique dealers! *Above all* remember that you are supposed to be having fun and don't go off the deep end. Its not pretty :)
my experience with the Audionote CD2 and the VTL Deluxe 120 amplifier has confirmed that changing tubes can affect an audible difference.

the CD2 uses a 12 volt tube. it is a 1990's design CD player. I have treid 12AU7, 12AT7, 12AY7, 12AX&, and 5751.

i currently use a GE 5751 black plate.

the VTL uses 12AT7 input tubes. I prefer RCA 12AT7 black plate or 1960's Mullard.

the VTL is a design from the late 1980's.

It is possible that the older tube designs are more sensitive to tube changes, than current production tube products.

Again, i believe the circuit design is key.

Here's another example. I had a Cary AES preamp. I tried many NOS 6SN7 tubes, hardly noticing a difference in performance.