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

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 :)

Tvad and Neebee, there is no doubt that I support my design decisions !! :)

-And the reason that I made those decisions is based on the experience that I had with the tube types I mentioned. The 6DJ8 was a popular intrumentation tube (look in an old Tektronix scope sometime) and is very good in that application. Unfortunatly, the tube was not originally intended for audio and while it is very linear, it has a micorphonic tendancy that is profound relative to other tubes.

Despite that it has been popular in audio for a long time. However if one pays attention to what it and other tubes do, it will be seen that any equipment that uses it will have a coloration particular to that tube- you all know what I am talking about. The fact of the matter is that audio (high end anyway) equipment ought to have *no coloration at all* so if you want that you will have to start with a less troublesome tube. It really is that simple.

I was influenced to try the 6SN7 after seeing the the MFA Luminescence preamp (which was extraordinary in its day) as well as the Vacuum Tube Research Labs stuff. It was easy to discern why the tube had/has a serious following.

So I was just letting you in on what I have found to be true over the years (I think most of the more respected people posting here do exactly the same). I hope this is not troublesome.

Ah- OK. The 6SN7 will *generally* offer you smoother sound with less 'hash', especially at higher volume levels where airbourne vibration can affect the tubes. Detail will be the same and 6SN7s will generally allow you more drive ability as they will handle higher currents better than 6DJ8s. Lifespan seems better too. The 6SN7 will make less gain, but often that is not an issue in driver circuits, line sections or DAC outputs.

Another way to describe the difference is that circuits with 6SN7s sound more refined, while those with 6DJ8s sound more brash.

Mrtennis, you might reread the caveats I included earlier. However I might also point out that this experience is from working with literally thousands of tubes. I do agree that the tube type ought not be the criteria for selecting a component, although I would hesitate at buying any new component that uses tubes that are out of production or doesn't sound right with current production. That is one reason we switched to 6SN7s- the current production sound much better than the current production 6DJ8s.

Of course, we have our own reasons for thinking that we have a handle on transparency that allows us these judgements :)

FWIW we tried several types in circuits that were identical in topology, but optimized for each tube. The differences became very clear.