It's been 2 weeks since the GOTL has landed with me and I've been soaking up the awesome combination of tone, technicalities and features it has to offer. I've kept in the 4x Melz 6H12C tubes and have gotten to know a lot of driver tubes with it. Having spent all my listening time with these outputs, I feel confident to begin evaulating the differences between some driver tubes, without confusing tonal changes with the amp or the Melz's native sound. The Melz themselves are great; I found they have superb detailed high frequencies, great bass impact and slightly forward mids.
One particular driver family which I have really gotten to know better with the GOTL is the 12AU7- I was previously unimpressed with these guys and found many of them unlistenable due to unpleasant treble distortion in the past, but the GOTL has largely cured this problem, and I'm having a great time rolling these tubes again. I found myself leaning towards the British Mullard ECC82 offerings to add some warmth to the Melz.
Incidentally, I have been in contact with a local hi-fi/head-fi enthusiast (not many of us where I am!), and had the privilege of receiving some incredibly rare, early 12AU7s on loan. In light of this great opportunity, I have spent the last few days doing some comparisons between all the Mullard ECC82 variants I have on hand. In short, I think they almost all sounded distinctly different, and to the extent that differences couldn't be due to batch variation alone. The older variants sounded better to me.
Tube 1: Hivac CV491
One particular 12AU7 has always stood out to me in the past, the Hivac 'smooth welded plate' CV491. The one I have is from 1953 and tests low but was noise free in the GOTL. Combined with the Melz outputs, the tone was incredibly lush and musical. Bass response was textured and expansive with plenty of slam, while retaining a sense of effortlessness. The treble was refined and smooth, without any hint of harshness whilst presenting so many layers of detail. I am a big fan of classical symphonic music, and this combination allowed me to dissect any individual section of the orchestra better than ever, while still retaining a coherent stage. The mids were perhaps the tube's greatest strength; the warm fullness and slight forwardness is extremely euphonic and made for easy listening without sacrificing any technicalities. Overall, one of the best combinations of clarity and detail I have heard! This set the bar for comparison for the other variants.
Tube 2: Mullard CV491 1593 change code
This was my first time hearing another 'early' Mullard 12AU7. This particular one is from 1954, square getter and long welded plates. It matched my expectations that it would sound similar to the Hivac- I couldn't pick any substantial differences, except in the bass, which I found was ever so slightly more extended. However, this was only really evident on a particular test track (E E Power Biggs playing Bach's C minor Toccata and Fugue), which has an organ going down to C0 at 16Hz. It wasn't quite as earth-shattering as a KenRad with graphite 6080, but I find that to be too
bass heavy. In other tracks, I heard no difference in bass quality.
Tube 3: Mullard ECC82 k61 change code
The first k61 sample I have here is from 1956, also with long welded plates and a square getter. I really did not expect to hear a great difference from the 1593, but they were obvious. I found there were less microdetails overall, and the bass was less focused in sound. The mids retained their smoothness and warmth, which made the overall presentation highly enjoyable still. However, it was clear to me that the 1593 gave more to the extremities, which resulted in a better sense of balance overall. This k61 needed more sparkle and treble detail, IMO, having heard the previous offerings.
Tube 4: Mullard ECC82 k61 (O getter) and Tube 5: Mullard ECC82 Gf1 change code
The 4th tube has always interested me, as it marks the exact period of transition between k61 and the Gf1 change codes. It has the characteristic Gf1 shorter plates, halo getter, and 4 digit date code dating it to 1960, but retains the old k61 change code. It also has a 'fattened' area at the base of the glass, while all other 12AU7s I have are straight at the base. Despite this difference in the glass, the internal construction and sound is identical to the Blackburn Gf1 ECC82. These are the most common Mullard Blackburn 12AU7s, and they again retain the highly musical mids of the earlier Mullards. However, I heard a big difference in the treble, which sometimes sounded harsh to me, and overall could not be described as smooth. The bass was still well extended, but not as impactful. For me, the presentation shifted to be more upper-mid to treble focused, but the result overall was not as airy, technically proficient, or sweet sounding as the earlier Mullards. I tried several Gf1 and Gf2 coded tubes between 1960-1970 production, and they all sounded the same. I picked out a uniquely letteredone for the photo; does anyone know what the J J represents? I recall reading it was a government code of some sort, but cannot info on it now!
Tube 6: Mullard 12AU7 Made in Australia
The last tube is a Mullard made in Australia, 1972. The only difference in construction from the British ones was a shorter top getter stem. I found it sounded very similar to the British Gf1 tubes, but with much more rolled off treble. The bass was also a little less present, but the characteristic mids were still not lost! I preferred this tube for treble-heavy music, and for mid-centric music with vocals. The Gf1 tube was still better for orchestral music overall with slightly better balance and bass support.
Overall, the early Mullards have become one of my favourite inputs, up with the best 6SN7s. Their scarcity and price are really inhibitory though, unfortunately. I will be looking into the dual 6C4 path next as an alternative, which should yield very good results if they are anything like the 6J5-6SN7 relationship!
I have been entrusted with selling tube 2 and 3 'for the right price'. Do PM me if you are interested! I am also on the lookout for the early 6C4 tubes; MOV L77 and early Mullard/Hivac CV133/EC90- perhaps mentioning them on this thread will cause more to pop up for sale
I also wouldn't mind a pair of reasonably priced Melz 6H12C to complete the sextet!