Tube Comparisons on VTL 7.5

I just did a bunch of comparisons on my VTL 7.5 preamp. A long process. These are all based on my listening opinions in my system. Some of you will benefit from this as it is an apples to oranges comparison. I know some of you are waiting for this.

First Straight to the results of my peference;
1) Telefunken 12AU7s - great mids.
2) Svetlana (from VTL) 12AX7s - live sound, very good mids
3) Siemens 12AU7s - Not my cup of tea, but good overall
4) EH 12 AU7s - Not very good at all

I did the comparisons for a few reasons. First the gain on the 12AX7 is very high under some circumstances (depends on the recording). Second, my preamp originally came (second hand) with the EH 12AU7s, so I purchased new 12AX7s. Then I felt the gain was slightly too much on some recordings. I then purchased the Telefunkens & Siemens to find out & learn more.

I matched volumes w. a test tone & SPL meter on all tubes first & kept the volume control at set volumes for listening so everything was mostly equal. There was no talking or sounds between demos to keep my auditory memory as intact as possible. I used an SACD sampler of Rock, jazz, & classical. I also switched back & forth multiple times on each track for each tube until I was more or less sure beyond a question.

The Telefunken just has amazing midrange. It is missing the last bit of treble energy & the very lowest bit of the bottom octave. It is very smooth sounding & you can listen all day. You really don't know you are missing anything until you listen to the Svetlana 12AX7s.

Now the 12AX7 has great midrange as well but it isn't as smooth or musical as the Telefunkens. It does have great dynamics and gives the VTL 7.5 a different personality. You don't get the sence of space or ambience as you do w. the telefunken. You do get definitely more solid bass which you feel in your stomach. The treble is alive & the mids are musical as well.

The Siemens has more treble energy than the Telefunken, but the midrange did't have that musical flow of the Telefunken. The Siemens also didn't have the dynamic flow of the 12AX7s.

Lastly the EH 12AU7. It just makes the VTL 7.5 sound lifeless or soft. It does have decent extension at the top end. The bass isn't up to the 12AX7. But most of all, it sounded lost in trying to make the music flow.

My final conclusion is that I need to figure out if a Telefunken 12AX7 will do it all. But honestly doing this comparison is a pain in the....

Anyone have any ideas of what is next I am open for input.
Correct me if I am wrong but I don't think you are supposed to deviate from stock supplied tube types (for instance substituting a 12 ax7 for a 12 au7, as they have different gains. Different brands of 12 au7s will give different sonic signatures. 12au7's have the lowest gain and lowest intrinsic distortion (compared to 12ax7s and 12at7s. To my ears, the Mullards are soft,warm, and lush sounding. Telefunkens are very neutral sounding, and detailed. The RCA's, and their premium variants, 5814s, and 6189's are kind of "in-between". The regular version Amperex tubes seem to me to have more "air" and top end detail than the others, and the premium Amperex 7316 versions are highly sought after (I have not heard them). My preference (IMHO) is for the Telefunkens, but your ears will tell you what you prefer,as, in your application, results may vary.
I too was unaware you could simply swap 12AU7's and 12AX7's but I have not tried this in my Aesthetix Callisto and Io components....I stay with 12AX7's.

Over the last few months, I have tried many 12AX7s from Telefunken, Mullard and Amperex over the stock Sovtek. I really like the Tele's also due to their awesome midrange. But the Mullard's seem to be a little more dynamic. I like the Teles in the Io and the Mullards in the Callisto. My favorite 6922 tube so far is the Mullard over the Amperex and Siemens as the Mullard has a very natural and smooth tonality but also quite lively; I have not yet heard a Tele 6922.
VTL (Luke Manely) recommends using 12AU7s if you are experiencing too much hiss. I doubt all preamps can accomodate the switch but the 7.5 sounds fine. I like having more steps in my volume control.

I am thinking of trying the Mullard. I have heard very good things about it. Maybe the slight extra dynamics will be what I am looking for.

I still need to try a NOS tube in the 12AX7 spot.
I did a comparison of Sylvania 3 mica 5751s, GE 3 mica black plate 5751s, and Mullard 12AX7s. I liked them all. The Sylvania have great mids but didn't have the bloom. The GE were also great with great extension & speed but no enough bloom. My 1956 Mullards were just right. It is difficult not to love this preamp w. these tubes in the circuit. Big beautiful images w. great bass and everything just sounds great
I did some experimentations in my 5.5 and came up with similar results as DGAD saw between the Svetlanas and telefunkins when i used NOS Mullard 12ax7 and the Svetlanas. The mullards gave up the details in the sence of room space and in the top end, especially cymbols, that the Svetlanas couldn't touch. The Svetlanas on the other hand gave the system more energy...toe tapping groove if you will, and more bass.

The Mullards also seem to have less gain or maybe it's just that the sound is smooothed out reletive to the svetlanas, maybe both, because i find myself turning the preamp up significantly lounder with the Mullards before i percieve things to be loud. Trying to talk to my wife with the system going suggested that the mullards may get louder than they seem.

I'd like to try the groove tube reissue mullard AX7 to see how they do relative to the NOS. $30 or $40 each for a balanced tube would be a lot better than the $100 ea for the NOS and they probably wont't go away.

the whole 12a family is pretty interchangeable techically speaking. see the referenced site if you want more info. Also look at the Phase inverter discussion to see the variation found in tubes. In the end tube rolling can be as crazy as chasing the perfect cable.

After you get all this info in your head it all boils down to what sounds good.

I was trying a 12at7 i stole from my 5.5 as a phase inverter in my guitar amp in place of an ax7 to drop the gain and increase the current some to improve dynamics. my amp hated the added current from the at and buzzed like hell. I'm planning on trying some NOS GE 5157 which have a similar gain but different current. I also swapped a couple of svetlana ax7s, that i also stole from my 5.5, a GE12ax7 and a couple of sovtec ax7wts i had in the various preamp positions of my guitar amp. Each swap changed the nature of the amp pretty dramatically.