Tubes Asylum

Well here's the other half of my presonal journey into the world of 12AX7s. Again all the usual caveats apply, these obseravtions are based on the sound of these tubes in my rig, in my room within the context of my taste. While I think these comments are somewhat generalizable the fact remains that given the variability of gear out there every tube has an ideal match out there somewhere so if I dont get excited about it that doesnt mean its a lousy tube - it just doesnt work as well in my rig as some of the others. Given that here we go.

Well, I tend to break the 5751 family of tubes into two groups in terms of sonic quality and capability. This is a bit arbitrary but it reflects my own preferences and experiences and the fact that I do find significant differences in sound quality among these beasts. The first group are the tubes that I find a bit less exceptional for various reasons - note that I'm not calling them bad tubes. In fact I find any of these superior to all 12AX7s (except perhaps for the Amperex) so they are in fact a great place to start your 5751 odessey. There are other reasons you may want to start with these tubes as well, but more on that later. In this first group I would include the following:

GE grey plate
Sylvania grey plate
RCA black plate (common??? version)
RCA black plate (silver clip??? version)
Siemens grey plate

Now a few general comments regarding these guys before I get a bit more specific. All of these tubes have a more focused and transparent sound than 12AX7s with less fuzz & grain and a greater sense of palpability (sorry about that, but its the only word that fits for me, Jack G would say organic here, I think) which is particularly noticable on vocals. When it comes to the way they present individual performers what you will hear from these guys a is a focused dimensional object rather than a flat cardboard cut out or fuzzy undefined image. This allied with the lack of the typical 12AX7 fuzz results in a more clear, open sound with good transparency and detail. I also hear a greater sense of layering in the soundstage with these guys than with 12AX7s. 5751s also tend to be warmer & smoother than some of the 12AX7s, in fact a few of these guys sound downright sweet in comparison. These traits are endemic to the type in my experience, but there are still significant differences tube to tube. Now, taken individually I hear something like this:

GE grey plate - 3 mica spacer version: A bit less refined than other 5751s, this is the entry level tube of this type. If you've ever heard the linestage of a CJ preamp made in the 1980s you've heard this tube since thats what CJ used in the line section of all their preamps for years. This tube can sound a little brash at times and is a bit less liquid than some of the others but its still a good tube. Common as water too, huge JAN stocks of this guy abound so he's cheap & readily available - typically $8 to $12 a tube. If you are messing with 5751s its worth grabbing a pair of these...

Sylvania grey plate - 3 mica spacer version: A bit more refined and grainless than the GE. Actually a pretty nice tube that was used in series the RCA black plate in the Joule Elektra LA 100 & 200 series preamps up through the mk 2 units. A little bit forward in balance in my experience as are all of the Sylvanias in this type (oh yeah, the best in this brand & family is yet to come! patience, patience...) Another JAN tube thats common & cheap. Once again if dabbling in 5751s, one I would put on my short list and definetly nab a pair of...

RCA black plate (common??? version) - 3 mica spacer version: OK controversy time. Common wisdom among 5751'o'philes is that this tube is the cats meow, the one to own, the audiophile's dream, the...uh, you get the point. But the odd thing is I have found multiple versions of this guy that vary rather dramatically in sound quality. I am using my own nomenclature here calling this the common version - I've seen this style more often than the others and is the type Ive bought from 2 national tube delaers. The way to distinguish it from whats to follow is to look at the top and bottom corners of the black plates themselves where they protrude through the upper & lower mica spacers. If all you see are the corners of the plates themselves, or at most small silver rectangiular tabs attached to the tips of the plates, its the type I'm referring to. Sound with this guy is quite good. He's smooth, warm, linear and sweet on top. If your system is bright and brash and needs a warming influence this quy could be the ticket, but still not in the league with the best of this type in my (not in the least bit humble) opinion. Another common tube, but it's audiophile reputation precedes it so it will typically set you back $12 to $18 or so - still cheap as dirt compared to those premium 12AX7s.

RCA black plate (silver clip??? version) - 3 mica spacer version: Talk about another kettle of fish. This tube is as different from the RCA above as any other tube of this type. Irony is I've NEVER seen any tube dealer distinguish between the two. To be fair, as near as I can tell, this guy is quite rare and I have only seen it (and bought it) at two hamfests. While the tube looks very similar to the description above it is distinguished by what I call silver clips. You will see these on the tips of the plates where they pass through the upper and lower mica spacers. That means there are 4 on top of the upper mica spacer and four below the bottom mica spacer, and you can't miss them - they are utterly unique in my experience and appear designed to clamp the entire plate / spacer assembley together (presumably to combat microphonics). These clips are chrome silver, about 1/5 of an inch long and wavy in shape - in short, they are big & obvious enough you can see them at a distance of 3 feet. Now, the sound. These guy is a very transparent and broad band tube, particularly extended on the high end with a very clear and open presentation. It also might just have the best soundstage of any 5751. The only downside? It can be a bit bright & relentless, I would only mate this with a dull, warm system that needs some spicing up. In my rig it sounds dazzling, but ultimately fatiguing. I dont wnow what to say about availability - the clip description is my own, I dont know of any NOS dealer who would know what you were talking about if you asked for it. Oh yeah I've found it in '50s RCA markings and in late 60's RCA new red logo livery, so I cant even comment on vintage. Oh yeah, when I have found it at hamfests, its relatively cheap - $6 to $12 a pop.

Siemens grey plate - 3 mica spacer version: Well I love Siemens 6922s, 7308s and even their 6DJ8s, but this is one tube I found a bit disappointing. Overall it is very linear, neutral almost to a fault and ultimately a bit sterile sounding. In my rig you could almost mistake this for a current manufacture tube. Somehow it just lacks a bit of that NOS magic that makes old tubes so fun - but then again if you have an overly romantic sounding system it might find a useful home in your rig. This tube tends to command premium prices ($20 and up) due to its lineage, but in my not so humble opinion it aint worth it.

Now thats it for the more commonly available tubes in this family. I would however, like to address a few issues that arise relative to these guys as well as a few other brands that are actually nothing more than relables of the above tubes before moving on to the esoterica.

First off - regarding 2 and 3 mica spacer tubes. The 5751 was somewhat unique in that its design originally added a third mica spacer (the round, usually serrated edge horizontal discs above and below the plates) above the upper mica spacer. In fact, every 1950's production tube of this type I've ever found has this upper disc which clamped the getter halo in place, apparently to combat microphonics. These early tubes invariable possess two metal strengthening rods between the plates which also helped lock the whole plate / spacer assembly together. But towards the end of the tube era that all changed. Ive seen RCAs, Sylvanias and GEs from the mid to late '60s and '70s that have only two mica spacers and no strengthening rods - apparently casualties of cost cutting towards the end of the tube era. And it gets worse. Ive measured quite a few 5751s for noise, gain & microphonics and I've always been struck buy how consistent, quiet and low in microphonics they are - the 3 mica spacer versions that is. The 2 spacer versions? Forget it. All over the map, noisy as hell, terrible for microphonics. Now I'm sure there are good 2 spacer tubes out there but the point is that while I would buy earlier 3 spacer tubes unmatched and untested, I wouldnt buy the 2 spacer versions that way - based on what I've measured & heard I just wouldnt trust them.

Now what about other brands? You will often see Motorola 5751s listed by some dealers. Should you buy them? Only if you need a pair of GEs and they cost no more than the GEs do, because thats what they are. Motorola never made them, they were OEM sourced tubes.

The same is true of another, more legendary brand - Amperex. Wow, could you imagine a 5751 made by this legendary company? It would have to be the best of all worlds and they are sold on ebay all the time... Well imagination is all you've got here, along with a common old GE that is. Unfortunately Amperex never made 5751s so we will never know what that tube would have sounded like - instead Amperex sourced them from GE and the sound is the same as a GE labelled tube - oh well, what might have been....Oh, and by the way, you will never see a Mullard, Brimar or Telefunken 5751 because they never made them. This tube was the domain of US manufacturers and the Siemens is the only non US brand I am aware of in this type.

Well, thats it for the more commonly available 5751s. Geez, this post is getting long & I havnt even touched on the second group of generally (but not always) rarer and even more desirable brands & types. I think that will have to be a story for part 3 of the 12AX7 odessey which I will try to post befor the day is out.

Please bear with me...

Joe