QUESTION: 12AX7 Considerations

[Mango Moon]

Hello Everyone,

I'm in need some good technical guidance on the re-tubing of a standalone Mesa preamp. Specifically, I need some feedback on the various types and designs of 12AX7 tubes out there, and their suggested or recommended application in a standalone Mesa Boogie preamp.

Basically, I've got a 1990 Mesa Boogie Studio Preamp that according to the schematic, calls for a total of four (4X) low-noise 12AX7/7025 tubes in the following locations and functions (excluding a lone 12AT7 reverb driver at V5):

FACTORY LOCATIONS/FUNCTIONS OF 12AX7 TUBES

V1A: Input and Tone
V1B: Input and Tone

V2A: Rhythm or Lead
V2B: Reverb Mix/EQ Send

V3A: Lead/Overdrive
V3B: Lead/Overdrive

V4A: Main Outs
V4B: Main Outs

So, my question is this, given all of the various renditions of 12AX7 tubes out there (i.e., NOS vs. new production, short plate vs. long plate, high gain vs. low gain, etc., etc.), what key points do I need to consider here?

For example, I've got all of the following 12AX7 tubes sitting right here in front of me new in the box, and I prefer a clean (no-distortion/no overdrive) sound almost exclusively, but which ones would you use in this particular preamp? Where would you place them? And above all, why?

1. JJ ECC83S (cost-effective, said to be of 'standard' gain)
2. Sovtek 12AX7LPS (said to produce a softer, mellower tone)
3. Tung-Sol ECC803S Gold Pin (said to be less gain but lower noise)
4. Sylvania 12AX7 Long Plates (said to draw more current with nice low end)
5. JAN/GE 12AX7 Long Plates (expected to be same as the above Sylvania's)

Do I keep the gain high at V3/V4 and low at V1/V2? Do I focus the benefits of the long plates on tone, etc. (at V1/V2)? I'm lost! Anybody?

To all who respond, thank you! You've helped make "one small step for man, one giant leap for mankind!"

[Enzo]

Tubes are subjective, I can't tell you one tube is better than another any more than I can tell you lobster is better to order than salmon. Just because I prefer one doesn;t mean you will.

You have the tubes in front of you? Then start at the first socket and stick one in it. Play the amp. Now stick the next type tube in that socket and play it some more. Do this until you have tried them all.

Now the next socket. SInce each socket does a different job, there is no reason to assume we will prefer the same tube there. Go through your collection again in the same way to see what tubes you like where. No need to over-analyze it, in my opiniion.

[Mango Moon]

Enzo,

As always, I appreciate your input and your opinion, but this question isn't about the subjective 'tone' of a given tube to one person's ear or another. Its about the fundamental difference in tube types (or designs) and their intended application.

Clearly, there must differences between the purpose and function of short- versus long-plate 12AX7 tubes, and clearly, there must be differences between low and high gain performance, and where they will best apply. That's what I'm looking for here. One can easily use a 12AT7 in a 12AX7 slot, but that doesn't mean that it was "intended" for that application, right? Somebody (somewhere) figured out that the 12AT7 was best for reverb driver and phase inverter, so that's the rub. Where do I use the different types of 12AX7 tube, "by design"?

[Enzo]

Well, I personally would not use the word clearly here. I do not think they designed the various tubes with V1 or V3 in mind. 12AX7s were used in thousands of different applications, and RCA didn't sell 50 types of them. What they did do was change their designs over time. SOmetimes it was to improve performance, and other times it had nothing to do with performance, but was aimed at improvong the manufaturing process and efficiency.

There were such variants as ruggedized construction for industry and military, there might be some with specially designed heaters that warmed up in a predictable manner for demanding powr supply applications like portable gear. These differences don't always translate into meaningful differences in guitar amps. A controlled warm-up characteristic is of no important to us for example.

When someone comes up with the spiral filament for example, it was to reduce hum. That is desirable in most applications, though it matters more in an input stage than in a phase inverter for example. But when JJ or Sovtek or whoever designs a tube, their wish is that you will fill ALL the sockets in your product with their tubes, not just the one.

Low microphonics is a desirable trait in any tube, and again, it matters more in the input stage than the phase inverter, so you would want to put the low microphonic tubes there. It is tempting to say that short plates are less microphonic tha long plates or some such - I assume that is the sort of declarations you are searching for - but it doesn't pay to over-generalize. It might be that one brand of long plates happens to be more microphonic than another short plate brand (And I am making up the example, I know of no such tendencies), but we don't really know that the length of the plate is the real issue. AFter all, you can find short plate tubes more microphonic than other short plate tubes as well. You may also find that RCA might have seen the ever increasing frequency bands their tubes operated in, and they redesigned some tubes so they worked better in HF applications than the older versions, the tuner in your TV set perhaps. Your guitar amp won't care, but TV tuner designers would, and would then continue to specify RCA tubes.

This is all just my opinion.

[Steve Conner]

Enzo,
Clearly, there must differences between the purpose and function of short- versus long-plate 12AX7 tubes

I'd say your logic is faulty. The type number "12AX7" implies an electrical spec which any tube bearing that number should be expected to comply with. The amount and type of tube innards used to achieve that spec is left to the discretion of the manufacturer. So broadly speaking all 12AX7s should indeed be interchangeable, and ECC83s too for that matter.

There will be variations: for instance the 12AX7 spec allowed for a gain tolerance of 20%. And there are little gotchas like the Sovteks with poor heater-cathode insulation that blow up when used as cathode followers.

And there are things poorly specified: for instance the small differences in plate curves that give the different 12AX7s their different flavours when overdriven.

But for any great difference in purpose and function, you'd expect it to be sold as a new tube with a different type number. (Maybe those Sovteks should be 12AX8s, or something.)

[6267]

It seems to me that most of the differences in tube rolling, especially modern versus old, are due to the fact that much of the modern tooling was designed to russian specs and not western ones. Some of the newer tubes like the reissues might be closer to the western specs, but the cheaper chinese & russian tubes are probably made on the old tooling. I can't for the life of me tell the difference between an RCA and a GE by ear. Tubes themselves vary quite a bit, too, so YMMV.

The other part of "tube rolling" is that when you plug say a 12at7 or 12**7 into a socket wired for a different variant is that the tube may be providing a lower or higher Zout, running at a different current, and/or operating on a different part of its loadline. To some extent, since most preamp stages are cathode-bias, the particular current draw of that tube plays a big role. A larger role, however, is played by the cathode resistor, the anode resistor, the following grid resistor, the coupling caps and the bypass caps (if any). I can tell the difference between a bypassed preamp and an unbypassed preamp a lot better than I can tell the difference between a GE 12ax7 and an RCA 12ax7a.

I'm still somewhat of a tube neophyte, having been born in the 80s, but I started out just tube rolling. Tubes have more than a passive association with hot air! It does make different sounds, sometimes, but you can generally make any given 12ax7 tube have the response you want, given that it's not got physical problems like bad H-K insulation or a bad cathode or something. Check out the grounded cathode tube section of tubecad.com, it's a little bit technical, but if you look at different 12ax7s as being divergences from the published curves, you should be able to adjust resistor values to get the appropriate effect.

[Mango Moon]

Enzo,

Thanks again - some excellent observations and viewpoints. Whether intended or not, you've at least 'begun' to address one of the questions posed here by speculating on something that numerous others have widely observed; that long-plate varieties of 12AX7s not always (but often) prove more microphonic than their short-plate counterparts, and that as a result, they may indeed be better suited for use in phase inversion rather than at the input stage. In the vernacular of the street, that's often called . . . "bingo!" Also, I think you're other observation about vacuum tube manufacturer's "wishing that we would fill ALL the sockets in our product with their tubes, not just the one" is precisely right and this would seem to be borne-out by the fact that most amps (including the two Mesa Boogie units in question here), have the exact same low-noise (7025) 12AX7 tubes in ALL locations. Does this mean that their choice of application was best or that they knew best? That's the part I can't say.

Steve,

My thanks to you as well for your comments, although I'll admit, I'm not quite sure what is "faulty" about the above referenced short-plate vs. long-plate observation with respect to microphonics, etc. It seems that directly or indirectly, we've more or less concurred here with what no less than six other industry professionals have said to me in recent weeks - that there is indeed 'enough' difference between 12AX7 types (e.g., low-gain, high-gain, short-plate, long-plate, NOS, new production, etc.) to enable someone to make an informed decision regarding their selective application and specific location within a product amplifier. I mean, you yourself have already alluded to meaningful differences ala the Sovteks. Thus, interchangeable does not necessarily mean identical or even similar. I have some ECC803S's here that are purportedly way less microphonic and also significantly lower in gain than some standard ECC83's, so it stands to reason (faulty logic or not), that the former might be better suited at the input stage as Enzo postulated here earlier.

[Enzo]

One problem with this sort of analysis is that the variation between tubes of an axact type - Long plate ruggedized by one maker for example - can be as great or even greqter than the differences between two differning types of tubes. In other words you might find a NOS SYlbaina with long plates and a new SOvtek with short that are closer to each other than two seemingly identical SOvtek LPS.

I did mention that when I labelled one type as potentially more microphonic than another, it was a completely made up example. I have no idea if one is more mic-ish than the other. Once we know a tube is more microphonic, only THEN do we decide where in the amp it is best left to work.

The fact that a Mesa amp comes along with the exact same 12AX7s down the row means only one thing - that was the tube they were using that week. It means they are not differentiating between tube applications. This is a production situation, not esoterica. The decision went no deeper than: this week we are installing SOvtek tubes.

MY point about the RCAs of the world wanting you to use all their tubes, not just the phase inverters made my point. If there were specially designed 12AX7s for each stage of an amp, there would be those 10 types of 12AX7 in the RCA catalog.

[zhyla]

Clearly, there must differences between the purpose and function of short- versus long-plate 12AX7 tubes, and clearly, there must be differences between low and high gain performance, and where they will best apply.

I think you're assuming the various manufacturers' designs for a particular tube type differed solely for performance or "tone" reasons. There are other possible reasons for various design changes. Ease of manufacture, cost reduction, reliability, product differentiation, etc - all reasons for an ECO. Changes in sound in a guitar amp are incidental in these cases.

You'd think if RCA or whoever wanted to change some kind of functional quality they would design a new tube type. I've never seen a schematic call for a 12AX7 of a particular brand or model.

[Steve Conner]

These are all good points, but I have one more, and hopefully it's worth $.02 to someone

My point is that the behaviour of the tubes under overdrive conditions was never really part of the spec. 12AX7s were designed as linear amplifiers a long time ago, before the Kinks ever thought of slashing their speakers, and they weren't really intended to be overdriven at all. It's just a happy coincidence that they react well to it.

I guess that nowadays, tube makers do give some thought to this. Maybe the different shapes and sizes of tube guts give different harmonic spectra, or something. But I have no idea what kind of effect there might be, or how to quantify it. I'm looking forward to driving the nuts off a TechTube E813CC http://www.techtubevalves.com/valves/e813cc.php to see what happens...

[Mango Moon]

I'll close this thread out by saying 'thanks' to all who responded here. Its been a great discussion and has gone a long way towards educating me (and perhaps others) on the manufacture, intended application, and use of 12AX7 tubes.

I think the bottom line is, time and design changes may indeed have altered such tubes over the years to a limited extent, but those changes were not necessarily intended to produce specific differences in function or tone, and that's great news! In fact, by in large, that's the answer I was looking for.

From here on in, I will be evaluating single 12AX7 tubes in a given application, on a generic basis, and then replacing the entire suite of such tubes with the brand that proves to sound best, rather than trying to accomplish custom applications for any given channel (i.e., V!, V2, V3, etc.).

Thanks again all!