12AX7, 12AX7A, 12AX7B, whats the difference?

[Jim McShane]

The JJ is MUCH more rugged, both electrically and physically,

Where did you get that information? On what do you base that statement?

Keep in mind the LPS can't handle cathode follower circuits, as mentioned earlier, unless you raise the heater voltage reference by a substantial amount.

That's misinformation. There was a problem with the earliest LPS tubes used in cathode followers, but I haven't had any reports of that in maybe 5 years or more.

The current LPS is a better tube than when it was first made.

 

[Jim McShane]

I believe that there was an issue with heater cathode voltage tolerance on LPSs. Be good to research this on applications such as CF where this might be an issue.

mike

There was an issue at one time. But that was the early production tubes from a long time ago, late 1999 or maybe 2000. I'm not aware of any issues currently.

 

[rooster18]

Where did you get that information? On what do you base that statement?



That's misinformation. There was a problem with the earliest LPS tubes used in cathode followers, but I haven't had any reports of that in maybe 5 years or more.

The current LPS is a better tube than when it was first made.

As far as which is more rugged, put them both in a tube guitar amp combo box, run it up to about 15W with a great deal of gain (think '80's - 90's metal, I won't even mention the newer stuff), and see which one squeals. Clean up the signal, plug in a Strat and see which one "pings" more on open E chords. I build guitar amps, which are much more abusive to preamp tubes than any hifi will be, simply because they run into great amounts of saturation and cutoff with higher voltages than you're going to see in a hifi.

That's from where I base that statement.

As far as LPS being better than when it was first made, that may be so, but a cathode follower putting 175-200V on the cathode will cook an LPS in no time. A JJ runs and runs, at least the 100-200 I've used do. Not misinformation, I don't get my info secondhand. I build amps, put them on stage, and cook the living crap out of them the way a good tube amp should be cooked. No politeness in my gear, I'm all about raw and rude.

rooster.

 

[resound]

As far as I know, from many years of experience without pouring over pages of numbers on the 12ax7 and its variants, Any 12AX7 variant will replace another. Maybe with different results, but they do the same job. Some may have been reported to act weird or not work but those would be bad tubes for one reason or another.
For absolutely sure, when talking vintage tubes, a 12AX7A is more powerful (higher gain) than a 12AX7.

EIA designation codes on tubes were sometimes confusing back in the day but they all mean something specific. Today's tube post # designations dont mean the same as they originally did. Today it seems its more of a company to company thing designating origin or power etc.

The "W" designation on older tubes refers to a ruggedized version. Not all W's are Military but all Military are W's even if their designation otherwise trumps the ruggedized designation of W. A JAN12AX7A is a ruggedized version but the prefix means military contract so ruggedized is a given.

Also, on some "W" tubes with bakelite or phenolic bases it can mean Low loss base. Those are tubes with brown bases (high fiber content to control heat) This is a ruggedized feature but not all W's have low Loss Bases either.

Get together a handful of tubes and start rolling.
12AX7 any designation
12AU7 any designation
12AY7 any designation
12AT7 any designation
Maybe 12AZ7......:scratch2:
5751
7025

........and a bunch of others. go to this tube search and enter 12AX7A. http://tdsl.duncanamps.com/tubesearch.php It will list a bunch of replacement #'s which will all work in the same spot but with different results.
The manufacturer recommends a tube for the application to maintain their designed sound parameters but there may be others that will swap right in and work great with just a bit of aural difference.

Thats part of the fun for me.

 

[rooster18]

Well, a few things here. While I'm not a hifi expert, I DO know 12AX7's very well, as I run them outside their rated parameters on a regular basis. This is a good way to find out whose tubes are more durable/rugged.

As far as the 12AX7A vs. a 12AX7 on the gain factor, the main difference was the spiral filament; the 12AX7A was much quieter. After the 50's or so, ALL 12AX7's were built to "A" specs, so it became moot. As far as any 12AX7 subbing for another, the Cathode Follower circuit is very brutal on the tubes. A 12AX7LPS has a 100V heater-to-cathode limit. They simply don't stand up to that circuit, which runs the cathode at the voltage of the plate of the stage preceding it. Also, the Electro-Harmonix tubes labelled "E-H" don't make the trip, either -- we burned out an entire sleeve of them in a Soldano SLO100.

Other than that, they're pretty much interchangeable, the difference being tone.

rooster.

 

[Jim McShane]

As far as which is more rugged, put them both in a tube guitar amp combo box, run it up to about 15W with a great deal of gain (think '80's - 90's metal, I won't even mention the newer stuff), and see which one squeals.

That has nothing to do with ruggedness. Tubes with large plates are more prone to microphonics due to the size of the plates. There are a few instrument amps I'll use an LPS in, but for most (especially combo amps) long/large plate tubes are a no-no.

As far as LPS being better than when it was first made, that may be so, but a cathode follower putting 175-200V on the cathode will cook an LPS in no time.

That is way beyond the spec for a 12AX7, the spec is 100 volts. RCAs can go up in smoke at 200 volts h-k.

I build guitar amps, which are much more abusive to preamp tubes than any hifi will be, simply because they run into great amounts of saturation and cutoff with higher voltages than you're going to see in a hifi.

That's from where I base that statement.

Okay, fine. I was referring to use in audio amps. and while guitar stuff may use higher voltages, they do tend to run lower currents.

JJ runs and runs, at least the 100-200 I've used do. Not misinformation, I don't get my info secondhand. I build amps, put them on stage, and cook the living crap out of them the way a good tube amp should be cooked. No politeness in my gear, I'm all about raw and rude.

Whatever. You aren't the only person who has some first hand knowledge of what goes on in a guitar amp. And your opinion of JJ stuff is not widely held, but I'm glad you like them.

Your opinions regarding the LPS are not valid in audio gear though. It's a fine tube for audio use.

 

[rooster18]

That has nothing to do with ruggedness. Tubes with large plates are more prone to microphonics due to the size of the plates. There are a few instrument amps I'll use an LPS in, but for most (especially combo amps) long/large plate tubes are a no-no.



That is way beyond the spec for a 12AX7, the spec is 100 volts. RCAs can go up in smoke at 200 volts h-k.



Okay, fine. I was referring to use in audio amps. and while guitar stuff may use higher voltages, they do tend to run lower currents.



Whatever. You aren't the only person who has some first hand knowledge of what goes on in a guitar amp. And your opinion of JJ stuff is not widely held, but I'm glad you like them.

Your opinions regarding the LPS are not valid in audio gear though. It's a fine tube for audio use.

I agree with you wholeheartedly on the first comment. I can't put an Ei in the first stage of anything but a lower-gain type amp, like a Fender-style circuit, without it ringing, and combos are kinda out-of-bounds. However, for some reason, the ECC803S seems to be able to take the vibration very well. I can't explain it, even JJ says not to do this.

As to the lower currents, you're may be right there, although most guitar amps actually use lower current than one would think on the preamp tubes; they tend to get biased colder rather than warmer to create the asymmetrical distortion artifacts.

I realize JJ is not a highly appreciated name in new tubes. In fact, I had a KT77 go out on me most spectacularly in my Dynaco. However, in the hundreds of JJ tubes I've used, I've only had 1 preamp tube failure. Their power tubes (especially nowadays) may be another matter, but I've had great luck with E34L's, EL34's, and their 6V6S's.

The spec may be 100V, but guitar amps regularly run the cathode voltage at 130-170V, and Soldano pushes the 200V level in places.

The LPS is indeed fine for audio gear. I just would keep it away from a CF circuit that ran the cathode at more than 100V. Oh, and as far as great sound goes, the LPS, to my ear, can sound as good as just about anything. I just can't use them as tube #2 in a Marshall, or most of the tubes in high-gain amps (which tend to use several CF circuits on the distortion stages).

Thanks for your input, though. When it comes to hifi amps, I can tinker with them, but I don't have the intimate knowledge that you (and MANY others) here have. Wish I did, but I need to get a few more years of hifi under my belt. Don't know if it'll happen, as I'm backed up for a couple of years on guitar amp builds.

Once again, thanks for the input. If it weren't for you guys, I doubt I'd be listening to my ST70 and PAS setup, nor my Thorens TT. You guys made all the refurbing possible!

rooster.

 

[Jim McShane]

Hey Rooster,

Thanks for a great exchange - that ends on a positive note too!

I don't get the chance to work with guitar amps all that much anymore - they've just been crowded out by audio stuff. I appreciate you sharing your experiences with us. Very informative and helpful, thanks again! :thmbsp:

 

[rooster18]

These exchanges are what lets me learn. A good example would be my cartridge. I run a Stanton 681EEE, which is not as popular around here as many others in the same price range, but I looked at the postings of the Stanton fans, and found them to be quirky, offbeat, and somewhat separate from most people as to their tastes and listening habits. So I picked one up, and it sounds great to me.

On the 12AX7 front, has Winged-C ever ironed out the problems with theirs?

rooster.

 

[tshoejohn]

Apparently the difference between a 12AX7 and a 12AX7A is their suitability for use in circuits with the heaters connected in series.

Here is a pretty good list of 12AX7*** and similar tubes with descriptions of each.
(No affiliation to SND TUBES)

http://www.vacuumtubes.com/12ax7.html