Differences between standard 12AX7, 12AX7A and 7025 tubes?

Hello to all forum members. As the title says is there really any physical difference between 12AX7, 12AX7A and 7025 tubes?
I know that a 12AX7 tube is the original design, a 12AX7A tube is a lower noise version of the original 17AX7 and so is a 7025 tube, but are they really made differently? Or maybe they are just standard 12AX7 tubes that after inspection if they get low noise ratings they selected and badged under the 12AX7A or 7025 designation?
I have tubes of these three kinds from several brands and I must say that a pair of Hitachi 7025's from the 70's I have are the quietests and with the lowest hum tubes I've ever heard, and their sound is outstanding. I bought them NOS a couple of months ago for a bargain price, around 40 $ the pair including shipping to Spain.
I'm very interested in your opinions.

 

The difference between a 12ax7 and a 12ax7a has to do with the way the heaters are wired. The 12ax7 can only be used in circuits where the heater circuit is wired in parallel, the 12ax7a can be used in series or parallel heater circuits. In modern gear you're unlikely to run into series heater circuits where your non-a 12ax7s won't work. I think the 7025 designation was for 12ax7 tubes made with spiral filaments which could reject hum better in circuits that use AC heater current. There are spiral filament 12ax7s made today -- like the New Sensor-made Sovtek 12ax7LPS -- not labelled 7025.

 

Thanks much for your reply, very informative, I appreciate it.

 

Having a look at my "tubes drawer" I realized I have an unused pair of Sovtek 12AX7LPS, I'll give them a try and check how these sound.
This is the pair of Hitachi 7025 I have, they sound great, smooth sounding, very linear, excellent gain and dead quiet, they are the quietest 12AX-type tube I've ever heard.

 

I've tried the pair of Sovtek 12AX7LPS I have, hum is low but they are quite noisy, noisier than any other 12AX7-type tube I have, and I have quite a few. Maybe I got a bad pair or they're simply not good and their quality control is nonexistent?

 

Quality control on current production tubes is very poor unfortunately. But some of these tubes still sound great, even despite this.

 

On my "tube drawer" I have a pair of matched (boxed) Psvane 12AX7 MkII and a pair of golden pins 12AX7 Gold Lions (new production). I've tried both and I'm quite surprised with the Psvanes, their very quiet and 'though I only let them warm up for 5 minutes (and they have around 100 hours of use on my previous Phono preamp, or less) their sound is quite good. The Gold Lions sounded well also, they too have around 100 hours of use or less, I let them warm up for another 5 minutes. They are not as quiet as the Psvanes, but they both sound fine. The problem I see with both Psvanes and Gold Lions is that if one knows where and who to buy NOS tubes and not on a rush, one can get much better tubes than the ones like Psvane, Gold Lion or Electro Harmonix, to name a few, for less money. I paid 30 $ for my pair of Hitachi 7025 including shipping to Spain, or 75 $ for 4 Hit Ray (joint venture of Hitachi and Raytheon in the 60's) 12AX7A and they both TROUNCE the Psvane and Gold Lion sound wise and they sure they will last longer than the new production ones. That's why I have a "tube drawer", I'm not going back to solid state Phono preamp, I love the combination of analog records and tubes, so I try to get all the good and affordable tubes I can for the future with my eternal flat broke economy. I'm quite happy with all my NOS scores so far, both price and sound wise.

 

I agree that the quality control is not as good as vintage tubes from the golden era of tube equipment. That being said, some of the newer stuff, from JJ, Sovtek (and all the various names they have purchased ie Mullard, Tung Sol etc), Sino and others can have some good sounding tubes, getting matched sound from more than 1 tube is a bit harder. The vintage tubes, if you find real ones that are testing good, tend to be better, some even can get you to audio nirvana!

 

I haven't use a 12AX7 LPS in a while but I always thought they were good sounding tubes. In fact I remember one day tracking a guitar solo with a little Vibro Champ and listening to the great creamy tone on the playback and thinking it was great that I had an old 60s Mullard in the preamp slot only later to discover that I actually had an LPS in there. In hifi I thought they were nice, balanced and, yes, quiet.

 

I have a couple of matched pairs of NOS Sylvania 7025's, and a new production "special" 7025 from Shuguang, IIRC. I bought them specifically for their "low noise" description, after the original 12AX7's went bad in my phono. Prior to buying all these 7025's, though, I'd bought and popped in a matched pair of Genalex Gold Lion 12AX7's, and they've been doing so well that I've yet to try out the 7025's. When I do - I will share my impressions.

 

Please do, I'm interested in how these Shuguang 7025 sound.
I have a pair of new (under 100 hours of use or less) Psvane 12AX7 MkII and I tested them today for the first time on the EAR 438 "clone" I own, their dead silent, almost no hum (my Phono preamp is not grounded, I don't have any grounded wall socket in that room) and an excellent signal to noise ratio. I only left them warm for about 5 minutes as I was busy doing other stuff, but they sounded good to my ears. But I'm not changing them for the RCA 5751 I'm currently using.
Maybe these Psvane 12AX7 MkII that came matched in a box are exactely the same as the Shuguang 7025 you are talking about, they sure come from the same maker and factory.

 

BTW, I think it's worth mentioning, I don't think there's any specification of a 7025 that's different from a 12ax7. They're called lower hum and lower noise tubes, but if you look at the old spec sheets, there's no specific design element or electrical characteristic a 7025 is speced for that a 12ax7 isn't -- even the spiral filament thing, I don't know that every 7025 had spiral filaments -- never mind any potential other noise or microphony lowering construction elements (like shorter plates or more or thicker spacers or something). And the spiral filament thing may or may not give less hum depending on the application.

These days, with current tube production and QC being so variable, I wouldn't automatically assume there's any difference in construction or performance between a tube labelled 12ax7 and one labelled 7025. And with old tubes, at this point, I wouldn't assume that any one 60 year old 7025 is going to be lower noise than any one 60 year old 12ax7.

 

Quoting from my RCA receiving tube manual (RC-28), it says under the 7025 description:
"This type is identical to type 12AX7A except that it has a controlled equivalent noise and hum characteristic."

I interpret this to mean that these most-likely were originally just factory-selected 12AX7As which happened to meet or exceed certain noise specifications required for this particular designation.

Whether this particular noise specification is being genuinely honored with any of todays tubes is highly doubtful though. And I personally suspect that it is presently represents little more than a way of creating a marketing distinctions for different models of 12AX7s which happen to be sold underneath the same brand name.

 

I think there's a definite correlation between the gain characteristics of specific 12AX7 tubes and the noise...i.e. the tubes with more gain tend to have a bit more noise in them.

Higher gain = tolerable in guitar amp; not as tolerable in stereo amp.