The Big 12ax7 Tube Test

By Bradley Simpson

tubesI've just spent about the last 8 hours drinking coffee, swapping tubes, playing guitar, and generally trying to deafen myself with a range of tones from ear-meltingly awesome to teeth-grindingly abysmal. I'm passing on my observations to you so you can get the same results minus the hearing damage. Here's a list of the tubes I'll be talking about:

Mullard (NOS)
Telefunken (NOS)
GE 5751 (NOS)
Tung Sol (NOS)
Phillips 12ax7A (NOS)
Groove Tubes 5751 (New)
JJ ECC83S (New)

First things first - some info on how I went about testing the tubes:

I'm running a stock late 2007 Vox AC30CC2, which has the Wharfedale speakers, both of which have been fully broken in. This amp is the second update to the new CC series, so it's got the non-soldered speaker cables, fixed tremolo and effects loop, and is generally more reliable. This is a very bright amp, so please take this into consideration when reading my impressions of each of the tubes. The amp has a 'Normal' channel which is based on the pre-top boost AC30s and is very low gain, and also a 'Top Boost' channel wich is built upon the more common Top Boost AC30 circuit. This channel has a lot more treble and also more gain than the Normal channel.

More info here for anyone that's not familiar with all the controls:

In every case, the amp had the output bias set to 'Hot' and the smoothing set to '44 uf Modern.' Guitar-wise, I'm using my Special Edition Epiphone Firebird V reissue, as this gives a good clean chime, but can also push the amp into overdrive moreso than a strat or tele. For testing clean tones, I was going into the 'Normal' channel, with the master set to '6' and the channel volume set to '4.' Brightness switch was in the on position, and Tone Cut was set to '0.' This channel has no EQ controls. For overdriven tones, I was going into the Top Boost channel, with the Master at '10' and the channel volume set to '8.' Tone Cut was again set to '0' whilst the EQ was set to 'custom' with Bass at '4' and Treble at '6.' In every case, the tube being tested was placed into V1, which is the tone control for both the Normal and Top Boost channels. V2 and V3 were occupied by JJ ECC83S tubes, whilst the poweramp was fitted with four matched JJ EL84, with a JJ GZ34 rectifier.

Ok, that's enough of the background nonsense, let's get stuck into the meat and potatoes. In no particular order:


NOS Mullard 12ax7:

The 'holy grail' of NOS preamp tubes. I'd never heard one of these before, so I was interested to see what all the fuss was about, and I wasn't disappointed. This tube gave a very warm and open clean tone, which was also very harmonically rich, with a lot of very nice 'chimey' overtones. Plenty of headroom to be had here. There was no trace of the "ice-pick" tone that can occur when you hit the strings hard. The bass was very tight and clear, and did not become muddy as I started pushing the Normal channel toward overdrive. Switching over to the Top Boost channel, I found that the overdrive tone was very transparent, but also quite thin, and with little in the way of singing musical feedback. Also, the tube proved to be quite noisy, with a noticeable hum and hiss coming from the amp at idle.

Overall, I'd say this is an excellent tube for clean tones, but the overdrive left me feeling a bit flat.


NOS Telefunken 12ax7:

Compared to the Mullard, this tube had a noticeably less open sound to it. Balancing this was a slightly more harmonically rich tone, with a lot of chimey Vox clean, although it quickly gets into ice-pick territory when you start to attack the strings (more so than any of the other tubes I tried). The bass was again very tight, and didn't muddy up as I pushed the amp toward overdrive, although there was noticeably less string definition across the board. Clean headroom was about average compared to other tubes. The overdrive tone was still quite thin, although not as bad as the Mullard, and the harmonic richness proved to be a bit too much. Whilst the OD had a nice singin tone to it for single-note runs, it proved to be a bit too complex for chord work, which made anything more than powerchords sound quite messy. Overall, this was a very quiet tube, with only a little audible hum.

I wouldn't complain if someone gave me a few of these, but I wouldn't go out of my way to get a hold of them. Definitely better than the new production tubes, but toward the bottom end of the NOS spectrum.


NOS GE 5751:

Not a good tube in the AC30. This had a very open tone, which I liked, but was very harmonically bland. No chimey cleans to be had here, and there was a lot of ice-pick action. String definition was poor, and the bass is best described as 'flubby.' The overdrive tone was very aggressive and hard-edged, but very sterile, with not much musical feedback. On the upside, it's a very quiet tube.

If I never have to listen to one of these in my amp again, it will be too soon.


NOS Tung Sol:

I've previously used some new production Tung Sols in this amp, and quite liked them, so I was expecting good things from this tube. Unfortunately, I was again a bit disappointed. The tone was again very open, with nice harmonically-rich cleans and a lot of warmth, but the tone itself was very dark, with little chime and a flubby low end. There wasn't a lot of headroom to be had either. The overdrive tone was quite muddy, which wasn't surprising given how dark the clean sound was. Single-note tone was sweet and singing, but the musical feedback was very inconsistent. Some notes would sing out into feedback, whilst others seemed to be badly 'choked' by the tube to the point that I could hear the note cutting away prematurely. This tube was a little microphonic though, which may explain the strange behaviour.

This tube was definitely better than the GE 5751, although not by much. It is simply too dark and muddy, even in the AC30.


NOS RCA 12ax7:

I can't stress this highly enough. NOS RCA ARE THE BEST SOUNDING 12AX7 TUBES EVER PRODUCED! I'm going to buy up enough of these tubes to keep my AC30 running at least until the Large Hadron Collider sucks us all into eternity, possibly even longer.

The clean tone from this tube was head and shoulders above even the Mullard. Harmonically rich, with more chime than the Mullard, but no noticeable ice-pick on the attack, even when I started pushing the amp hard. The cleans were also much warmer and more rounded than the Mullard, but without sacrificing any of the beautiful Vox chime. These were also the most dynamically responsive of any of the tubes. With the Normal channel pushed hard, I could ease up on the strings and get a totally clean sound, or dig in and push the amp way out into a solid, warm crunch. The low end was very well defined, with excellent string definition, and managed to be noticeably looser and more open than any of the other tubes, but with no discernable flubbiness. Headroom was good, although not quite as high as the Mullards. The overdrive tone was thick and warm, with excellent harmonic richness, and a beautiful singing feedback that was surprisingly easy to control. Chords retained their!
character and string definition, even when pushing the amp into a hard overdrive, and there was no muddiness to be had anywhere on the neck. This was also the quietest of any of the tubes, NOS or new production.

I seriously cannot recommend these tubes highly enough. I doubt I will ever use anything else in my amp.


NOS Phillips 12ax7A:

First things first. This tube suffered from the most terrible tooth-grinding tube rattle I can imagine. This could be a one-off for this tube, but don't say I didn't warn you. What about the sound itself? The cleans were nice and harmonically rich, and had a nice chime to them, and was second only to the RCA and Mullard tubes in string definition, although the bass did get a bit muddy when pushed hard. The tone was also less open than the RCA or Mullard tubes, but still better than any of the other tubes. Also, the highs came across as being a bit less warm and more brittle than some of the other contenders, but this could be tamed a bit by rolliing some of the treble off. Headroom was about average. The overdrive tone was wonderfully thick and warm, but was very lacking in sustain and musical feedback, to the point that the notes felt noticeably 'choked' compared to many of the other tubes. Overall, this tube was among the quietest that I tried.

Assuming the tube rattle was just an unfortunate fluke for this tube, I'd put this in solid third place behind the RCA and Mullard offerings. However, the gap in tone quality between the Mullard and this tube was quite large.


New Production JJ ECC83S:

These are the standard short-plate ECC83 that eurotubes will provide you with for all of about $8. Not bad tubes by any stretch of the imagination. A bit better than the NOS GE 5751, but not in the same realm as even the NOS Tung Sols. Clean tones with this tube had very little harmonics, and practically no discernable chime. Top end was brittle, even with the treble rolled back, and the overall sound was very closed and two-dimensional. The low end was tight and well defined, with no flubbiness that I could hear, even when pushing hard. Headroom was about average. The overdrive was very thin and buzzy, with practically no sustain or musical feedback. Chord work with overdrive sounded a very buzzy and weak (think along the lines of a tubescreamer with the tone wound out too far). Idle noise was low, but not as quiet as the NOS RCA.

For the price I guess you can't complain, but these don't hold a candle to the NOS tubes.


New Production Groove Tubes 5751:

Another rattler. The tube wasn't microphonic, and has had very little use, so I can only assume the design doesn't agree with the combo design.

The standout feature of this tube was the impressive headroom. The counterpoint being that you're going to sacrifice a significant amount of overdrive when you wind the volume out. The clean tones were nice and warm, with a tight and focused low end and a nice open tone, although there was less chime and harmonic richness than the JJ ECC83S. String definition was quite poor across the neck, and there was not much in the way of dynamic response. Overdrive was similar to the NOS GE 5751, in that is was very hard-edged, with very little warmth or harmonic detail. Sustain and feedback was poor.

If I had to choose one word to describe this tube it would be 'sterile.' I could see that this might fit quite nicely in a high-gain setup, with the hard-edged OD, but if you're looking for a warm and round tone, you won't find it here.


I'm not a fan of objectively ranking tone, but if I had to list these tubes in order it would look something like this:

1. RCA
2. Mullard
3. Phillips 12ax7A
4. Telefunken
5. Tung Sol
6. JJ ECC83S
7. Groove Tubes 5751
8. GE 5751

That's all of the tubes I've tested so far. If I get around to testing any more I'll add them in here as well. Hopefully this is useful to a few people. It's always hard to describe differences in tone, so I've tried to keep to the basic descriptions that most people seem to use.

Thanks for reading!