The famous 6L6 GC Powertube

The forgotten tube

If we analyse the HiFi market today, searching for an amplifier witch is constructed around the 6L6 GC penthode, we have to face the fact that there is near to nothing on offer. In the opinion of many HiFi and High End enthusiasts this tube is made for musicians and their guitar amplifiers – not good enough for serious listening purposes.

This is completely wrong!

Some of the most respected Mcintosh tube amplifiers used the 6L6 GC, the amazing Mc30 monoblocks come to mind or the Mc240 stereo amplifier. The 6L6 GC has one great advantage compared to KT66 or KT88 as also EL 34, you can get new old stock quality without searching for months and spending crazy sums. The supply will vanish in the next couple of years but here and now there is still a great chance to get a matched quad of nice vintage 6L6 GC tubes.

The 6L6 has a very long history till it reached its last incarnation the GC variant, which is the most powerful example of them all. The tube was originally made for military applications and for that reason it is a very robust kind of a tube, the construction had to withstand a lot more when used in applications of the US Air Force, as we would ever could think of in our music reproduction systems. There is a good chance that a vintage set of well matched 6L6 GC (NOS) will work for many, many years of regular use in your power amplifier without the need to be replaced.
Something modern tubes from China or Russia can only dream of.

The Air Tight ATM4:

But all that is worthless if you won´t find a good amplifier to put the fine glass into it. One of the rare examples of exceptional modern built tube amplifiers constructed arround the famous 6L6 GC or one of their equivalent the 5881 will be the Air Tight ATM4 manufactured by the small Japanese company A&M Ltd. The founder of this company Atsushi Miura was born into the audio industry, because his father, who begun to wound transformers for the Japanese audio industry back in the 30ties of the last century, became the head of the Luxman Cooperation, a brand name well respected in the history of HiFi or High End. Under his fathers guidance, Miura made his way through the company and ended up as an audio designer, who was responsible for some of the most famous tube amps this company had to offer. Miura learned the “art of audio” from the ground up, and took over the reins at Luxman. In the 80ties the company was sold to the Alpine cooperation with now biasing towards the more commercial aspects of that industry, something which Miura san was not intended to tolerate, so he left Luxman and founded his own brand, A&M LTD, with the brand Air Tight and Acoustic Masterpiece.

A living legend:

Miura san is one of the last living legends in the famous Japanese tube scene today, which had to suffer so many losses in the last couple of years, the passing of Kondo San as also Shindo San comes to mind. Very sad news for those of us, who want to enjoy a classic tube amplifier with that typical Japanese attention to detail and therefor a sound characteristic, which is hard to describe. Those amplifiers are maybe not the last word in neutral music reproduction, but they can deliver a sound which becomes a sort of hyper realistic picture of real music played by real musicians. Very often you can forget with these amplifiers from Kondo, Shindo or Miura San (Air Tight), that you are listening to your HiFi setup, and not to a real performance. They share all in a different way the same goal – you should relax and you should be able to take the musical performance reproduced from a vinyl record or CD for something real. To reach such a goal, there are some tricks, these old Japanese masters put into their “music machines”.
It could be a very common practice, that such an amplifier will bei the last one in your live – forgotten is all the technical stuff, all the technical reference data delivered by the most famous High End companies today – instead you get something very seldom found in the enthusiastic HiFi scene – you get satisfaction.
No “upgraditis” anymore – you relax and you start to enjoy music – instead of that typical self-questioning – is there maybe too less air around the cymbals ? – could the singer maybe need some more sparcle at the top end? – is the bass dry enough – and is it deep an powerful enough?….all that comes to a sudden end and you will listen to music.

The ATM4 form Air Tight will remind you from its outward look to some designs made in the 60ties (last century), the Marantz 9 monoblocks comes to mind. It is an amplifier with no exposed tupes, which could be a good thing if you have pets or little children…..

A very classic no nonsense design which takes his beauty out of a puristic “form follows function” appearance.
The front plate is dominated by a round bias meter (a quote from the Marantz 9), which shows the bias (DC) balance between the two pairs of output tubes. Under a screwed-in plate you will find the corresponding bias – balance potentiometers. In normal use this potentiometers are covered by the bespoke aluminium plate with the engraved manufacturers name. Left of this plate we will find an input switch, the ATM4 comes as most of the Air Tight amplifiers with two sets of line input terminals. If you just want to use two sources with these amplifiers, they could be used as a very puristic integrated amplifier. On the right hand side of this input switch we will detect two input gain potentiometers. Use them to adjust the power amplifier in terms of gain, to match the characteristic of your preamplifier – and you are also able to use this feature to match the whole amplification system to a cartridge with very high output (EMT JSD series) or just the opposite – a cart with extremely low output (AN IO for example).
The potentiometers are hand selected Alps Japan Blue Valvet potis, which are high quality parts – so there is no need to get rid of them, they are acoustically invisible, a precise gain matching feature ist worth to keep them in the signal flow!!! The second pair of input jacks could be also used together with a CD player or DA converter with adjustable volume on board. In such a case, you can skip the line preamplifier and connect the digital device directly to the power amplifier.

On the right side of the amplifier we will find the bias switch, which is used to select the two pairs of tubes displayed on the bias meter. And of course there is also an on and off switch.
The rear panel is fitted with 2 pairs of RCA input jacks, and 2 speaker terminals with the possibility to configure two impedance settings. The factory standard is a 4 and 8 Ohm tap, which could be also configured internal to 8 and 16 Ohm. Last but not least we will find the IEC power socket for a dedicated power outlet cable.

Tube rolling:

If we unscrew the top cover of that amplifier, we will see a special feature – the Air Tight ATM4 is able to be comfigured as a monoblock amp (we will need two of them in this case) or in stereo operation mode. For the Mono conversion we have a toggle switch together with a switching knob both lettered with Mono or Stereo – keep in mind – usage of these switches wil presume to shut down the amplifier first.

In Mono operation mode the output power is doubled, so the ATM4 is able to handle even more complex loads.
The tubes implied under the top cover are:
4X 6L6 GC or 5881
2X 6CG7
1X 12AT7 /ECC81

It is a classic Mullard design layout were the splitter tubes (6CG7) are also used in the driver stage.
The one and only line level gain stage is accomplished by a single 12AT7 double triode, which gives us an input sensitivity of 1,5V, that means you need 1,5V to modulate the amp to its maximum output power, which reaches its maximum at 2X 24W in Stereo Mode or 2X 48W in Mono operation.
If we compare this figure with the famous Mcintosh MC40 amplifier, we have to consider, that the MAC has a maximum output power of nearly 2X 40 watts with the same output tube compartment.
Miura San opted with his design for a higher damping factor to handle more complex loads, for that purpose he used no global feedback but a good amount of local feedback implied into the ATM4 amplifier which results in  a very good “drivability”.
In other words – the ATM4 ist designed with electrostatic speakers in mind, like the Quad ESL 57 or the ESL 63 for example.
The ESL 57 needs just around 15 – 20 Watts to reach its maximum sound-pressure level – but it is everything else than an easy load.
Please read my article about the Quad ESL 57 Quad ESL57, for the closest approach to the original sound. Part 1 you will get much more information about that topic.
One of the most asked questions regarding the famous british electrostatics would be the question, “which amp is able to drive them whith ease an precision?” – the ATM4 will be one possible answer to that question.
And it is a much, much better choice as the original Quad II monoblocks designed together with the ESL 57.

Signal flow and design:

With the 6L6 GC we will face two very famous specimen of this famous output tube, the RCA 6L6 GC Black Plates and the General Electric 6L6 GC Grey Plates, double side getter.
Both are very good choices to get one of the finest 6L6GC ever made.
If we listen to the basic sound of the Air Tight, which is very, very transparent and open, I would give the General Electric 6L6 a slight advantage in terms of smoothness and a grain free treble.
The RCA variant is much more “zippy” on top, which does not correspond as good as the GE6L6 to the basic sound of the ATM4.
One of the reasons for that openess and transparency might be the legendary output transformers from the Japanese manufacturer Tamura.
Those output trannies are one of the finest options available today – and are broadly used in the Air Tight portfolio. Another amazing power tube option could  be also  the Tung Sol 5881 – the tube might look small from its appearance – but the sound is  big and bolt with a smooth sparkle on top, which Tung Sol also implied in its very famous 6550 black plates.
With the splitter /driver stage, we have to find a tube, which has an equally good performance in both stages.
There are two very good options for this tube position – one option will be the RCA 6CG7 clear top, and the other one is the Sylvania 6CG7 metal shield grey plates. The RCA clear top gives us a more sophisticated treble, there is more resolution in this frequency range and a very good ability to render recorded space. If you choose the GE 6L6 GC output tubes this will be a fantastic match. If you choose the RCA Black Plates 6L6 GC the Sylvania 6CG7 might be the better option.
The last tube we haven´t spoken about till now is the 12AT7, which represents the input gain section. This is not such an easy task as it seems to be. To get the most out of this tube position, I have to give some more detailed descriptions about this tube and the problem to find one, which fits our needs in this amplifier.

If you think about the 12AT7 you can basically divide the whole bunch of them in two sections. One category is more suitable if the 12AT7 will be used as a driver stage, such design we will find in new Mcintosh amplifiers like the MC 275 MK IV – VI.
Mcintosh skipped the utilisation of the 12BH7 which was originally used in the old vintage MC 275 as a driver tube for the KT88 and replaced it with 12AT7 in this stage.
12AT7 tubes which are very good at this specific task are the US made types such as RCA 12AT7 black plates, Sylvania gold brand black blates or Tung Sol 12AT7 black plates.
If you use one of them in an input stage of your amplifier or preamplifier, you are in for a big surprise – gone is the ability to render a wide soundstage or the ability to place sound sources in front of the speakers or behind them. The whole spatial performance will be negatively affected.
Also the treble performance and the resoulution are very restricted – in one word – it sounds bad – really bad! – But if you use one of those famous US tubes in a driver stage, it is an amzingly good choice – maybe one of the best you can get. This is one good example for the importance to know which job in a given circuit a specific tube has to do and  why the opinions about the quality of such a tube differ so much.
One person tells you a black plates RCA 12AT7 is heaven on earth – and the next calls it crap…..
This makes us aware, that we alway have to tell the people in which part of the signal chain in an amplifier we listen to a specific tube – because the conditions in different stages of an amplifier are very specific – and the same tube could be bad in one stage and shines in another.
With the Air Tight ATM4 we can nearly skip any US 12AT7 the only exception is the Sylvania gold brand, black plates 12AT7 which is a good performer.
If we dig deeper, we face now the whole European ECC81 elite at our disposal.
But it is also not an easy task to get the right tone out of these ECC81 –  at the end it is one of the most complicated tubes to roll.

Mullard had just one very good ECC81 it is portfolio –  the very old dark grey (nearly black) plates design with 3 round side holes (plate construction) made between 1955 – 1958 – and those are rarely seen today in the used market. And this Mullard ECC81 might be very good – but its sound is too polite in the treble area to work well in our Air Tight ATM4.
The exact opposite direction will be adopted by the Siemens Halske E81CC double getter support, triple mica – this is one of the most analytic ECC81 and also one of the most dynamic tubes in its class….
But – you guess it already – it is too much for what an Air Tight ATM4 might need!
There are some rare variants which can sound outstanding but in spite of hunderets of different well made ECC81 just a few options are really outstanding with this amplifier.Lets start with the cream of the crop….this might be an expensive option, and it is also a very, very rare tube – but the Valvo Hamburg made 6201 blue print, gold pin, pinch waist is a hell of an ECC81 tube!!!
Maybe this is the best or one of the best ECC81 I ever listened in an input or gain stage of an amplifier or preamplifier.
It has it all – resolution, dynamic, deeb and articulated bass, a very sophisticated treble without being too analytic, an amazing spatial sound and more so – unique qualities in the department of rendering micro dynamics and inner detail which is hard to beat!
But this little “super hero” is sooooo rare and sooo expensive – that I will give you also more common alternatives – but if you want the best – the Valvo will be my first choice!
The second best is also not very common – it is the same Valvo described above but without the pinch waist glass structure.
Beneath these exotic tubes it is also a fantastic idea to use a Telefunken ECC81.
If you can find the first series with the slanted O getter, it will be a very good choice, as it will be the Amperex Bugle Boy /Philips ECC81 with the large O getter.

Coming back to another output tube option – the tung Sol 5881 will be a very good alternative to the 6L6 GC from General Electric. The Tung Sol will deliver a warmer tone with a unique character in the upper midrange – I would call this characteristic somewhat a creamy presence with a colourful and complex sound character. The Tung Sol 5881 might be not as common as the 6L6 GC – but it is worth to search for it. Do not be surprised, if the appearance of this tube might differ somewhat from the picture above, this tube came with a large variation of base colours, and also the mica clips at the side changed over the years.

How does it sound??

The Air Tight ATM4 will give you with the following tube setting a sound quality, which is completely amazing:
4X General Electric 6L6 GC double side getter, grey plates
2X RCA 6CG7 clear top
1X Valvo 6201 blue print, gold pins (pinch waist) or Telefunken ECC81 slanted O getter.

It will perform in such a powerful way in spite of its apparent restricted output power of “just” 24 watts per channel, that you will be stunned.
Be it the Quad ESL 57, a pair of BBC LS3/5a or some 12 or 15″ classic Tannoy with Alnico drivers, the ATM4 will handle them all with great easy and with a sound quality normally not associated with the 6L6 GC power tube!
The sound is very controlled, slightly on the analytical side of neutral, you will never think of an overly warm and rose tinted tube amp – instead you get tremendous speed and accuracy.
The bass is immensely fast and articulated, and without the slightest emphasis on the upper bass region so many tube amplifiers could be characterised of . It is a bass performance a lot of very sophisticated transistor designs would be proud of.
In the midband there is this difficult to describe miracle happening, a kind of “being there” – sound, a floating midband, which is as natural as breathing. The treble has some seldom heard three dimensional characteristics with a very rich palette of different colours.
The resolution is mind blowing and to all that the ATM4 adds the ability to render recorded space in such a realistic way – that you might forget the existence of the walls in your listening room.

The treble performance is one of the reasons, why this amplifier is a great match with the somewhat dfensive treble performance of the Quad ESL 57.
Those stats do not like overly warm amplifiers – to get a realistic top end, you have to match the ESL 57 with an emplifier, which sounds very open and uncompressed in that area.
But if there will be the slightest glare in the high frequency spectrum, you will detect that in less than a second with these legendary speakers.
There are so many transistor amplifiers, which seem to have a tremendous resolution in the treble – but they sound harsh and awkward. The ATM4 ist one of the very rare examples, which can easily compete with the treble performance of the best class A transistor designs at the very low impedance settings the ESL 57 brings along – but shines without any hint of grain or synthetic treble performance.
In other words the Air Tight ATM4 is one of the best amplifiers I have heard till now teamed up with the Quad ESL 57 or the BBC LS3/5a!!!

Classic design:

The ATM4 is made in a very high quality fashion.
The whole casework is made out of welded steel.
The bottom of the amplifier is constructed with a thick sheet of copper – the whole enclosure is able to shield the electronic parts inside of the amplifier – the copper bottom plate has also the function to isolate the amplifier from vibrations.
The casework is extremely sophisticated – all tubes are located on top of the  main case, the whole electronic is located inside of this case, the power transformer, an EI core type, is shielded and placed beneath the output trannies on the right hand side of the tube compartment.
The whole circuit is made without any use of printed boards!!!
If you open the electronic section of the amlifier you will detect top class components selected with a certain sound in mind. The whole construction is made by hand and the quality and craftsmanship is of the highest standard – those amplifiers are not cheap, but there is a great chance, that you can pass them on to your children – a timeless piece of audio gear!

Some Tips:

Air Tight amplifiers do not like to be treated in the newest fashion of upgrades achieved with “High End” gadgets and accessories the HiFi industry has on offer.
These amplifiers do not like any isolation base or replacement of their original feet with spikes or other constructions. Put them on a stable wooden piece (solid wood is preferable) of audio furniture as companies like Box Furniture in the US or LignoLab  from Germany offer to you – and they will perform perfectly.
Miura San made the amplifiers and preamplifiers in a very special way, they bring their own damping devices with them – the heavy welded steel chassis and the special applied copper bottom plate is everything these type of gear needs.
The same could be said about the usage of tube accessories like tube dampers –  spend your money elsewhere!!
Any damping gear like HRS isolation devices for example will make the sound slow and overly ripe, the leading edge sounds compressed and the whole sound looses focus and speed.

The usage of heavily screened power cables, more so with some magical boxes in between the cable structure is a also not recommended with these classic amplifiers. Use unscreened power cables or – my tip – the traditional Belden power cords and you will receive the best performance.
Keep in mind that Miura San designs his gear with pure copper only – so keep it that way – in my experience a mixture with copper and silver as a conductor will result in  a strange behaviour of upper frequencies,  which is seldom predictable. So there might be a great chance, that the newest fashion in silver cable development will end up at selling platforms like ebay.
Keep it simple – use Belden interconnects made with pure copper, or if you want to have something special, which I highly recommend, try to get some Chris Somovigo designed cables made with a copper tube inside. Chris made in the last couple of years a confusing amount of cables branded under different names like “Stereovox”, “Stereolab” or “Black Cat”.
Not all of his designs are worth to be tested – but those with the copper tubes inside are one of the best matches, you will find for your Air Tight gear.
Beside Belden and Somovigo designs, there is also the German company Auditorium 23, which developed a range of interconnects and speaker cables, which will also sound excellent!

To make a long story short – skip power conditioners, exotic power cables, silver wired interconnects or speaker cables, forget about all the expensive and exotic damping devices and platforms – my strongest advice is: Let this type of gear as it is!
It is fully developed and it does not need any help from any HiFi accessory company!
To prove that: In the Air Tight stable there is no latest fashion. It might be no surprise, that some designs Miura developed 30 years ago, are still unchanged in the companies portfolio – something which is quite unique in the audio industry!

All classic Air Tight amplifiers are made with the 600 Ohm standard in mind. So any of these classic power-amplifiers will have an input impedance of 100 Kohm and needed to be paired with a preamplifier with 600 Ohm output impedance or less  – to get the full frequency range this gear is capable off.
For example the very, very good Hovland HP 100 preamplifier with its unusual high output impedance will be not a good match with Air Tight power amplifiers, you will get a rolled off bass performance.
A logic step would be to mate these amplifiers with their stablemates  – the also amazing Air Tight preamplifiers – but that is another story for another day….

Stay tuned

E. Strauss