Tube Rolling the Xduoo MT-602

For those interested in a full review of the Xduoo MT-602, please check out my review.   What is below is not a review, but a comparison of tubes used in the MT-602 and how they impact the sound quality and signature of the device.   I have previously discussed the 6J1 tube in my article on Tube rolling the Ta-02 and Ta-05 amplifiers so some of the background material presented here may look familiar to those who have read that previously.

If you want to learn a bit more about tubes before diving into the 6J1, read the introduction to tubes article here.    Some of the block below is included in the intro article, but it bears repeating.


First a few notes that apply to all tubes:

When removing on seating tubes, never rock the tube in the socket as this is a great way to loosen the socket.  Often poor tube performance is a result of poor connectivity.  Seat and remove tubes by holding the unit in one hand, and the tube in the other and moving the tube vertically.

Never trust an ebay seller who says a tube is “Just like” another tube type.  Unscrupulous sellers will tell you tubes can be interchanged in order to dump what they have in stock and don’t care if it destroys your equipment as it will usually take long enough to do so that you won’t be able to put it back on them.   When in doubt, consult the spec sheets.

There is no such thing as a tube that sounds good cold.  All tubes have to warm up to perform and will require an hour or more of time to do so.  All testing done here was with tubes that had been preheated at least an hour before listening.

Tubes may need to having matching internal sections (dual triode or dual pentode for example) or be paired  (two tubes that are equal) in order to perform well in a circuit.  This depends on design and the ability to adjust bias, current, etc. outside the tube.    Generally, the simpler the design, the more important having well matched tubes is.   More complex designs may have other compensation mechanisms.  In this case, the MT-602 requires that tubes be fairly closely matched to channel imbalances do result.


Now on to the main attraction:  The 6J1:

While known as the 6J1 in China and Russia, in the west this tube is more commonly known as the 6AK5 or 5654, and also as the EF95 in Europe.   The tube was originally developed by Western electric for early radar systems and was used extensively in IF amplifiers during WW2 through the 1950s.  The tube is a 7-pin miniature pentode that has the grid placed closer to the cathode than standard and uses a very fine mesh for the grid that gives it excellent high-frequency performance.   The drawback to this close spacing is that thermal drift can become an issue as the tubes heat up.  Expansion of various components can lead to differing tolerances, plate voltages etc. and can be audible if QC is not extremely tight.   This is one reason the military grade tubes (JAN 5654W marked) are so popular as they were both improved to limit this issue, and very stringently tested at the time of production.   Commercially, the tube was widely used in VHF amplification and was commonly seen in TV sets and radios and while these applications still require a good deal of precision, QC varies between brands and eras among the commercial tubes. Audio use has generally been in guitar effects pedals, microphones, pre-amps, and headphone amplifiers as it has a lower output than needed for larger wattage applications.

In our use case, the 6J1 is used in pairs since they are a single pentode and only support a single signal (microphones and guitar pedals often require a single tube).    Getting a pair that is relatively balanced is important as getting two different strengths of tube can lead to large channel imbalances that are audible.  Generally, once tubes are paired, they should stay together as a pair so they age similarly.  Once a pair is split up, it is best not to recombine them without retesting them first to be sure they are still well matched.

Note:  JAN is joint army/navy and indicates the tubes were originally packaged for military use in the US.  CV (Common Valve) is a similar designation used by the UK military.  US JAN tubes often will specify a maker on the box while it was strictly forbidden to label the tube with a maker’s mark for tubes sold as CV.   Lot numbers can be used to trace CV tubes back to their original maker.

In general, the 6J1, EF95, 6AK5, and 5654 labeled tubes are considered the standard models.  The 6J1p-EV, 5654W, 6AK5W, and CV4010 were built to military standard often with longer lasting components and to tighter standards and will command a premium as a result.   The 6096 was built as a premium tube as were the RCA Command tubes and generally will command a higher price yet, and from there prices can go into the stratosphere for rare tubes and exotic makes or early tubes by a specific maker.


There are a wide variety of options for the 6J1 including:


6J1 This label is most commonly used by Chinese makers Peking, and Shuguang
6J1P-EV This label is most commonly seen on Russian produced tubes from Voskhod and Novosibirsk
5654 This was the original western name used in the US with common brands including, Westinghouse, RCA Command, National, Amperex, and RadioTechnique
5654W The W designation was an improved 5654 version and is most commonly seen as military tubes (JAN) made by RCA, GE, and Sylvania
6AK5 The 6AK5 was another western designation with brands including Tung-Sol, Siemens, National, and Ericcson
6AK5W An improved 6AK5 with typical brands including Raytheon, Mullard, and Telefunken
CV4010 British Military designation (Common Valve) most commonly seen made by Mullard
6096 Premium 5654 Model often seen made by GE, Tung-sol, Sylvania, and Mullard
EF95 European designation often seen made by Phillips
403b Early western designation used by Western Electric


I have tried to use a good selection of various tubes to wring the most out of the MT-602 but please know going in that it is entirely possible to spend more on a pair of tubes than you did on the amplifier.     Also worth noting, the similarities between the Ta-02/05 article and this one basically end here as while some of the same tubes were tested, the results are not the same between the two products and what worked well in the Ta-05 may not be a top pick in the MT-602.

I tested the original 6J1 Chinese tubes to give a baseline before introducing  CV4010 Mullard,  6ak5 Siemens and Sylvania (I used 3 pairs of Sylvanias from different eras of production), Several 5654(w) models by RCA, GE, Raytheon, and Amperex  ,Voskhod 6j1P-EV, 6AK5W Telefunkens, 6096 GE branded, and 403 (a and b) Western Electrics.   This gives a pretty good cross section of what is available but is by no means all inclusive.       Remember, that this is my opinion and others will undoubtedly feel differently, and these observations are only applicable to use in the MT-602.  Use in other devices may yield entirely different results.

My notes here are focused on differences from the stock 6J1 Shuguang made tubes that ship with the unit.


CV4010 Mullard:

While Mullard has a reputation for warm lush tones, the 5654 is not particularly tube sounding and in this case those buying these thinking it will get a more lush tone will probably be disappointed. The Cv4010 did have a larger stage than the stock tube and slightly improved low end extension by comparison but was middle of the pack in overall performance in this application and near the top of the pack in price so better options exist.


6AK5 Siemens:

Siemens tubes can be several different makers depending on location of purchase and specific tube, my 6AK5 are Valvo made and badged as Siemens.  Where the Mullard added a bit of low end, these add at the top with clearer detail up top and a larger stage as a result.   These do sound a bit brighter than the stock due to that increase, and may not be a favorite for some but for those looking to get the most detail out of the MT-602, these should be tried.


6AK5 Sylvania (1952):

I was digging through some tubes I had bought years ago and found some Sylvania made 6AK5s that were blue tip and marked Goodyear aircraft, Ohio so were likely from the a stock made and tested for use in Super Corsair fighter.  I decided to give a pair of these a try as I found several pairs and was really surprised at the results.  The stage expanded considerably and they had good end to end extension so in many ways combined the best of both the Siemens and Mullard I had tried.    I tried a 2nd pair to be sure I hadn’t picked a pair that performed above the norm with the same results.


6AK5 Sylvania (1964):

I decided to try some additional Sylvania made 6AK5s as 1950s blue tips are not easy to find and may not be available.  I wanted to know if the magic was due to the tight matching of those hand selected tubes or something in the Sylvania itself.   I found a pair of standard 1964 made tubes in my stash and swapped them in.  Again, holographic stage and great extension on both ends.  It appears that 1950s through early 1960s Sylvania tubes may well be the best option for the Mt-602.


6AK5 Sylvania (1976):

Next up, a newer production Sylvania and proof that by 1976 the magic was gone.   While still at least as good as stock, these didn’t have the stage or the extension (especially at the low end) of the earlier made pairs.  My suspicion is that with the company having changed hands several times, these later tubes were not made by the same hands and quite possibly not even in the same factory.  They don’t behave like the earlier tubes and are a marginal improvement on stock.


RCA JAN 5654W:

Now jaded by the early 1950s Sylvanias, the RCA didn’t sound as good as it might have had I tried it first.  It is still a marked improvement in stage and cleaner than stock but didn’t have the same extension as the Sylvania.    Good overall performer and well worth trying especially if you can’t get the Sylvania at near the same price.


GE JAN 5654W:

Next up was the JAN 5654w US Military tube manufactured by GE.  These were not a marked improvement over the stock and changes were minor at best.  One pair tested had pretty severe problems with microphonics, while the other 3 tried simply did not improve the stage, extension, or clarity enough to justify spending money to replace the stock tubes.  These might be a good spare pair to replace the stock when they do eventually need replacement if they can be found inexpensively, but better options abound.


6AK5W Raytheon:

Having been disappointed with the performance of these in previous tests, I did what any insane person would do and bought a couple more matched pairs to be sure I wasn’t just using a poor sample.   While neither of the new pairs howled as badly one one pair I originally had,  they also failed to impress and simply were not a big improvement in any way.    Save the $20-30 a pair you’d spend on these and stick with stock.


5654 Amperex:

These were an interesting listen.  The stage is not as large as the Sylvania or RCA,  they didn’t have the lows of the Mullard, or the highs of the Siemens, but they do have a bit of warmth and better mids than any of them.   For those trying to add warmth or a bit of mids to the mix, these would make a good choice.


Voshkod 6j1P-EV:

These were designed for Russian military use with upgraded tube heaters, heavier glass, and vibration resistance built in.  Most were installed in radar arrays with little concern for audio properties.  The good news, they should be sturdy and long lasting,  the bad news, they are not a substantial improvement on the stock 6J1.


6AK5W Telefunken:

These were sort of typical Telefunken as they have good extension and stage and a nice very clean presentation.   They don’t introduce any warmth and as such come across a bit bright.   Sound is well detailed and speed of transients is quite good.    The problem for me is these end up sounding slightly clinical and sterile if very well detailed.    This ignores the fact that these command a premium price as well.    If you must have the cleanest presentation, grab these, otherwise save a few bucks and you’ll likely get something a bit more musical to boot.


6096 General Electric:

The 6096 a premium 5654w sold mostly for military application.   The 6096 were better extended than the stock at both ends and very fluid.  Stage was not markedly bigger than stock though and detail level was roughly the same.   These wree probably the most fluid of the bunch so those wanting the most tube sound will likely prefer the fluidity of these.   For me, better options at lower prices exist unless you value fluidity over detail.


403 Western electric (1945 Navy stamped):

Again after last time, I went out and found a few additional pairs of early 403s.  Unlike the Raytheon though, this was a worthwhile endeavor.  The early 403 is different from the later 403b in that it has a higher current than the later but is still within spec for use in 6Ak5/5654 sockets.    I found a cache of 1945 made Western Electric 403s in USN marked packaging that I was able to make a deal for so have several pairs to test with now.  All test as new old stock on my Hickok and all are fairly well matched.   The 403 had good extension on both ends and a bit of extra warmth compared to stock and similar to the Amperex.  Stage was improved as well but not as much as with the Sylvania or RCA.    These have an earned reputation as a great tube, but others are a better value when price is added to the equation.


403b Western electric (1962):

These have a lower heater current than true 6Ak5/5654 tubes and are more correctly labeled a 5591 but will usually work well in 6AK5 circuits.    These did not share the warmth of the earlier 403 but instead had good end to end extension and a nice clean presentation. Stage was only slightly above the average, but for those who value a clean signature these make a solid option.