Tube Rolling the Xduoo Ta-02 and Ta-05 Amplifiers

Currently, there are seven models in the Xduoo line that use tubes with an eighth on the way.   Those currently available use the 12au7, 6J1, 6C19, or some combination of the three tubes.  The soon to be released TA-30 will add the 5U4G to that list of tubes (it also uses a pair of 12au7 pre-amp tubes).     I will be covering each tube individually, and then discuss the models that use that tube as the hardware around the tube will influence how a tube behaves.      The first on our list is the 6J1 tube used in the Ta-02 and Ta-05 amplifiers.

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.

There are tons of opinions of what is the best tube and just like opinions of what is the best car, more often than not, the answer is “it depends. What do you want to use it for?”  What sounds great in one configuration may not sound good in another so no one tube is perfect for all uses.


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:


6J1This label is most commonly used by Chinese makers Peking, and Shuguang
6J1P-EVThis label is most commonly seen on Russian produced tubes from Voskhod and Novosibirsk
5654This was the original western name used in the US with common brands including, Westinghouse, RCA Command, National, Amperex, and RadioTechnique
5654WThe W designation was an improved 5654 version and is most commonly seen as military tubes (JAN) made by RCA, GE, and Sylvania
6AK5The 6AK5 was another western designation with brands including Tung-Sol, Siemens, National, and Ericcson
6AK5WAn improved 6AK5 with typical brands including Raytheon, Mullard, and Telefunken
CV4010British Military designation (Common Valve) most commonly seen made by Mullard
6096Premium 5654 Model often seen made by GE, Tung-sol, Sylvania, and Mullard
EF95European designation often seen made by Phillips
403bEarly western designation used by Western Electric


Since the Amplifiers we are discussing are budget products, we will limit our discussion here to those tubes that can be purchased for less than $50 per pair.

The Ta-02 and Ta-05 can be discussed in a single discussion as they are realistically the same amplifier.  The difference is the TA-02 has a single input while the TA-05 has 2 and the Ta-02 has a smaller footprint.  Inside both is a hybrid amp with the 6J1 serving as pre-amp and a BD139/BD140 Transistor pair in push pull providing the power stage.   Both have exactly the same spec sheet from an audio perspective with differences in size, weight, and internal vs external power conversion making up the differences.


In my Ta-05, I tested the original 6J1 Chinese tubes before swapping in Voskhod 6j1P-EV, GE and RCA branded JAN 5654W models, CV4010 Mullards, 6AK5 Siemens, 6AK5W Raytheons, and Telefunkens, 5654 Amperex, 6096 GE branded, and 403b Western electrics.   This gives a pretty good cross section of what is available.   My notes here are focused on differences from stock.    Remember, that this is my opinion and others will undoubtedly feel differently, and these observations are only applicable to use in the TA-05 and Ta-02 amplifiers.  Use in other devices may yield entirely different results.


The first thing I noted in my testing is that changing tubes didn’t always seem to make any difference at all, I suspect that the design somewhat limits the contribution the tube is making to the output as it was at times difficult.

Voshkod 6j1P-EV:

The Voshkod tube is supposed to be a step above the stock as it was designed for Russian military use with upgraded tube heaters, heavier glass, and vibration resistance built in.  Admittedly, the Russian makers had no concerns about audio use as most of these tubes were built for radar arrays.  I found these a slight improvement over stock in that the low end has better depth and the stage is a bit wider.  Mids and treble are changed very little from the stock tubes.


GE JAN 5654W:

Moving to the US Military version was a similar experience to the Russian made tubes in that the GE was not a marked improvement over the stock and changes were minor at best.  I ended up trying several pairs and these tubes seem to be very hit or miss as some sounded worse than the stock while others performed about equally or very slightly better than stock.


RCA JAN 5654W:

I popped these tubes in and powered the amp up expecting that an hour later I would hear more of the same.  It would stand to reason since they were made to exactly the same specifications as the pair I had just tested.   These were a pleasant surprise as the stage was better in both width and depth, extension was improved on both ends, and overall they sounded a bit cleaner.   These might be the best value pick as they can be had for < $10 each as new old stock.


CV4010 Mullard:

Sticking with military versions, but switching to another continent, the next up was the Mullard CV4010.  These did not fare as well as the RCA and were only marginally better than stock.  Mullard’s reputation is for warm mids and lush tones, but at least in this instance it was not substantially better than the stock Chinese tube.


6AK5 Siemens:

Next up, the Siemens 6AK5 badged model.   Siemens tubes are a bit of an anomaly as they were made in several factories depending on model and will have the characteristics of their respective makers rather than having a consistent signature across all products.   I did a bit of searching and comparison and it is very possible that these are actually Valvo made 6AK5s badged Siemens.   These were a nice lift for the Ta-05 as they definitely had more top-end than the stock tubes and a larger soundstage as well.   If there is a drawback, these are not as warm as the stock tube and while accurate, they can sound a bit bright as a result.   These were my favorite in the Ta-05.


6AK5W Raytheon:

Having now hit my stride with a few that were significant improvements over stock and knowing the 6AK5W is an “improved” model, I expected good things.  I was disappointed.  These were the only tube of the test that was microphonic audibly and just plain failed to perform.  They may be great in other applications, but the two pairs I tried were not happy in the Ta-05.


6AK5W Telefunken:

These were the 2nd place spot right behind the Siemens tubes as they had similar air to the top end and stage.   Again, a bit like the Siemens, these did not introduce any additional warmth to the mids and will sound a bit brighter as a result.    Honestly, these are so close to the Siemens tubes that sonically I’m not sure I could tell the difference in a blind test.  With the Siemen’s tubes commanding less of a premium, they get the nod, but if for some reason the Telefunken made model are less expensive where you are, grab a pair – they don’t disappoint.


5654 Amperex:

Another good upgrade to the stock tube, the Amperex had good top end and a bit warmer mid-range than the Siemens or Telefunken versions.  These had a slightly smaller stage than the other two, but for those looking to boost warmth in the mids these make a nice compromise.   This would be my choice if pairing the Ta-05 with a source that was slightly clinical leaning.


6096 General Electric:

I expected the 6096 to behave as the earlier JAN 5654W tube had as they are both made in the same plant and to the same specs.   The 6096 definitely has a different personality than its military sibling though.   These were better extended than the stock at both ends and very smooth in their presentation.  Those looking for a tube that gives that classic “Tube” sound in the Ta-05 should give these a try.  Not quite as detailed as the Tele or Siemens, but very fluid.


403b Western electric:

These were the oldest tube of the lot and I had mixed emotions about even including them in the test as I only had a single pair so whatever the outcome, my sample would not be reproducible with a 2nd pair for confirmation.   The 403b did have some improvements over the stock as the mids were a bit warmer and the top end slightly more open.  In the overall, I don’t feel they did as much as some other competitors in this round up, but that view might change with a larger sample size.


Hopefully, this gives readers a good starting point for upgrading tubes in their own Ta-02 or 05.  If you don’t have one yet and are interested, give Xtenik a visit.