Tube Rolling the Xduoo Ta-26

First off – thanks to Xduoo for sending this beast for review.    A full review of the Ta-26 can be found here.


This article is instead aimed at what tubes work best in the Ta-26.  The Ta-26 uses a time-tested combination of tubes, so we can learn a lot by looking at what has worked well in other amps that use the same pairing.  The Darkvoice 336SE uses the same tubes and, with slight modification, the Bottlehead Crack can as well.   A lot of information about tube rolling those two Amplifiers relates directly to the Ta-26 as well.  Countless modifications and adapters that have been tried along the way as well.     Understand from the outset that if you are a tinkerer and want to adjust bias voltages, trade in for better capacitors, etc., you will likely be better served by one of the other amps that uses the same tube complement. The Ta-26 uses two PCBs with all components mounted, which is more difficult to alter due to construction of the board and the case is just large enough to fit the boards.  If instead, you are after the sound of a Bottlehead Crack at half the price and without the attendant hassle of having to construct it, the Ta-26 will fit nicely.

As with all of the other tube articles, I’ll first cover some background on the tubes used and discuss some alternative naming conventions and similar tubes.  After that I’ll get into which versions work best for what application.

The pre-amp tube used in the Ta-26 is listed as the 6N8P while the power tube is the 6N5P.  Understanding that these are the Chinese designations for both tubes is important because there is also a Russian 6N5P, which bears no resemblance to the tubes used in the Ta-26 and will not function in it.  To avoid confusion, I’ll refer to the tubes by their US equivalent names from here forward.  What Xduoo calls the 6N8P is what the west refers to as a 6SN7GT and the 6N5P in western vernacular is a 6AS7G.


6SN7GT history:

The 6SN7GT is a dual triode tube with an 8-pin octal base and medium gain (20) similar in some regards to the 12AU7.  It is a direct descendent of the 6J5 single triode and was often substituted for pairs of 6J5 in later designs.   (See Williamson Amps for one such example).   You will sometimes see the tube referred to as just 6SN7 although technically it was never named just that.   RCA released the 6SN7GT in 1939, which was an era when many of the 6S series were metal tubes and the GT suffix was used to denote short bottle glass tube instead of metal.  To my knowledge, there was not a 6SN7 metal tube version unless you consider the South American produced  6SN7GTC.   There is some debate as to whether these are actually a 6SN7 at all and they quickly faded out of the scene and are not well regarded or sought after except for tube collections.   So the 6SN7GT was born a glass tube unlike many of its siblings that started out as metal cased versions and gradually became GT versions later in life.

In addition to the 6SN7GT you will see later variants labeled as 6SN7GTA (1950) which updated the anode dissipation from 2.5W to 5W and 6SN7GTB (1954) which was an updated GTA with controlled heater warm up for use in equipment with 600mA heaters such as the 6SN7W, 6SN7A and 6SN7WGT, which were military designated tubes with stronger vibration resistance and taller bottles in early production examples.  Later production military tubes reverted to the basic 6SN7GTA design.   Sylvania’s early 6SN7W used a metal base and a strengthening rod between micas while other producers 6SN7WGT used thicker micas and a heavy miconal base to strengthen the tube.

Another thing to note is most early 6SN7GTs used a graphite coated glass that kept electrons from accumulating on the glass and impacting the tubes performance.  Later changes in glass made this unnecessary and by the late 1950s most makers had discontinued the practice.

In addition to those tubes labeled 6SN7GT you will also see variants labeled as 1633, VT-231, 6180, CV1988, ECC82, B65, and 13D2.   US production ended about 1980 as did most production in western Europe and Japan.  Russian made tubes from that era will often have “made in Holland” or “made in Great Britain” printed on the glass as companies tried to avoid import restrictions since there was still a market for replacement tubes but no legal provider in operation.

The biggest competitor to the 6SN7GT was indeed the 12AU7 as it was smaller and took roughly half the heater current of the 6SN7GT.   The 12AU7 was never fully successful because of the 6SN7GT’s higher cathode emission and plate dissipation which made it better suited for certain designs than its competitor.

Today the only current sources for 6SN7GT tubes are Russian, Eastern European, and Chinese made versions but since the 6SN7GT wass one of the most commonly used tubes for several decades, many old stock versions are still readily available to choose, from various sources.

There have been a lot of falsehoods spread regarding the 6SN7GT over the years that need to be addressed as well.  One of the largest is that the VT-231 carries some sort of mystique and is worth more than its 6SN7 labeled counterparts.   There is no difference between a VT-231 and a 6SN7GT made by the same factory at the same date.  VT-231 was just a military stock number and came with no changes in design or improved testing to differentiate them from the civilian versions.   The next is that there are many substitute tubes for the 6SN7GT that will work.  Often we see 5962’s listed as drop-in replacements and while they may work in some circuits a quick look at the spec sheet shows they are not the same tube.   The 5962 was developed at the request of the military because the artillery pieces were destroying 6SN7GT tubes with the vibrations they produced, so the 5962’s are a smaller, stouter, and electrically similar tube, but not a direct replacement electrically.      Likewise, it is also common to see ECC32 and ECC33 tubes labeled as drop-in replacements for the 6SN7GT and again, electrically they are not the same and may damage your gear.   Another commonly listed substitute is the 6F8G, which is realistically a 6SN7 with a grid-cap.  The problem with the 6F8G is it requires an adapter or a re-wire of the device to use it in the 6SN7GT socket and most sellers conveniently forget to say it isn’t a drop-in replacement.

From a buyer’s perspective it is important to know who made the tube, not just who’s label is on it.  Sylvania made tubes were rebranded as a number of other brands including RCA and Tung-Sol.  Tung-sol labeled tubes may be made by Tung-sol in the US, relabeled Sylvania made tubes, or later Russian made production depending on the era of production.    Remember that military stocks were often labeled at the factory before shipment to the military for acceptance.  Those that failed still carry the JAN label so the military markings should never be accepted as an indication of quality.  Always test tubes yourself or have something knowledgeable and trustworthy test them before paying a premium for any tube.    Also be aware that due to the popularity and cost of rare models a number of fakes have appeared on the market and more arrive daily.   The most recent re-label I have seen was a Chinese made metal base tube branded as a much earlier scarce US made model.   The easiest way to be safe is use a 6SN7GT, 6SN7GTA, 6SN7GTB, CV1988, B65 or 13D2.   There are loads of them available from the 1940s through current made tubes and prices range from a few dollars to a several hundred depending on the rarity and desirability of the tube.


Next up, the Power tube or 6AS7G:

The 6AS7G is a dual section power triode and was first introduced in 1946.   The parent of the 6AS7G was the 6B4 power triode, which was in turn the child of the classic 2A3.   The 6B4 is a single triode with an octal base so sort of bridges the gap between the older 2A3 UX4 single Triode and the later 6AS7G.   Early plans for the 6B4 called for an indirectly heated cathode to improve microphonics but whether due to the war or some other unknown reason, the production version of the 6B4 retained the directly heated cathode of its predecessor.     The 6AS7G shares with its predecessors, good linearity and the ability to deliver a lot of current, which make it very suitable for output transformer-less amplifiers like the Ta-26.      This is exactly the role RCA, the tube’s developer saw for it as early documentation lists the tube as an Amplifier output tube although it saw much usage as a voltage regulator in both commercial and military gear.

Some form of 6AS7G has been in constant production since the late 1940s with only small changes to the design over that time.   The most commonly found versions are the 6AS7G, the straight bottle version 6AS7GA, the military ruggedized version the 6080, the British CV2523, the European ECC230, and the Russian 6N13S (also called the 6H13).  There are also the 6520, which can be thought of as a cross between the 6AS7G and the 5998.  The 6520 uses 5998 style construction but similar specs to mμ on the 6AS7G.   The 5998 and 5998A are often listed as direct replacements for the 6AS7G but have a higher mμ and slightly different internal construction.   The 5998 may be preferred if more power is needed, but may leave little usable volume control with more easily driven models.  Likewise, the 7236 is often listed as a substitute but it was designed purely as a voltage regulator for computer tape drives and has a lot less heater to cathode insulation so may be more failure prone.  The 7236 also shares the higher gain of the 5998 so I tend to avoid it as well.    Two other tubes that are often mentioned in discussions of 6AS7G equivalents are the 6528 and 6336.   These two have even bigger departures from the standard 6AS7. Both require twice the heater current and the 6528 has the highest mμ of the lot at nearly double that of even the 5998.   The 6528 and 6336 should not be used in the Ta-26 as they will over tax the power supply.

Because the 6AS7G was used in commercial and military applications as a voltage regulator, there are millions of this tube available, but an awful lot of them were never meant for audio applications and some of these tubes can be howlingly microphonic as a result.  Beware of industrial tubes that claim to be “equivalent to” or “direct substitute for”  the 6AS7G as there are tubes out there with the same pin-out and very different current requirements and output curves.   If it isn’t one of the tubes listed here, I’d avoid it as there are plenty of good 6AS7G tubes out there at reasonable prices that don’t risk damaging your amplifier.


As always seems to be the case with these articles, there are only about 25,000 possible combinations of tubes that can be used in the Ta-26 and prices range from $10-15 dollars at the low end up to more than triple the cost of the Ta-26 at the top end.   It seems a bit foolish to put a thousand dollar tube in a three hundred dollar amplifier so I have tried to limit the choices to things that are a bit more reasonable.  I’ve also tried to limit the discussion to tubes that are obtainable.  There are a few rare carbon plate monsters out there, but finding one takes more than a little searching and if you do happen to find one, buying it takes us back to that earlier point.

This article has been a long time coming and has been postponed more than once.  When I first got the Ta-26 and its stock tubes, it was grainy, gritty, and generally not much fun to listen to.   So I did what any other complete lunatic would do and popped in my best 6SN7GT and 6080 and cured what ailed it and did my review.   I then set out to find tubes to do a tube rolling article.  I had a few in my collection but most of my collection uses either 6922 or 12a_7 tubes in the early sockets so I didn’t have as many good 6SN7s already in my collection as I did some other tubes.   It took about 6-8 weeks to get enough tubes in to start really listening but then I ran into another problem and that is that nearly half the 6SN7 tubes I bought were garbage.    I didn’t skimp and try to get the cheapest ones on ebay.  I bought from dealers I had worked with previously and had some faith in.   Even then a lot of the tubes were badly imbalanced, and what was being sold as NOS in some cases clearly was not when the tubes were tested.    What passes for “tests new” these days is highly questionable and one should be prepared to test tubes when they arrive and buy from sellers that take returns.   Because I won’t buy from sellers without a good return policy, I wasn’t out a lot of money, but I did lose a good bit of time sourcing what I had initially thought would be easy to find tubes.  Finding good older tubes for the 6SN7GT socket is going to take some time and effort and you either need access to a tube tester or a high degree of trust in the seller.   There are a lot of good tubes out there to be had, but there are also an even larger number of junk tubes for the 6SN7GT that are still on the market that should have been tossed in the dust bin long ago.     The other sad fact is that the cost of the tube had little correlation to the likelihood it was good.   Some of the least expensive tubes I bought were fine while some of the most expensive were not what they were purported to be.   With nearly every audiophile and guitar player out there looking for good 6SN7GT tubes, be prepared to pay market price if you want anything worth using.


We begin with looking at the 6SN7GT tube socket.   We can rule out B65,  6SN7GTY, VT-231, and 13D2 tubes for various reasons.   As mentioned before VT-231 are not special in and of themselves but being a wartime designation means all genuine VT-231 tubes are pre-1947 and thus fairly limited in availibility.  The B65 were Osram made and are extremely scarce.   The GTY was Brimar made and fairly rare and expensive.   The 13D2 like the VT labeled tubes commands a premium that makes them less than realistic for our budget.


6SN7 Tubes tried in the Ta-26 were:

Maker Type Notes:
CBS-Hytron 6SN7GT  CBS-HYTRON Black Plate This was the surprise of the test as these have tended to be middle of the pack performers in other things and are mid-priced.   This had great extension on both ends, lots of detail, and good speed.  Overall best in my testing. very good tube  – very musical good detail   – #1st
Current Russian Production ElectroHarmonix 6SN7GT,  Tung-Sol 6SN7GTB ElectroHarmonix was only ok – of three purchased, one was highly microphonic.     Tung-sol seems to be same construction but tighter quality control as 4 purchased all were quieter than EH.   In general none of these newly made tubes had the mid range quality of the older tubes and lacked the fluidity of the better older tubes.
General Electric 6SN7GTA Black plates good extension at both ends, nice smooth delivery but maybe not quite as much detail as some other tubes in the list.
6SN7GTB Gray plates Solid tube, but not as good as earlier GE 6SN7GTA for top end extension and has some grain in treble
Raytheon Raytheon black glass flat plates The Raytheon proved to be a very good tube as well with good extension on both ends but slightly less bass impact than the RCA.  Good fluidity and detail throughout the range, and a nice balanced top end. One of the most realistic tonalities of the lot.
RCA 6SN7GTA Silver Label Black Plate Good end to end extension, good detail and great mids,  solid imaging  as well.  May be a little syrupy for some listeners.
6SN7GTB Orange Label All of the RCA 6SN7 variants were good, but this GTB didn’t quite deliver what the earlier GTA did.  It still had a nice midrange with good warmth and fluidity but lacked a little authority in the lows and rolls off a bit earlier up top.
Shugaung Stock tube (Chinese 6N8P) The stock tubes were a mix with the 6N5P (6AS7G) being acceptable but the 6N8P (6SN7GT) being lousy.  With all the garbage tubes I tested, it was far from the worst of the lot but it did suffer from some micrphonics during warm-up and was grainy and not well extended.
Sylvania  6SN7GT black t-plates   3 Hole (Bad Boy) There is a lot of hype around “Bad Boys” with some saying only the 3 hole plate is truly it and others using a timeline to determine which tubes qualify.  Regardless of how one qualifies them, they were no better than the other Sylvania tubes from the same era.  Save your money and get the Chrome or yellow label
6SN7GT black t-plates 2 hole Good low end extension and slam, Good detail in the mids,  good top end as well,  stage is somewhat smaller than some others.  Good overall tube
6SN7GTb Angled T-Plate Chrome Dome Big low end, can get a little too warm depending headphone in use.   Good detail retrieval while still having a smooth delivery, Mids maybe not as detail rich or refined as earlier Sylvanias.
6SN7GTB Yellow Label Black Plate Good lows and mids, clean presentation with good detail, Top end is a little rolled off compared to best options.  Good stage and imaging. .
Tung-Sol 6SN7GT Mouse Ears (Motorola branded) good end to end extension and great detail.  Will be a bit analytical for some as they are not as fluid as the RCA or Sylvania but have better detail retrieval and imaging.
6SN7GT Round black plates This is another extremely hyped tube and again, I failed to find any great improvements made by it over the less expensive mouse eared or white label Tung-Sols – save your dollars and get one of them instead.
6SN7GTB White Label Similar in character to the Mouse ear’d earlier Tung-sol, detailed, clean, slightly dry and well extended.


I settled on the CBS Hytron 6SN7GT as my favorite of the lot and set to work getting 6AS7G varieties for testing.    It should be noted that RCA and Sylvania made a lot of 6AS7G tubes that were rebranded as other makes.   Philips bought the Sylvania plant so those have the same internals as well.   It is quite easy to buy eight or ten 6AS7G tubes only to find out you only have two manufacturers represented.    One of the biggest issues with the power tubes is how quickly they can recover from big hits in order to deliver the following sequence with equal authority.    I found that none of the power tubes changed the signature as dramatically as the driver tube (6SN7) did, but there were some distinct differences in the overall because of the changes to the 6AS7G.     I have limited the discussion to the 6AS7G Coke-bottle style tubes, the 6AS7GA straight bottle tubes, and the 6080 industrial version of the 6AS7GA as these all use the same heater current.    Beware of 6520s as they are supposedly a 6AS7G with balanced sections and most I’ve seen command a premium but do not guarantee the tube is any better for audio use.    Likewise the 7236 industrial tube has a higher mμ and was intended for use in voltage regulators so never tested for audio use.  Also be aware of tubes like the 6528 and 6336 that are often listed as substitutes but have very different heater current needs and can over-tax a power supply designed for the 6AS7G.    The Tung-Sol 5998 and 2399 have the same heater current as the 6AS7G but do differ in curve shapes an mμ and between that and cost are excluded here.  (average 5998 cost is roughly $125-150 as I write this).

Below is the list of 6As7G variants tried:

Maker Type Notes:
General Electric 6080 Middle of the pack.  Decent lows, mid-forward presentation with good but not great top end extension.   Better than Sylvania 6080 but still a bit lacking in dynamics and not on par with the Tung-Sol 6080.
JAN-CG-6AS7G This tube commands a premium over the GE 6080 even though both came from the same plant.   I found the 6AS7GA  to have better top end extension than the 6080 but the mids are actually a bit more recessed and the lows are quite similar.  I still find these to lack the dynamics of the better options.
Mullard Philips miniwatt branded 6080SQ Mullard made tubes have a reputation for being warm and syrupy and this one certainly lived up to its billing.   It is quite warm and rich, but isn’t as resolving as the better options and winds up sounding a bit lacking in transparency.   Those looking for a smooth, warm sound will appreciate the Mullard while those looking for detail and extension would do well to look elsewhere.
Philips 6080WC (USA Made) made in the same factory as earlier Sylvania tubes, these are essentially a Sylvania 6080 with a metal base instead of the earlier black and brown base models, and they sound every bit a Sylvania.
RCA JAN 6080WA Biggest plus to the RCA is a warm rich tone but the mids are recessed and treble extension is not great.    Not as warm as Mullard, not as transparent or extended as Tung-Sol.   These and the Sylvania are probably the easiest to find, but not really better than the stock tube.
JAN 6AS7G These can be founded branded RCA, GE, Raytheon, a ton of smaller brands and even badges as Sylvania. Look for flaps above the plates, thats a dead giveaway its an RCA produced 6as7.  In purchasing tubes for this review I ended up with a GE, an RCA, and a Raytheon that all share RCA construction.  These were all better than the RCA 6080 with better end to end extension, a bit tighter low end and better clarity in the mids.  Still a little glare and grain to the top end, but a vast improvement over the RCA JAN-6080-WA which disappointed.
Russian Commercial 6H13C All the Russian made 6AS7G production at present comes from the same Svetlana factory (now named New Sensor)  but QC seems to vary a good bit.  I tried a Sovtek tube which has the same internals as the earlier mil-spec tube for this review and found similar performance.  Extension is good end to end and bass is tight and well controlled.  Mids are well detailed and have good transparency but can show a little glare at times and the top has good air but does have some grain to the treble.   When these were dirt cheap they were a great buy.  Depending on price they may still be.
Shuguang 6N5P Honestly, one can do worse here.  Extension is fairly good at both ends, mids are a little recessed but detail is good, and Air is not bad.  Tube lacks the liveliness of the best options, but is still considerable better than things like the RCA 6080 I tried.
Svetlana 6H13C Older mil-spec OTK stamped winged C tube from Russian military surplus.   A few years ago you could buy a case of these for $20,  now they are a bit more but still very reasonable.  This was one of my favorite tubes in the test with good extension at both ends, good slam at the low end without control loss.  Mids are well detailed and have good clarity and the top end has good air and sparkle.  The only thing that keeps this tube out of the top spot for me is that it doesn’t quite have the finesse of the Tung-Sol.
Sylvania 6080 Gold Brand I have had these in my collection awhile so they may no longer be price realistic for the Ta-26 but since I had them, I tried them.  While most of the 6080s were very tight sounding, the Sylvania was warmer and more relaxed.  It gives up a little clarity in favor of musicallity. The bass has good slam, mids are lush and warm, and the top end is well extended with good air.  If you can find a “Gold Brand” 6080 for a decent price, it would be my pick over the Mullard for a warm tuning without getting too thick.
6AS7G I went through three different tubes to find one that wasn’t noisy in the Ta-26. I’m not sure if this is an incompatibility with the amp or just bad luck on my part.  Once I found one that wasn’t noisy, it had good low end extension but was somewhat loose.  It is a Mid-centric  sound with good fluidity at the expense of a little detail.   Treble extension was only average and there was a little grain to the treble as well.    Not a big step up from the stock tube.
Tung-Sol 6AS7G This tube will tend to be a little polarizing as it has good end to end extension but not quite as much slam at the low end as some and gives the tube a bright overall tone despite not having the absolute best extension of the group.   With the right tracks, this can be great as female vocals and strings really shine.  With the wrong tracks, it can be a bit harsh and glare is apparent.   For me this tube is worth taking a chance on because mids and vocals are the best of the lot and stage is as good as I’ve found.  If you can find one at a reasonable price, its well worth a try.
6080 The Tung Sol 6080 seems to be more common than its 6AS7G brother and prices are generally a bit more reasonable as a result.  This is the best value tube of the lot as it betters the Winged-c and still manages to keep price down to something less than insane.   Great extension with good punch, as good a stage as it’s sibling with plenty of depth, good width, and enough height to give it a 3d feel.  Mids are well detailed with good dynamics and there is only a hint of grain in the treble.  Yes the 5998 or GEC 6080 are better but at $300 a tube, they should be.  This is the budget king.
Westinghouse 6AS7G Westinghouse tends to get overshadowed by the bigger names but they made some great tubes in the 1950s and early 1960s.   My sample tube was made in 1958 and was less expensive than the Sylvania and Tung-sol branded 6as7gs for a new old stock tube.     Extension was good on both ends, bass is well controlled with some slam but not as dynamic as the Winged-C or Tung-sol.   Mids are a little recessed but still have good clarity, and the top end is well detailed if rolling off slightly earlier than the top tubes.   Overall a solid option but fairly similar to the stock tube.


For me, the best spend for the Ta-26 is either the CBS-Hytron 6SN7 or the Raytheon 6SN7 Black Glass in the front socket and wtihout doubt the Tung-Sol 6080 in the back.    If the Tung-Sol were not available the Sylvania 6080 Gold Brand would be my next choice.

Good luck, and happy listening.