Tube Rolling the Xduoo MT-601

The Xduoo MT-601 is a small pre-amp and headphone amplifier that utilizes a single 6N11 tube.  For a full review of the MT-601, see this article.   The MT-601 is a hybrid amplifier that utilizes a single tube for the pre-amp stage and a solid state power amplifier following it.   This gives the user a chance to introduce some tube flavor into the sound and some ability to tune the amplifier by tube rolling.   One of the downsides to saving space by using a single tube is that the individual triodes in the tube must be well matched or one can expect channel imbalances.    In this case, some of the pairs of tubes used in earlier articles are not good choices here as while the tubes are matched, the individual sections within them can vary considerably.   When the two triodes are averaged in a dual tube design, this doesn’t have an impact, when a single triode is used for each channel it does.

That also brings up the point that finding a single tube with well balanced triodes can be just as costly as finding pairs of matched tubes.   Because of this, the MT-602 that utilizes dual 6J1 tubes may actually be less costly to feed in some cases.

I’ve written a good bit on the 6DJ8 family of tubes before so will skip a repeat of it here.  For those interested in more background material, see my Little Dot MK3se tube rolling article.    Suffice it to say that nearly every maker of tubes has produced some version of the 6DJ8 at some point between 1958 when it was introduced and today.   It was a very popular tube with just about everybody and has been produced in enormous quantities for military and industrial applications.   With popularity comes price increases and any time a tube becomes valuable, some unscrupulous seller will relabel, or outright fake the more desirable models.  Be careful in purchasing expensive tubes as I’ve seen some fakes that even went to the extent of gold plating the pins to look more like the real thing.

There are a lot of options ranging from inexpensive 6DJ8, 6N11, and 6N23P models through astronomically priced CCa models but for sake of this article, we will be focusing on tubes costing no more than the cost of the amplifier as it seems unlikely that many people will buy a $89 amplifier and then install a $499 tube.       That still leaves us with plenty of good options so read on.

I have broken the tubes out by rough cost per tube but remember that finding a tube with well matched triodes in important and that tube may cost a few dollars more than average.  Remember also, these are all compared vs the stock 6N11 tube that ships with the MT-601 (listed first below)

 

$20-50 range

Chinese 6N11	This is the stock tube shipped with the Xduoo MT-601 and it actually does fairly well in this application.   It has good lows and mids, but its downfall is the treble where it is a little uneven and grainy.   Stage size is good but not class leading.   Many will probably be well served by this tube and never want to look any further, but improvements can be had.
Sylvania 6DJ8	I had to try these first after being really impressed with the Sylvania 6ak5 in the MT-602.  No such luck here.   Extension was maybe a bit better than stock, but stage was not substantially improved, and tone was not richer.  These shared the grain in the treble with the 6N11 as well.  These have been good in other devices, here they were only average.
Amperex / RCA 6DJ8	I tried both an Amperex branded tube and an RCA branded tube made by Amperex NY with very similar results.   These both produce  good mids and highs but are slightly lacking in bass impact.  Stage size is about average as is overall extension.  Good but not great.
GE (Clear Glass) 6DJ8	These have been a consistent disappointment in pretty much everything I’ve tried them in.   they have good lows but from there up they can’t compete.    Mids are somewhat recessed and treble is grainy and aggressive.  Stage is average at best as well.  Save your money and stick with the stock over these.
GE (Black Glass) 6DJ8	Black glass was an early attempt to block RF so these are early 1960s tubes of a different construction than the clear glass listed above.   These were better than their clear counterpart in that balance is much better, extension is good on both ends, and stage is slightly expanded compared to the 6N11.   These were a mild improvement over stock, but an improvement all the same.
Reflector 6N23P	These were all built to military spec as they were not offered for civilian sale and as such these are closer to the 6922 than the 6DJ8 in many respects.   These have good extension, really nice mids, and great stage.  I expected good things as this tube has impressed me before and it did again.     These are cheap and plentiful for now so it might be worth grabbing a few spares.
Voskhod 6N23P	Very similar to the Reflector made 6N23P but with more grain in the treble.  Once given some time to burn in, some of that grain disappears but it can take quite awhile to do so.   The Reflector is a bit cleaner in my listening so while this tube is also above average, I’d shop for its Russian stable mate first.

 

$50-$75 Range

Phillips 6922	Very crisp and clear with good extension at both ends, good stage dimensions and very little coloration.  Those looking for the least “Tuby” sound will want to stop here.  Not a lot of warmth but great transparency.
Genelex Gold Lion 6922	These did well in this application with good lows and mids but still some grain in treble and a bit less stage than the top performers.  These have rarely lived up to the hype surrounding them in my experience and here again are only middle of the road performance to top end prices.
Raytheon 6922	These were one of the best in the bunch with good extension on both end, really nice lows, a bit of warmth in the mids, and good top end.    Stage was a bit larger than the stock as well.   These were never as numerous as some other brands but are worth seeking out and often less expensive than some 6922s.
Mullard (West Germany) 6922	These don’t command typical Mullard prices because they are not your typical Mullard.  They are made by Siemens and sound much more like the German tubes are known for.  Well extended on both ends, well detailed with good stage but no warmth to speak of and a bit clinical at times.   Another good option if you want a non-tube sounding tube amp.
Reflector 6N23p-EV (swgp)	I love this tube in other 6922 amps so had to try it.    In short, it is not spectacular in this application. I expected these to step the little amp up to another level and show me what it could be and instead it showed me that either the tube or amp is limited to about what the Raytheon 6922 delivered at a much higher cost.    These were still good with solid extension, nice mids and well detailed treble, but at more than double the cost of many others in this review, they simply did not perform up to the price.

In a previous article, I mentioned the PCC88 or 7DJ8 as possible alternatives as they often work well in circuits designed for the 6DJ8 but do have a higher heater voltage requirement.  Because of the power supply design of the MT-601, I think it is a poor idea to substitute in anything that places higher demand and thus did not attempt to use any of these in my testing.   They may work fine, then again they may overtax the power supply and in my estimation, it isn’t worth the risk.

There are way too many options out there for 6922 based amps to possibly test them all, so there is a good possibility that the best tube for this amp is still out there somewhere waiting to be found.    There are some good options among the ones I tested, a few surprises (both good and bad) and the ultimate winners were   the Reflector 6N23P as the best budget model and the Raytheon 6922 as the best of those tested.    Try either, I think you’ll be happy with that they bring to the MT-601.