BRZ A20B 6N2-6Π3C SE Amp vs 6L6GC 5881 EL34 12AX7

Click pics to enlarge. Top, on the amp, Mullard box-plate 12AX7 (on adaptor) and Tung-Sol 5881. Front, from L to R, Valve Art Chinese EL34, East German RFT EL34, Mullard xf1 large brown base EL34, Executone 6L6GC, 6SN7 adaptor.

Review: BRZ A20B 6N2-6Π3C SE Amp, Part II, Tube Rolling
Letter from Shenzhen (23-14): Digging through my Hong Kong Tube Stash
Overview: 6N2/6N1 Tube Rolling Guide, Part I

Part I gives details of the amp and slight modification.

Part III is quite substantial and a must, as it covers Tube Rolling of 6S_7 types, Capacitor Mods and Preamp Matching!

Note that basically Part II and III combined gives a detailed Overview of 6N2/6N1 Tube Rolling!

Left, adaptor for 12AX7 https://m.tb.cn/h.5kfN5Fb

Tube Rolling As the closest equivalent to the 6N2 is a 6AX7, which nobody has, one would want adaptors that would allow use of the much more ubiquitous 12AX7. They differ only in how the heaters are wired (the 12AX7 heaters can be wired either 12V or 6V). and the adaptor's small circuit board takes care of the conversion. One can then also roll in the extended lower gain 12A_7 family for different effect. Without need for adaptor one can also roll in pin compatible but similarly lower gain 6CG7 (like a larger 6V U7). One can also roll in 6SN7/L7 (like an even larger 6V U7+) with yet another adaptor. These 2 tubes, especially the latter, draw more current but the amp should be OK with them.

Right, adaptor for 6SN7 https://m.tb.cn/h.5PQ8s5u

Tube Adaptors for 6N2 The ingenuous adaptors have a small round circuit board at the bottom, where the pins are soldered to the underside and the socket clips on top. The small circuit board takes care of the conversion, basically rearrangement of pins. For some, for a premium, there are deluxe versions with a metal surround. My cheap versions are just wrapped in heat-shrink. One should be gentle when handling these. As the top socket is soldered onto the bottom circuit board, one should grab the top portion when when inserting tubes, and grab the bottom portion when inserting onto the amp. This is to prevent stress on the solder joints.

Please note that I'm not one of those 3-noters (one who A/Bs tubes and proclaims the gospel after very short clips - many of these people in HK; yes, I have seen some ass turn the amp off and on after 3 notes of 蔡琴). If a tube is not downright bad, I listen to quite a few albums before even considering switching. Short A/Bs are the quickest way to proclaim one knows something when one doesn't.

ROUND 1:6CG7/6DJ8/12AU7/12BH7

First, I just happened to have some of these old stock tubes around (for testing of the Douk T08 Pro as substitute for 6N1). These are all pin compatible but all of them are lower in gain than 6N2.

The first tube I tried were the 6CG7s. Immediately noticeable was that the gain was lower than the 6N2 (Beijing). I had to crank up the preamp (6J4/6P3P) quite a bit to achieve the same volume (finally, a use for these high-gain preamps!). The GE has that lightly veiled quality one so often finds with that brand. The Japanese Selectron was superior, combining nicely the crispness of the 6N2 with some 6CG7 warmth. Very good (if you have the gain in front)! Then I rolled in the 6DJ8 (Amperex Holland). Same low gain. I heard the typical cleanly sorted sound of the 6DJ8. Good but a little too light and I preferred the Japanese 6CG7. Then the adaptors arrived. I did not have an X7 on hand and so used the 12AU7 (Australian Radiotron). Gain was about the same as 6CG7 and the sound was very nice, just a little lighter. In time, I may test this out further. Then I tried the higher gain 12BH7 but did not like the thickish sound (often the case when used as U7)

As I go to HK now perhaps once a week to run errands, a few days ago I took the opportunity to return to my old house and dig through my stash of tubes. I didn't spend that much time on it, and quickly fished out a few 12AX7s, 6L6GC  and EL34 types for testing. I didn't include KT66/7581/7027 types due to concern about resistor ratings. They are unusually small (see pic in Part I) and there was a TB buyer comment that said the amp broke down because he used the wrong tubes (he didn't specify but I'd guess it's a power tube) and he had a pic of KT66 on the amp! Not exactly confidence inspiring! And forget about KT88! And, since the amp runs Ultralinear, I'd forego my favored 6L6G/GA too. I don't think I have a WE350B in HK (a few in NYC). Note too that the tubes I took are not necessarily my favorite tubes, just what I could find quickly.

12AX7s. From L: Mullard Long Plate; Likely GE with support rod; RCA long black plate; Telefunken smooth plate, and Mullard box plate.

Round 2: 12AX7

The first 12AX7 I put in was the Mullard Long Plate. It had about the same gain as the 6N2. I was shocked by the sound - rich but audibly veiled. No go. I then slotted in the tube second from left in the pic. Its grey ribbed short plates and support rods almost make it look like a GE 5751. Much better sound, clear and sorted. Then it was the turn of the Mullard box plates. Surprise, a very even performance! Just as clean and sorted, but with a little extra body. Good. Next was the RCA, which was a little less clean at the edges, slightly less focused, but imparting a somewhat larger sense of presence. RCA 12A_7 types have never let me down (unlike GE and even Mullard). Finally, the Telefunken smooth plate. Initially I was tickled by the litheness and seemingly enhanced upper harmonics, especially noticeable with strings. After a while, one realized that all came at the cost of lower mid solidity. One more thing, with more dynamic large orchestral music, the Tele is more parched and restricted than the others. These findings are all par for the course, and consistent with my previous understanding of these tubes.

Round 3: 6L6GC/5881/EL34

First was EL34. The Chinese Valve Art's had long ago been used in my (re-issue, VAC made) Marantz 9 to very good effect. The sound was very good, a little cleaner but also a little less emotive, and so commensurate with the Russian 6N3C. The Mullard xf1 large brown base EL34 was quite different. Compared to the Russian and Chinese tubes, there was significantly more microdynamics and a mellifluousness. Unfortunately, it too has a slight veil that I did not quite like. The Double D getters are my favorites but I have yet to find them. Slotting in the German RFT cleared up all the fog but also lost some subtlety - still reasonably good nonetheless. I was surprised when I introduced the Tung-Sol 5881. Previously these have not been favorites (as you can see in my review of Elekit TU-8800; as mentioned in Part I, that article also voiced some of my tube preferences, worth reading of you like to roll tubes). The 5881 is very controlled (too much so sometimes) and cleaner than the usual 6L6 types, but in this amp with just enough sparkle at the top to make it sufficiently lively. Surprisingly listenable. At first,  the 5881 handily trumped the Executone 6L6GC (likely Sylvania) that followed, which sounded just plain. But after Part III was first published a few days ago, I rolled in again the Executone (Sylvania) 6L6GC); holy, it overtook the 5881 in performance! Perhaps my better driver tubes on adaptors help, or perhaps tubes unused for the longest time need a bit of re-running in too. So I'm adding these lines to tell you how tube rolling results may change as things change. It's too bad that I could not try my preferred 6L6G/GA in this amp - they likely may not do well in UL operation. For reasons mentioned above, no higher current types were tested.

Comments

• The Basics and Value of this Amp This is an imperfect, but eminently likeable, amp. Grant you, I bought it for NO good reasons (at least I thought so) - I just wanted to see what I'd get for this ridiculous price, and I was really quite pleased, to say the least! BRZ is a company that builds super-budget products. They are all encompassing, manufacturing everything from DIY boards to chassis and both ss and tube amps. That's why they can undercut everyone. Now, these are built to a ridiculously low point, so expect occasional hiccups and less than ideal parts quality. DIYers can possibly reverse engineer and improve parts quality. Caps are easy but resistors are more problematic. Keep that in mind when you read about my cautions.

• The Basic Sound The Heart is in the Right Place. That is important. Even with stock Beijing 6N2 and Russian 6N3C the sound was musical and pleasing. That tells me the transformers are OK in quality (at least for now; and their FU-50 amps holds up very well in my system)! In fact, in this setup here in SZ, the sound is arguably even more musical than the far more expensive Elekit TU-8800 in NYC with Klipsch Hersey (if my memory holds).

• The Driver Tube Technically, the 6N2 is like a 12AX7. Use of just one triode is not the best driver for 6L6 or EL34 (better for EL84). In conversations with Shawn P, he remarked that his much more expensive (but still bargain by American standards) Decware EL84 and EL34 amps also use the 6N2 (although the latter uses 2x), and Decware advises Preamp Gain. This sounds right, but not for those stuck with passive "wire without gain" beliefs! So, it'd be good if you have a preamp with gain. The preamp sure helps, but there is intrinsic limitation on how much half a 6N2 can swing. So, sometimes, adjusting the volume knob on the preamp is a little tricky. Phase? Each channel being driven by a single triode, Should this amp be phase inverting? I don't know and BRZ doesn't give me an answer. I don't have a scope, so I just reversed the loudspeaker cable leads. That seems to focus the sound more in the center, leading me to think this amp is phase inverted.

• Who is this amp for? It's a good Entry Level amp (and the stock tubes sound quite good), particularly for those starting out on the SE path. Those people can roll tubes a little bit but I'd not go crazy on that, as the cost would be disproportionately high compared to the cost of the amp. While it's surprisingly good (my Beydas 3/5 are not so efficient), it is not for rock fans. It benefits from a Preamp with good gain. With the power tubes, unless proven otherwise, I'd like to err on the safe side. Given the official BRZ recommendation and the looks of the small resistors, I'd say this amp is NOT for the ambitious Power Tube Roller. Don't use power tubes that draw significantly more current than 6L6GC and EL34. I'd NOT use KT66/7581/7027 (and KT88)! Due to the Ultralinear operation, I'd also not use lower rating tubes, like 6L6G/GA. It's a restricted window. This amp is also for those who, like me, have a stash of EL34 and 6L6GC type tubes. With real world loudspeakers, I'd withhold recommendation for those who want to use it as an integrated amp - better results with a preamp with gain.

• Limitations Beside the practical tube use limitations detailed above, some re-emphasis on the sonic ones. As inferred above, due to limited driving power of the 6N2 (or equivalents), while a modest improvement can be obtained by tube rolling, do not expect night and day differences. While it certainly is not the most powerful amp (I estimate it to have a little less power than 300B), neither does it come across as wimpy or lackluster - the slight softness contributes a certain pleasing euphonic character. I have been listening to it happily ever since it arrived.

Part III will have Tube Rolling with 6SN7/L7, Capacitor Change, Preamp Matching, and more!

All Mullard. xf1 EL34 and box plate 12AX7. Tubes more expensive than amp.