Almarro 318B Overview 6C33C Audio Professor

Brief Review: Almarro 318B, vs A205A Mk II and AES SE-1

After I rehabilitated the Almarro A205A and AES SE-1, it was the turn for almarro A318B, but not before overcoming a hiccup. Let's start with:

Overview: 6C33C

In the last two decades, this tube, used on MIG fighter jets, became (in)famous. For an Introduction, read this article in tubetvr. If you look at the spec's, unlike most tubes, this beast is low plate voltage, high current and dissipation. As mentioned, the big variations in tubes favor individual fixed bias. It requires an ultra-stiff power supply, which is why, for such an interesting tube, there are fewer than usual DIY efforts. Otherwise, its ability to deliver 20 watts in SE configuration and hence to drive real world loudspeakers would render it even more popular. But there are people who advises against using this tube for audio, particularly with fixed bias, as opined in jacmusic, which is an even better introduction. Suffice to say, frequent checking of bias is a must.

OTL In a way, the 6C33C is most famous for its use in OTL amps. I have never heard the Atmasphere Novacron, one of the earliest OTL amps. And no, I have never heard the ridiculously expensive Lamm. In HK, I heard many times, always to good effect, the Italian Graaf GM20 and Modena (see my OTL Overview and also here). Another friend also uses the original Tenor OTL Monoblocks (this one) to drive his Watt/Puppy and it was OK (still a mismatch imho, but then I don't take to Wilson, particularly the incoherent Watt/Puppy).

SE The Almarro A318B is actually not the first SE 6C33C amp that I owned. In HK, I still have an old 100V Japanese Audio Professor amp, which has Tango transformers and uses 4 x 6J5 (= 2 x 6SN7) to drive the 6C33C (see right pic, borrowed from the net; click to enlarge). Its sound is excellent and warmer than the Almarro. I once took it to my friend Paul's place, where I matched it with the Graaf GM20, driving the B&W 802N. The two sounded similar in tonality and detail retrieval, but I preferred the SE for its better bass.

Push Pull This is the least often seen. From the start, BAT has always had a PP model in the lineup (see Herb Reichert's recent review of one). I have heard it at shows, pretty good (usually shows don't yield good sound).

Almarro A318B
This amp is quite famous and needs no introduction. For details, read the 6moons review. The A318B uses a little NFB. I have owned this amp for a long time, but have never formally written it up. Recently, my unit developed a problem. As I have recounted in a previous article:

"...Almost a year ago, I took out my Almarro A318B for a run. The machine heats up the filaments for almost 2 minutes before unmuting itself. Soon I heard popping sound in the left channel and shut it down. The same the second time. It also would not bias (recommended is 0.18V; no more than 0.21V). I got frustrated and put it back into the box. Recently I decided to get a new matched pair of 6C33C, and they are not cheap (over $100). I installed the stock Russian 6SN7 and 6SL7 and ran the new tubes. Great music, no bias problem. The next day, I installed the same 2 old-stock N7/L7 which I have kept in the box, expecting a better sound. Guess what, a pop in the left channel! I went to test the N7/L7. The L7 tested great. The N7 passed but for one triode I saw the meter drift erratically in both directions. There! That 6SN7 is faulty! Yes, tubes can measure well and still be faulty, particularly when warming up. I exchanged it with a RCA 5692 and the sound was perfect. Humbled, I reflected on why I had assumed it was the 6C33C that was at fault. Why had I not considered the possibility of either of the small tubes being the culprit? Perhaps the biasing problem compounded my confusion. Yesterday, I swapped in the original pair of of 6C33C and, guess what, they worked perfectly. And this time, there is no bias problem; in fact, they bias just like the new pair. Well, what happened back then? Unlikely as it is, I could only surmise that the faulty tube somehow messed up the bias. Well well, this is as it should be. I never used this amp that much and the stock tubes still should have plenty of life left (especially since I now run them at only 0.13-0.15V bias; mind you in NYC my area voltage can be 126V, dangerously near the tolerance limit of gears designed for 117V +/- 10%). I could have saved $100 if I had been more thorough before..."

In the forums, some complain of popping noises. Some of these cases could be due to the 6SN7/L7! In my experience, the stock Russian 6H8 and 6H9 are not reliable at all, and early failures are not uncommon. In HK, I had another friend who had one and used it with the stock tubes for a long time with no problem. Even though my listening sessions are long, there is no bias drift. So, provided you check the bias frequently (easy in this amp), imho it is a reliable amp.

What about the sound? Well, of all the amps I have used with the Harbeth P3ESR SE, the A318B was the best. It surpassed the Akitika GT-102 in texture and color. Everything is just bolder and fleshier. Plenty of power. In comparison, the lower powered Almarro A205A Mk II and AES SE-1 seemed relatively subdued flatter. Not very tubey in sound, it is nonetheless highly listenable - a great amp and a classic by now, especially for those who need a little more power. Too bad Almarro is no more.