41MXP SE amplifier Part 2

Supply

Choke input is the way to go for most SE A1 amplifier with moderate current (below 100mA). Two good reasons:

1- Ripple is very low behind a choke, equal to 

V _Ripple = V_out / 6 π² f²LC√2

almost a perfect sin wave with very little transients in power transformer unlike input capacitor filtering.

2- Rectifier choice is greatly facilitated as we don’t have to deal with complex calculations mandatory for operation with capacitor input supply. Usually, tubes makers provide all needed information for choke input in their datasheet. You will always be safe if you stay well within the max peak inverse voltage and peak plate current.

For basic ripple and time constant calculation refer to LCR Phono preamplifier Part 3

 

Step by step construction

CNC aluminum top plate, epoxy coating, teflon soldering standoff, silvered solid wire and Belling Lee vintage sockets

Holco, Kiwame and Allen Bradley resistors. Aerovox V161 coupling capacitors (IMHO best for the price with ITT PMT/2R when you are looking for a neutral well balanced sound).

Sub chassis holds high voltage capacitors and second filtering choke

I was willing to build an all-British valve amp. Chosen rectifier is another staunch workhorse: the venerable UU5/VU39, this is a full wave rectifier found under so many code numbers (1821, 441U, APV4, CV1039, CV3760, DW4/350, MU12, UU120/350) and used for decades by the British services. Plenty of good ones still available. These were the ancestors of the world famous 5Z4G, GZ30 and GZ32.

Also tried a Philips AZ50, fine rectifier with ample sound-stage but slightly more obscure and a Cossor 43IU, in my opinion the very best on voices.

On the bench for a few tests

100Hz & 10KHz squares

VU39 versus 43IU

Listening report

Setup: Garrard 301, FR24/Entré EC30, UA7082/AT33 Mono, C3g phono preamp & 6J5 line preamp. Speakers are the homebrewed Klipsch Altec multicell combo

For me this amplifier combines the R120 clarity and the PX4 warmth, with more punch. Low end is ample and well defined, but a little rearward. For sure the Klipsch do not help, but I am not frustrated at all and can listen to cello or piano solo without the feeling of instruments lacking of body. Mids/heights are detailed, subtle, refined, never harsh or tiring. Cymbals are just like in the real life and trumpet rendered with excellent articulation and correctness. Jazz voices are perfect, nothing else to say. From that point of view it is the most natural and sensual of all the amps I have. The ML4/41MXP is a combination I can listen effortless for hours even at high-level.

I built several amps until today and I can affirm that this one is a top contender along with the Philips MC1/60 and the Visseaux A710

Vinyls, vynils & vynils encore

41MXP SE amplifier Part 1

Among the numerous triodes suitable for an audio project few are indirectly heated. Saying they are sonically inferior to the directly heated ones or less audiophile would be restrictive and unfounded. Main reason is that they are not numerous and nowadays scarce.

One of the most notorious is the French R120. A PTT tube beloved by Japanese who bought almost all the existing stocks decades ago. French where not the only ones to develop such tubes.

German RL12T15 is another good example of a very linear triode who prove to be a great sounding tube. Its somewhat limited use comes from a proprietary socket a nightmare to find. American industry was not left out by offering the 6A5G created on the basis of a 6B4G, itself derived from the famous 2A3 / 6A3.

What about the British? They produced some of the most remarkable and sought-after triodes ever, mostly of the direct heating type, but not only. Few people have heard about the 4033X or the 41MXP. The last one being the most interesting. Specifically developed for audio, it is a real champion. Bought for cheap years ago, prices skyrocket as stocks evaporated. However, when you find a set it’s worth the money for a nice SE project.

Ugly box for a gem...

The 41MXP comes with or without carbonized anode, same sound.

Original datasheet does not give much information and I couldn’t find the usual Ip/Up curves. Not a real problem, Cossor gives the best working point, load and voltages. We can see that the 41MXP is not a very demanding valve, 200V on the anode at -12,5 V grid. Unusual for a triode that generally bias much deeper, it eases construction a lot.

 

Remarkable fact, internal assemblage reveals a second grid factory tied to the anode. This feature supposedly intended to increase HF extension by reducing Miller’s effect is seldom seen. The above cited R120 and the DA30 are other examples of such arrangement.

Presented as a super power tube, a well-founded superlative, this cute triode possesses a lot of delicacy and definition paired with a loud and authoritative voice despite a modest 1.2W output power.

To match such talent, I needed a driver of equal quality. My choice stopped on the ML4, another great British valve completely underrated and left aside by the audio community without reason. During the golden age of vacuum tubes, it was the power horse of many amplifiers. It’s low mu and great linearity where useful to drive tubes like PX25 & PX4. With an internal resistance of 2.8K this tube is suitable for transformer or choke coupling. I chose the second option because at about 180V, high voltage is too low to properly load and bias the ML4 with a resistor.

I have on my shelves a pair Tango TC 160-15 audio chokes. Nice old Hirata iron bought decades ago and a perfect choice for the task.

 

Mass production GEC CV1732, grey plate and tubular glass envelope. Where intended for replacement in professional equipments.

Nice looking and great sounding Osram ML4.

Another GEC CV1732, smoked bottle, black plate in typical military boxe.

schematic is very straightforward and the parts choice will determine the final result, especially coupling capacitor and OPT.

 

more to come...