May 29, 2021

41MXP SE amplifier Part 1

Among the numerous triodes suitable for an audio project few are indirectly heated ones. 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 280V, 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...

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