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A few aficionados have used it, but despite very good results it stays a marginal tube. Main reason is the flea power you can expect in SE setup. It is in the same league as the 10, about 1,2 W. Not an issue with efficient speakers.
Those who have built an amp with are unanimous in saying that this little bottle contains a treasure of refinement and delicacy. It may even be better sounding than the 45 with a deeper and tighter low end. From this point of view it can be considered a 45 + 1 because it has an extra grid. However, it is not a tetrode and the data sheet confirms its use in triode only. The characteristics and operating point depends on how this additional grid is used. Low mu triode with G2 connected to the anode, high mu with both grids tied together. Only the low mu setup is practicable in hifi, especially in single ended operation.
A parenthesis to say that some rare tubes have the particularity of an additional grid connected to the anode. Best known are the French R120 or the British DA30 and the 41MXP. All sold as real triodes.
The 46 is a piece of history, an old tube developed in the late 20s and sold by all major American companies. It saw long service and use in radio receivers and was made in France under license by Visseaux, Radiotechnique and Mazda. Even if the 45 and the 46 are very similar, they are not interchangeable mainly because of the UY5 socket of the second one, but not only. Anode voltage, bias point and filament current are different.
Mine were probably made before WWII. A set comes in the usual cart board wrapping typical of military storage, the other one in Visseaux commercial blue and red box.
The 46 is pretty easy to drive, a favorable point to pick a good sounding driver among the numerous medium/high mu triodes available. For this purpose I chose a 7 pin miniature double triode primarily intended for computer use. The E92CC and it's lower mu relative the E90CC are inexpensive tubes with excellent tonal qualities and reliability. Paralleling both units gives a rather high transconductance tube, means low noise. Linearity is good and when properly loaded distortion is obviously very low.
I used a shared current coupled stage as driver. It is the same arrangement implemented in my phono preamplifiers. This setup widely used in professional equipment is to be considered a single unit providing high gain under low impedance. This helps achieve a more solid and defined low end from the final tube. Calculation is pretty simple as long as both tubes are Gm matched. From that point of view the TFK E92CC were made with tight tolerances, it helps a lot to sort two good pairs.
I first calculated the anode follower load for gain, bandwidth and distortion paying attention to keep enough grid headroom. Then used the same resistor in the second tube cathode. Stage is self balanced, there is nothing else to do, just check that voltage swing stays in the linear area of both unit. Additionally measured the CF bias which should be equal to the AF stage. This setup works great with high Gm triodes running some current, but can be tricky to implement with tubes like ECC81, ECC83...
Setting anode resistance to 22K gives a very similar AC load due to the CF high input impedance. Gain in the first stage will be G = µRL / RL + ρ = 34 and overall one about 33 with the cathode follower.
The 46 is biased according to manufacturer specifications : 250V, -33V, 22mA. Preferred load 7K.
A 1.5K/10W resistor will make the job. I personally favor non inductive ones and used a Sprague Koolohm along with Philips vintage LL capacitor. These electrolytics are super reliable with great sonic qualities. Other components are Allen Bradley, Kiwame & Beyschlag resistors, Aerovox V161 coupling capacitors. Supply uses SEL PMP and ICAR Polypropylene capacitors.
During the European Triode Festival 20in21 in Bellême I bought a gorgeous pair 46 globe by RCA Cunningham. A dream for any tube lover.
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