Mar 17, 2020

Visseaux A710 SE amplifier Part 1

New post, new tube. Time to discover one of the finest directly heated triodes ever.
One that favorably compares to the very best and sought-after tubes like PX4, AD1, RE604 and RS242.
One that belongs to a glorious and vast family of triodes but so unique it can be considered a family by it’s own. One so rare that just a few aficionados know its existence but probably never held one in hand. One with a beautiful shape and sound, maybe THE most beautiful sound (at least to my ears…)

I could use all the superlatives to describe the magnificent sense of music this triode hides in its glass envelope I would be far from the emotion I fell each time I listened to this beauty. Let me just say this tube always brought gold to my ears. I built an amp decades ago, kept it for a while and sold it because I did not have the speakers to go with. Fortunately, I kept a few on hand and now it’s time for a rebirth.

No more suspense, this triode is a 10 but not anyone. Among the numerous varieties produced for more than half a century by almost all actors of the tube industry, the Visseaux A710 holds a place apart because it outperforms IMHO all the other ones by his liveliness and his musicality and is only disputed by the RS242_E130. It is probably the fact of its oxide coated cathode that produces a soft glow when lights up. This tube is also fitted with and unusually large bulb, large plate and can be confused with a 50 at first glance. Despite these characteristics it is a true 10, and we can’t expect more than 1W/1.5W when properly loaded and biased. Don’t be fooled by this flea power, it plays really loud. This watt drives my La Scala effortless whatever the music.

To extract the quintessence, I will use a very simple circuit. The best would have been a simple step-up transformer to directly drive the tube. An impossible quest regarding the gain ratio and impedance. A second-best choice is a very linear and neutral driver combined with a high quality inter-stage. I have a very positive opinion about Japanese transformers an acquired a nice pair Tamura B5002 for this project.
It can handle 10mA unbalance current with a good bandwidth. This helps a lot to find a suitable companion.


I need a triode with a relatively high Gm, a low ρ and a µ ~ 20.
Good candidates are E182CC, 12BH7A, 5687 excellent in the VT25/VT25A push-pull, 6350, 6463 and 6840.
The two last are the most interesting but the GE 6840 is hard to source (I just have one in stock). No regret, the 6463 is an excellent performer - full data here -
This double triode primarily intended for computer use is not stated for audio. Believe me it is absolutely untrue.
Punchy, clear, linear (this could have been first) the 6463 is very close to a 6SN7. In fact, one unit is exactly a paralleled 6SN7 and what goes in is what comes out, not more not less.
Loaded 10K@10mA / -8V, I have my power horse that will drive the Visseaux with high gain and low distortion.

6463 load and bias

and A710 working point

Amp is almost drawn, just have to decide the A710 best load, 10K or 14K, remembering that theoretical optimum operating conditions for a perfect triode are :

                                    Load                   RLoad = 2 ρ
                                    Grid bias         Eg = 0.75 Eb / µ
                                    Plate current      Ib = Eb / 4 ρ
                                    Max power     Pomax = Eb / 16 ρ

In the real life power is lower and can be graphically determined by relation

                               Po = 1/8 ( Emax – Emin) (Imax – Imin)

1.4W with 10K load and 1.1W with 14K
Guess my ears will judge as a last resort

more to come, stay tuned...

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