Analogue revived !

841 (VT-51) Direct Coupled to 300B SMPS B+

If at first you think that this is a DRD amp, look again. It is not. No slow, giant, heat inducing cathode resistor found here! The output impedance of the SMPS supply is less than 10 ohms and is voltage regulated. The same low impedance supply that is feeding the plate choke of the 841, and driving the grid of the 300B is also directly connected to its cathode. In fact, the stacked SMPS supply can easily deliver over 300mA of current to any of its voltage taps, instantaneously and continuously, while maintaining voltage to +/-1V! The ultra low impedance supply means the SMPS filtering capacitors are essentially out of the signal path. This gives incredibly good low frequency coupling… better bass than any electrolytic or oil capacitor could provide. Bias protection for over-current of the 300B is provided by the multiple voltage regulated taps that the stacked SMPS supply offers, not just the current draw of the 841 driver tube. Additionally, the thoriated filament 841 tube will begin drawing current before the oxide filament 300B tube. A large sonic improvement would be found by using a true audio rated low DCR plate choke, wound by Bud at Onetics, to load the 841 plate. A low DCR plate choke would improve dynamics and recovery of both tubes better than the inexpensive 200H chinese choke I had laying around. Even-so, listening tests show that this breadboard amp destroys my Welborne Terraplane DRD monoblocks in every audible way that matters, while putting out more wattage and dissipating less wasted heat.

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3 responses

  1. Stephen Doyle

    Despite being a non-techie, I continued to be intrigued by the simplicity of this circuit. Intuitively, it should result in minimal attenuation / corruption of the signal – perfection is achieved when there is nothing left to take away. How does the 841 compare to the 6EM7, or in other words can this be implemented using the 6EM7 dissimilar triode for voltage gain and driving the 300B? The revered Coincident Frankenstein mk II transformer-coupled 300B SET uses the 6EM7:

    ‘The Frank was built over a period of years that consisted of trial after trial and combination aftercombination. He doesn’t rely on computer aided design, he relies on his ears. One tube he ended up with is a rather unusual choice. I asked him to explain; “The choice of the input driver tube was far more critical and it is largely here that the Franks are set apart from its competitors. Instead of using the ubiquitous 6SN7 which is cheap and simple, years of research lead me to the 6EM7 super tube. This octal tube has so much current capability it can put out 2 watts as a single ended output tube. Used as a input /driver this enormous current capacity provides so much signal to the 300B that distortion is reduced by a factor of 10 and prevents the 300B from ever being current starved no matter how dynamic the signal. Using the 6EM7 is an expensive proposition( another reason other manufacturers shy away from using it) since the tube requires its own power transformer and power supply.”‘ (http://www.coincidentspeaker.com/Frankenstein-3000B-SM-2009-Full.html)

    Also, would normal tube amp power supply filtering apply here with SMPS? Pi or double-Pi filtering?

    August 3, 2012 at 4:17 pm

  2. Hi Stephen,

    I have never heard the Coincident Frankenstein, but may have a chance to do so soon. The 6EM7 I am familiar with, and it sounds fantastic. The 6EM7 is a dissimilar dual triode, so that means three stages instead of the two in the Wander, causing a slight loss of transparency. I really love the sound of the 841 choke loaded, more than even the 6EM7. Because the 841 is directly heated, it is not as quiet as the indirectly heated 6EM7.

    With a tube rectified supply, the dynamics will be much ‘slower’ than the SMPS. Yes, you can use traditional Pi filtering after the SMPS, but then you are giving up what makes the amp unique. I prefer simple tube rectification over having to use Pi filtering after an SMPS.

    Have you heard the Coincident Frankenstein?

    Thanks!

    August 3, 2012 at 10:10 pm

  3. Stephen Doyle

    I haven’t heard the Franks mk II – only know them by Arthur Salvatore’s assessment (which I trust) at http://www.high-endaudio.com/RC-Frankenstein.html. The best system that I’ve had the privilege to hear was Roger Herbert’s, the man behind http://www.wyetechlabs.com . He was using a Jean Nantain Reference Lenco (http://www.idler-wheel-drive.com/ and http://www.high-endaudio.com/RC-Lenco.html), his new Ruby preamp, prototype Ruby parallel 211 Ruby SET monoblocks (I think he mentioned he might go to quad 300B monos) driving his uber-rare Tannoy Churchills.

    I live 4.5 hours from Coincident, so I hope to get there and hear the Franks some day.

    How does the 841 compare to the 6EM7 in terms of the current it supplies the 300B? Conceptually, the Franks’ signal path should travel through the 6EM7’s 2nd triode and the interstage transformer. While hearing is everything, I might suspect any comparison would be an issue of the Luddite incredible circuit simplicity v. Franks’ 6EM7 high current – low distortion.

    I recall Arthur S. being rather surprised that the Franks’ passed his demanding subwoffer / low bass test with such high marks: “I asked [Isreal] Blume [President of Coincident] to play two of the most demanding orchestral LPs that I’m aware of when it comes to challenging subwoofers. These two LPs are the original Mobile Fidelity UHQR versions of The Planets (Saturn) and The Pines of Rome (Catacombs)… To my utter shock, they played them well, with no loss of control at any time, and the volume was “normal”. Now don’t get me wrong. I’ve heard both of these passages with more force, authority and weight, but never with more harmonic detail, cohesion and control. This is a singular and indisputable achievement. It is what has excited me more than anything else, because it is, to my mind, a true and real “breakthrough” in audio, meaning I believe there is (or may/will be) another alternative for audiophiles who are unhappy with the current choices offered to them.”

    Cheers,

    S.

    August 3, 2012 at 11:23 pm

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