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Posts tagged “300B

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.