any years ago when I was designing the Amity, Takuji Yamamoto of the Direct Heating Society of Japan urged me to try direct-heated drivers for the 300B tubes. Although the Amity ended up sounding different than the Sakuma-method amplifiers - more "American" in style - I very much appreciate Sakuma-san's poetic approach to amplifier building.
I never forgot Takuji's comment that getting the "tone" right on a 300B required a Direct-Heated-Triode driver. Putting on my Tektronix hat, DHT's have remarkably low distortion compared to nearly any other tube, and peak-current capability is far in excess of more conventional drivers. DHT's just sound better, more natural, more clear, and most of all, more emotionally engaging. "Presence" and "vividness" are the impressions that come to mind.
But the inevitable awkwardness of AC hum induction, microphonics, more complex power supply, etc. etc. kept me back from looking into it. I went through the Raven and Aurora before even considering it. At an intuitive level, the idea of a three-stage amplifier composed of an input 5687/7044/7119 driving another 5687/7044/7119 driver somehow just felt wrong.
Vacuum tubes are good at doing different things; a driver is not at all the same application as an input tube, which is really a preamp-style voltage amplifier and input buffer. Driver tubes have to confront the shock of power-tube grid-current whenever the amp clips, which happens more often than people think. They also have to deliver a massive voltage swing to the power-tube grids - more than two or three times the swing required by pentodes, combined with a heavy capacitive load. In the Karna, each VV32B (or equivalent) is biased at 100V; this means 282 rms volts of drive are required for the pair, and at low distortion - preferably less than the 0.3% distortion of the VV32B's themselves. This is a tall order for any driver. By the way, that's why high-end DHT amplifiers are sonically all over the place - very few have sufficient drive capability in the driver stage. Instead of hearing the transparency and directness of the DHT power tube, you hear an overloaded RC-coupled driver instead.
