|The subject of appropriate driver tubes for direct-heated power triodes is quite esoteric to most consumers. Investigating it just cursorily presents immediate reasons however why 'famous' tubes like 300Bs may sound anything but famously in certain implementations. From Lynn Olson's Nutshell Hifi pages, we learn that "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 which is more than two or three times the swing required by pentodes, combined with a heavy capacitive load. In my Karna amplifier, each VV32B* (or equivalent) is biased at 100V; this means 282 volts RMS of drive are required for the pair and at low distortion - preferably less than the 0.3% distortion of the VV32Bs 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." And, "the most colored-sounding (and highest-distortion) triode is the input section - the choice between a 5687, 7044, 7119, 6H30 or other similar triode is essentially subjective."
*The VaicVV32B, much admired also by Joe Fratus of Art Audio and used in his Diavolo, was a high-current super 300B in the vein of today's KR's 300BXLS and EML's 320BXLS.
Against this background, Japanese audiomaniacs experimented with power triodes to drive power triodes. A famous Western example thereof is the big Cary where a 300B drives an 845 or 211. In Lynn Olson's Karna amplifier discussion quoted here, a 45 drives the output tube: "The distortion from the DHT driver is far lower than any IDHT tube tried - in fact, the distortion from the 45 is so low, it cannot be distinguished from the signal coming from the input stage (with measuring equipment with a noise floor of -130dB). In this amplifier, almost all of the distortion from the 4th harmonic on up is coming from the input stage, not the driver nor the output tubes. Direct-heated triodes, especially the 45 and 300B, have extremely low distortion. The sonic result is vivid tone colors and remarkably transparent sound, limited only by the sonics of the transformers... I was curious just how robust a DHT driver was. Well, the driver can drive the 300B grids 30 volts (!) positive, with no crossover glitch as it passes from Class A1 to Class A2. In Class A1, it delivers 15 watts at 0.3% distortion and in A2, 30 watts at 3% distortion. The driver is so powerful, it doesn't mind the onset of power-tube grid current and the 300B stays linear as it enters the positive grid region. This gives the amplifier the subjective headroom of a 30 - 100-watt amplifier, not what you'd expect from 300Bs."
If we extrapolate from these quotes, the voltage swing potential of the driver tube is absolutely critical to the performance of the output triode. Furthermore, its harmonic distortion character will overlay heavily on the power triode. Lynn Olson again: "The 5687, 6900, 7044, 7119 and ECC99 family are at the head of the line for low output impedance, wide voltage swing capability, high current and low distortion. As for modern tubes in the same family, I've heard good things about the Sovtek 6N6 and 6H30." And pertaining to our subject most directly, what Olson says next paraphrases the crux of the issue: "Generous driver design is vital. Most commercial tube amps only have 1 or 2dB of headroom in the driver circuit so the entire amp clips at once. This leads to longer recovery times and exaggerating the audibility of clipping. I prefer 3 to 6dB of headroom so the driver can retain its (voltage) linearity even when the output stage is deep into clipping.
"Perhaps more important is adequate current and low output impedance in the driver. Much of the amplifier coloration is actually in the driver and is a result of not enough current to properly charge the grids of the output tubes. I give Arthur Loesch credit for pointing out that the 'sound' of different DHT output tubes is greatly exaggerated by not having enough current in the driver. With enough current, DHTs become more transparent sounding and begin to lose the characteristic colorations they are known for. This implies that grid current is present during much of the duty cycle and is quite nonlinear. The more current available and the lower the source impedance, the less important this grid-current nonlinearity will be.
"The capacitive load which the output tube presents to the driver changes the load line into an ellipse. This pushes the driver into its nonlinear low-current region - once per cycle for a SE amp, twice per cycle for a PP amp. Pentodes have both less capacitance and half (or less) the voltage requirements of DHTs, so the driver requirements are relaxed (EL84s are particularly easy to drive, needing only 10 volts). Direct-heated triodes, on the other hand, are much more severe loads, more capacitive, needing twice the voltage swing and worst of all, exposing the driver distortion since DHTs have such low intrinsic distortion of their own. The sad fact is that many commercial DHT amplifiers, certainly all the RC-coupled designs, have more distortion in the driver than the output tube! What's the point in spending hundreds of dollars on a handmade 45, 2A3, 300 or 845 if all you hear is the driver distortion? But that's why DHT sound quality is all over the place - the driver section isn't up to the job."
In short, much finger pointing against the soggy, highly colored sound of certain 300B amplifiers should really be aimed at their driver stages. I'm certain Shigeki Yamamoto would concur 100%. Yet the language barrier between us limits the extent to which more technical matters can be intelligently discussed. I hence took the liberty to use Lynn Olson as an unwitting stand-in. I trust he'll forgive me and consider it for a good cause.
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