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As I was going through and checking bias for the preamp tubes in my salvaged Hammond M2, I quickly realized that something like four preamp stages are sharing 1 cathode resistor! This seemed strange to me, and I quickly chalked it up to bean counters saving change. I left it alone, each tube seemed to be doing ok and the organ was sounding good (after replacing a few 1M+ "100k" carbon comps...).
I was thinking about this, and I think they may have been on to something though. In a theoretical example, imagine 4 preamp tubes. Most of their cathode bias is established across one (of course much smaller in value) cathode resistor. In sine wave conditions, the current draw of the "in-phase" preamp valves counter-acts the current draw of the "out-of-phase" stages...this would cancel the "common mode" cathode signal between the two, allowing the resistor to approximate a CCS without the inherent degeneration in an unbypassed Rk. Each phase pushes current the opposite way, stabilizing currents. I guess in a more modern design, we might want to have all small resistors from this "CCS" to the later tubes to establish more headroom, but still keeping the current cancel effect.
So I was wondering if this is a lost design technique, or I'm just overthinking it. Good idea/Bad idea? Some of the early tube designs seem to entirely avoid the 60s and onward design canon, while still sounding pretty neat.
I was thinking about this, and I think they may have been on to something though. In a theoretical example, imagine 4 preamp tubes. Most of their cathode bias is established across one (of course much smaller in value) cathode resistor. In sine wave conditions, the current draw of the "in-phase" preamp valves counter-acts the current draw of the "out-of-phase" stages...this would cancel the "common mode" cathode signal between the two, allowing the resistor to approximate a CCS without the inherent degeneration in an unbypassed Rk. Each phase pushes current the opposite way, stabilizing currents. I guess in a more modern design, we might want to have all small resistors from this "CCS" to the later tubes to establish more headroom, but still keeping the current cancel effect.
So I was wondering if this is a lost design technique, or I'm just overthinking it. Good idea/Bad idea? Some of the early tube designs seem to entirely avoid the 60s and onward design canon, while still sounding pretty neat.
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