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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: Re: Experiments with "UltrPath" connection posted by Pär on March 6, 2001 at 02:15:39:
There is more going on than being just another filter cap. It samples what's happening on the B+ and amplifies it through the output stage. If the power supply drops, the "UltraPath" capacitor forces some of that into the cathode which then also drops the cathode voltage, increasing Vgk. Increasing Vgk then tries to turn on the output tube further which can cause more drain from the supply and drop the output even further. This is thus a positive feedback path. It serves to counter a loose power supply and props up the dynamics. Very fast changes are also being sampled all the time, which means the small changes in signal are being driven harder. This seems to be the reason microdynamics improve across the spectrum.The amount of this effect is dictated by the beefiness of the power supply. A supply that is well regulated and low impedance will offer less effect by an "UltraPath" connection since it doesn't move as much with the signal. The UP connection offers the most help to lightly filtered supplies. The UP connection is an addition to the regular power supply caps, not a substitute.
As far as trying to get the "optimum" cap, there's a lot going on with that. The voltage divider seen at the cathode between UP cap and cathode bypass would seem to work well, but I have found in practice that it doesn't necessarily work out that well. HF buzz seems to get through more than LF hum from B+.
The 45 circuit uses "excessive" capacitance, but it did not pose a problem with the well filtered RCLC PS filter. The 9uF capacitance on the PP amp was more of a problem for buzz amplification despite the fact that there was supposedly little being coupled into it - the cathode bypass was 60uF and the gain of the stage supposedly about zero (common mode rejection at the cathodes) and yet it amplified all HF buzz quite a bit.
It's difficult to simulate and difficult to analyze this kind of circuit. I think the "UltraPath" cap is really not critical in value from my experiments, but what is critical is a low buzz power supply; i.e. tube rectified, choke-input or resistor-input power supplies work well.
The ultimate in pacing (microdynamics) seems to have the following formula: Well filtered power supply: high order RCLC or LCLC but with small C's for quick charge/discharge and a 10uF or so high quality wideband UP cap, and PP beats SE when done well. The power supply will modulate a lot due to the small C's, but that will cause large positive feedback into the cathode with the UP cap. It may be "too good" if this goes too far. This is pretty much in the realm of art, or voodoo engineering right now.
The mod is easy to try. Get a 10uF high voltage quality SCR cap and connect the B+ to the bypassed cathode and listen. If you like it, great, if not, take it back out. It won't hurt anything.
Kurt
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Follow Ups
- Re: Experiments with "UltrPath" connection - Kurt Strain 03/6/0111:07:01 03/6/01 (1)
- Re: Experiments with "UltrPath" connection - DrM 19:22:44 03/6/01 (0)
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