Finely tuned sports car, yep. I'm a much better engineer than I am a guitar player, although music has always been such a big thing in my life. But it is a privilege to own such an amp. I'm not worthy, I'm not worthy.....
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Moderators: pompeiisneaks, Colossal
The power grids are held at 0V when clipping. Grid current is flowing (although not enough to drive a +ve voltage into AB2 operation) and charging up the PI to power grid coupling caps. With the downstream end of the coupling cap clamped or held at the power clipping threshold of 0V, the cap charging up is shown by the PI plate waveform top ramping upward. This is standard behaviour of a LTP PI driving a power stage to clipping. It shows up the same in my 18 Watt and will in any other push-pull amp.cxx wrote:Looking at the PI output waveforms when clipping shows a ramp at the top that seems to be a result of the variation of the cathode voltage. The el34 grid waveforms don't have this. Is this the result of the grid clamping or is it an anomaly of measuring at the PI plates?
This is always the case with a push-pull output stage. When one side reaches cutoff, the signal voltage is coupled through standard transformer action from the side still conducting to appear on the cutoff side.cxx wrote:The v4,5 plate waveforms and output are pretty much the same with v5 inverted.
No, it is totally unique to this amp. Yes you hear it - it is a key component of the tone and also reduces the clipped output power to be closer to the clean output power compared to a full headroom squarewave. This is the mixed mode distortion I've been on about. PI clipping sneaks underneath the power grid clipping threshold and becomes dominant on only the lower side of the output signal. The upper side remains power grid clipping dominant. The slope is kinda reverse of what happens as described above with power grid clipping. With the PI clipping having moved under the power grid clipping threshold, the upstream side of the PI to power grid coupling cap is held at a constant voltage (actually it drifts up slightly as the charging current through the cap backs off, and therefore the voltage drop across the PI plate resistor backs off) and the downstream side charges or discharges depending on how you think of it through the power grid leak resistor to the bias supply. When the signal goes up to PI clipping the signal cap is lifted (no change in voltage across it), and then because the PI output is flat from PI clipping the cap charges up so its downstream voltage heads toward the bias voltage (like any RC filter - won't pass DC). That shows in the output as a curve heading back toward 0V, but of course before it gets anywhere near steady state the signal flips the other way for the next half of the cycle.cxx wrote:The waveform at 3 isn't square which would be about 55 watts but more of a trapazoid which would be closer to square than sinusoid which would be about 50 watts. The funny thing is that as the wave becomes more square at higher volumes (the sides are more vertical) the lower part develops a curved ramp instead of a flat bottom. This would reduce the output power. This seems to happen most about 6.5 on the volume. Not sure you could hear this effect though. I can't see a reason for this in the inputs to v4,5.
So where does this curved part of the waveform come from? Is it on every amp that clips?