but here are a few possibilities to consider!Aiken on Swirl.
A: "Swirl"is a dynamically-changing, slightly "phasey" sound as a note or chord decays, which is common to some tube amps. Typically, "swirl" is caused by a midrange "dip" or varying duty-cycle change in a clipped square wave that changes position as the note decays, giving a sort of mild phase shifter effect.
What happens is that first the phase inverter or output stage clips and produces a flat square wave. As the note decays, the signal level decreases, and the midrange frequencies start getting "unclipped" (either by the fact that their frequency band level is lower, or by phase cancellations due to the unequal phase shift with respect to frequency caused by tone controls and other RC phase shifts that occur in a gain stage) and show up as a "dip" in the top of the square wave, which will move back and forth along the top as the fundamental and other harmonics shift the operating point. Even if the clipping ratio isn't extreme enough to show the "dip" on the scope, the duty-cycle of the square wave will usually be dynamically changing as well.
Since the preamp stages are all AC-coupled to each other, the operating point shifts as the signal gets smaller, due to slight "blocking" distortion, where the gain stage clamps the top peak to a point slightly above it's cathode voltage, while allowing the wave to still increase in the negative direction. As the signal decays, it shifts upward and changes the duty-cycle of the clipping. It is this ever-changing shifting of the operating point that causes the "swirl" effect. The trick to good "swirl" is in the correct staging of the gain and frequency breakpoints of each gain stage in the amp, particularly in the phase inverter and output stage.
A similar effect can be caused by too much drive from the phase inverter to the output tubes. As the note decays, a riding "buzz" can be heard coming in and out. This is crossover distortion aggravated by too much signal swing to the output tube grids. Reducing the signal levels at the output of the phase inverter will cure this.
Another cause of a "swirly" sound is a parasitic oscillation that is riding on the output signal, causing intermodulation distortion
Lastly, power supply ripple causing intermodulation artifacts when it mixes with the audio can create a "swirly" sound. This is usually an annoying, non-harmonically-related overtone that rides on the note. If you look at the signal on a scope with the timebase set long enough to see a few cycles of the mains frequency, you can see the 120Hz (or 100Hz) full-wave rectified triangular ripple superimposed on the waveform. If you are looking at a single cycle of the input signal, you can see a "fuzziness" on the top and bottom of the waveform. This effect is most noticeable when the amp output stage is overdriven and the power supply sags. You won't normally see or hear it on a clean waveform..
