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Hi, I was wondering if anyone can help me with my amp problem? When I play a chord through the amp as the chord starts to decay a swirling noise with a slight distortion through the speakers is heard. It almost sounds like you have a delayed affects turned on but I am playing the guitar amp clean. I have tried all the outputs to the amp and this problem is also heard on Channel 1 & 2 & 3 also if you plug a guitar into the back of the amp where you have a right and left channel for a drum machine or similar equipment the sound can still be heard through the amp. It is heard on both speakers and so the problem must be something that filters through all inputs! Could this be either an input capacitor about to go or an output capacitor or could it be as suggested here in this article:
What is "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.
Is this article on the right tracks to my problem and what do I need to do to get rid of the problem? Obviously this article was I believe written for a tube type amp and I know that mine is a solid state type but can this problem still be caused to my type of amp or am I on the wrong track?
Or could it be the Gate Resister as in this article below which marries to the above article?
The driver transistors drive the square wave signal into the gate resistors (the resistors connected to the gate terminal of the power supply FETs). In the photo, resistors R924, R928 and R934 are the gate resistors for 1 of the 2 banks of power supply FETs (3 FETs per bank in this amp). In most amplifiers, the value of the gate resistors is between 27 and 100 ohms. These are a bit higher at 120 ohms. It's common for the gate resistors to fail when the power supply transistors fail. As I mentioned before, the gate often shorts to the drain (terminals 1 and 2 of the FET -- look at the IRF1010EZ datasheet, page 9). When this happens, the drivers have to work against the internal short of the FET to try to pull the gate voltage down. It's essentially impossible because the terminals are fused-together internally and terminal 2 is essentially connected directly to the B+ terminal of the amp. If the drivers are tough, the gate resistors will fail. if the drivers can't handle the current (when trying to pull the gate voltage down), they will fail (unless the gate resistors fail first).
Thanks in anticipation to the answer to my question
What is "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.
Is this article on the right tracks to my problem and what do I need to do to get rid of the problem? Obviously this article was I believe written for a tube type amp and I know that mine is a solid state type but can this problem still be caused to my type of amp or am I on the wrong track?
Or could it be the Gate Resister as in this article below which marries to the above article?
The driver transistors drive the square wave signal into the gate resistors (the resistors connected to the gate terminal of the power supply FETs). In the photo, resistors R924, R928 and R934 are the gate resistors for 1 of the 2 banks of power supply FETs (3 FETs per bank in this amp). In most amplifiers, the value of the gate resistors is between 27 and 100 ohms. These are a bit higher at 120 ohms. It's common for the gate resistors to fail when the power supply transistors fail. As I mentioned before, the gate often shorts to the drain (terminals 1 and 2 of the FET -- look at the IRF1010EZ datasheet, page 9). When this happens, the drivers have to work against the internal short of the FET to try to pull the gate voltage down. It's essentially impossible because the terminals are fused-together internally and terminal 2 is essentially connected directly to the B+ terminal of the amp. If the drivers are tough, the gate resistors will fail. if the drivers can't handle the current (when trying to pull the gate voltage down), they will fail (unless the gate resistors fail first).
Thanks in anticipation to the answer to my question
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