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This problem has caused me troubleshooting grief in the past, but I have been aware of it now for over 10 years, and it's something that all techs and would-be techs should be aware of.
Many years ago, I had an issue with a Music Man combo that just sounded "wrong". Could not put my finger on it. FYI, it was loaded with an EV12L, whose magnet assembly was right up against the 6CA7 power tubes. On the scope, it had a subtle, yet ugly distortion of the sine waveform when testing, accompanied by the symptom of serious rounding of the peaks when trying to push it to clipping. To make things MORE confusing, it all disappeared when I removed the chassis for troubleshooting. After messing with it for awhile, I decided to push the chassis into the cabinet while monitoring the sine wave output. As the tubes butted up drew closer to the speaker magnet, that was when the "magic" happened. The fix? Switching to a less bulky speaker, in this case, a Celestion. With the magnet now several inches away from the tubes, the problem was solved.
The reason I mention this is that I see and hear about many techs and DIYer's shoehorning speakers into cabinets where the magnets are adjacent to the power tubes (the Champion 600 comes to mind). Tubes are electrostatic devices, and strong magnetic fields can cause irregularities in the electron flow, particularly in beam-tetrodes, where they affect the focusing nature of the beam plates, in the same way a magnetic field near a CRT screen can upset the trace. Anyone who has a scope can place a magnetic screwdriver up against the CRT face and watch the deflection of the electron beam trace to prove it.
This is just a piece of information to keep in your back pocket if necessary.
Many years ago, I had an issue with a Music Man combo that just sounded "wrong". Could not put my finger on it. FYI, it was loaded with an EV12L, whose magnet assembly was right up against the 6CA7 power tubes. On the scope, it had a subtle, yet ugly distortion of the sine waveform when testing, accompanied by the symptom of serious rounding of the peaks when trying to push it to clipping. To make things MORE confusing, it all disappeared when I removed the chassis for troubleshooting. After messing with it for awhile, I decided to push the chassis into the cabinet while monitoring the sine wave output. As the tubes butted up drew closer to the speaker magnet, that was when the "magic" happened. The fix? Switching to a less bulky speaker, in this case, a Celestion. With the magnet now several inches away from the tubes, the problem was solved.
The reason I mention this is that I see and hear about many techs and DIYer's shoehorning speakers into cabinets where the magnets are adjacent to the power tubes (the Champion 600 comes to mind). Tubes are electrostatic devices, and strong magnetic fields can cause irregularities in the electron flow, particularly in beam-tetrodes, where they affect the focusing nature of the beam plates, in the same way a magnetic field near a CRT screen can upset the trace. Anyone who has a scope can place a magnetic screwdriver up against the CRT face and watch the deflection of the electron beam trace to prove it.
This is just a piece of information to keep in your back pocket if necessary.
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