Seppo
Super Member
Thank you everyone for your help and suggestions. I figured this out and it was the wireless router.
The router was originally located something like 50 cm from the pre-amp, on the same plane so that the unshielded tubes saw the router across the amp. In this photo the white router is on the right and pre-amp on the left on top of the amp:
Now, turning off the router removed the crackling completely (even with the barely audible crackling w/ stock tubes). Then when I turned the router on, the crackling started exactly and only when the router started 2.8 GHz wifi (i.e. not when it was negotiating the base cable connectivity). So nothing wrong with the tubes.
Moving the pre-amp to the left, and the router on the floor, removes the crackling even with the Voshkod's, giving enough shielding and distance:
Interestingly 2.8GHz and 5 GHz are in the 6cm to 11cm wavelength range, i.e. in the ball park of the physical dimensions of these unshielded tubes. And seemingly Voshkod's are more sensitive to the interference. I have not measured, but I think they are a bit taller than the default ones, being closer to the wifi wavelengths.
The router was originally located something like 50 cm from the pre-amp, on the same plane so that the unshielded tubes saw the router across the amp. In this photo the white router is on the right and pre-amp on the left on top of the amp:
Now, turning off the router removed the crackling completely (even with the barely audible crackling w/ stock tubes). Then when I turned the router on, the crackling started exactly and only when the router started 2.8 GHz wifi (i.e. not when it was negotiating the base cable connectivity). So nothing wrong with the tubes.
Moving the pre-amp to the left, and the router on the floor, removes the crackling even with the Voshkod's, giving enough shielding and distance:
Interestingly 2.8GHz and 5 GHz are in the 6cm to 11cm wavelength range, i.e. in the ball park of the physical dimensions of these unshielded tubes. And seemingly Voshkod's are more sensitive to the interference. I have not measured, but I think they are a bit taller than the default ones, being closer to the wifi wavelengths.