We have found the 300B tube to be among the most problematic of all the power tubes in regards to microphonics. You can find excellent, inexpensive damping instruments for the 300B tube at Herbie's Audio Lab.
Herbie's Audio Lab
Warm regards,
Steve Herbelin
Herbie's Audio Lab
Herbie's Audio Lab
Warm regards,
Steve Herbelin
Herbie's Audio Lab