amp and unwanted speaker vibration

I like to seek opinion and advice from  Audiogon members.  When I connect my tube  amp (Lectron HJ50) to  my speakers ( Focus Audio Model  78) , unwanted vibration of speaker cone would occur.even  with all other equipment off.  The simultaneous vibrations from both speakers  would start to occur very rapidly and  consecutively   after I turn on  the amp for about 30 min.  However, when I plug the  amp power cord to a Belkin home theater surge protector 8 outlet extension cord (from Radio Shack), the vibration would start to occur in about 60 min, with initially only about 4 slow consecutive vibrations, then 5-9 slow vibration every 5-10 minutes.  This has been observed repeatedly.  So I think there is something wrong with my amp (it is more than 20 year old), although its performance is otherwise fine.   As the vibration  is delayed and reduced significantly by using the surge protector extension cord to supply the electricity to the amp,  I wonder if my electricity source is a problem. Would more expensive (presumable better) AC power cords on sale at Audiogon  solve the problem.?