hello, i have issue with my fender twin reverb, it is a 1972 or 73 not sure but no push pull button..when i bought it it was converted to a pre-cbs blackface and shoved into an original dual showman head sounds good of course.there is a hum coming from the reverb when it is cranked past 3..i took it to an amp tech..he told me that the hum is caused by the location of the reverb tank being close to the chassis...and the twin was not meant to be in such tight enclosure..it was designed to be in a 2x12 cab or 4x10 super reverb usually located on the bottom of the cabinet ..although he is a very good amp tech and he went through it and he could not find any other reason for it ...i just want to get a second opinion..i know there are some rare original twin heads. why those don't hum ?