Dentron/Amp Supply changed over the years - but I was not aware of the changes that you noted.
The eHam.net reviews talk about some differences between the 2500 and 2500Bhttp://www.eham.net/reviews/detail/1440
BAMA has the schematics for bothhttp://bama.edebris.com/manuals/dentron/mla2500/
The MLA-2500 is a heavily modified amplfier due to its usage fo the now discontinued Eimac 8875 tube.
B.N. "Bob" Alper - Svetlana Application Note on conversion to Russian 4CX400A tube (tetrode)http://www.svetlana.com/docs/TechBulletins/technoteNo14.html
Tom Rauch, W8JI has some comments (proper use of tetrodes) regarding Mr. Alper's articlehttp://www.dxer.org/w8ji.html
W4EMF mod to Russian GI-7BThttp://www.nd2x.net/w4emf-MLA.html
WB8WJU conversion to the Russian GS-35bhttp://www.nd2x.net/WB8WJU.html
BTW, after a large number of legal cases - as of 2003,
New Sensor/Electro Harmonix/Sovtek is the new owner of the Svetlana trademark.
The former owner of the Svetlana is now called SED (Winged C logo).