Useful and/or obscure information:  
   
New Approaches to Improved Tonal Reproduction, Pt 1
db Aug 73
1, 2, 3, 4, 5
Tube Types for Audio Use
Radio & Television News Nov 56
1, 2, 3, 4
New Transformer Design for Power Amplifiers
Radio & Television News Nov 56
1, 2, 3, 4
Presto Recording Corp Amplifier Model 41 A schematic 1