What most of us see as "Diff Amps", in this circuit are actually "Current Mirrors".
Pretty common in the op amp world, and you're right, they just don't look right in a tube circuit....
David McGown wrote:
Walt,
Still, unless the inputs are cap coupled to the preceding stage, the +/- grid inputs need to be at 0VDC. The cathode will be at a slightly positive voltage, with the 220K resistor to the -300V supply (resulting in 1.33mA current draw for both sections). So, in the first section, the plate for the - input is tied directly to +300V plate supply, and the + input has a 220K plate resistor. Lets assume that the grids tied together is an error, since the cathodes ARE tied together. So how does the circuit work from a DC perspective and keep the inputs at 0V? My first thought was that the schematic is in error. The circuit would be in DC balance IF there is a similar 220K plate resistor for the (-) input side, which means that both sections would have 150V drop through the plate resistors, 150V across the tube, the input grids at 0V. Both sections would have the same operating point (given matched sections). However, I did check with other schematics online, and am chastised that the schematic you posted IS correct for the Philbrick K2-W. I am stumped on what is going on.
David