by paart » Thu Aug 20, 2015 9:28 pm
I can’t answer why the change was made, but the original Dynaco units used a 1.35K precision resistor in most of the production units. In the very late amps, it was changed to 1.65K. Why? Who knows? I could only guess.
The actual cathode resistance will always be slightly lower than the actual resistor value, since it’s in parallel with the (R10) feedback resistor. The feedback resistor is connected to DC ground through the secondary of the output transformer.
The exact value of this cathode resistor isn’t critical.
Increasing the value of the cathode resistor will increase the “headroom” of the input stage, and decreasing it might cause the tube to operate in a more linear region, more of the time.
But then, a change in it’s value will also have a slight effect on both the positive and negative feedback loops…………..