Preamp Gain Stages: the Cathode Resistor. GAIN, TONES & THEORY!!!

A deep dive into the cathode resistor in a 12AX7 gain stage. What it's purpose and what impact do different values of cathode resistor have on gain and tone. This video examines the 2nd gain stage using my DDS40 clone, where a 3 way structure switch brings in 3 different cathode values. We also examine what cathode bypass caps are all about. Lots of theory here with dreaded load lines! But hopefully presented in an understandable way.


0:00 Play in - 10k cathode
0:31 Play in - 3.2k cathode
1:04 Play in - 2.5k//0.68uF cathode
1:36 Intro
2:48 The role of the cathode resistor
7:55 The 2nd gain stage in a 2204
9:23 Voltage readings
11:16 Plate load lines
14:15 Cathode load lines
18:51 10k cathode operating point
22:59 3.2k cathode operating point
24:54 Cathode bypass caps
30:02 Oscilloscope readings
33:27 Play out
33:52 Conclusion
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