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This is a schematic for a very simple guitar pre-amp. I understand the basic amplifier architecture except for those two 33k resistors in the middle, combined with the 220nF capacitor. Any ideas as to what those are for?

enter image description here

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    \$\begingroup\$ I'm not real knowledgeable about tube amps, so I'll make this a comment. That appears to be negative feedback to control the gain and linearity. See figure 5.6 for a circuit very much like yours. \$\endgroup\$ – JRE Aug 11 '16 at 15:46
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    \$\begingroup\$ Figure 5.11.1 is another example of negative feedback. \$\endgroup\$ – JRE Aug 11 '16 at 15:49
  • \$\begingroup\$ @crocboy: Do you already follow why the grid leak resistors exist? \$\endgroup\$ – jonk Aug 11 '16 at 18:20
  • \$\begingroup\$ Does this resemble an op amp feedback network to anyone else...? \$\endgroup\$ – ThreePhaseEel Aug 11 '16 at 22:14
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It's (capacitively coupled)negative feedback so the gain is controlled about 66K/1K = 66.

The feedback is applied to the cathode of V1a rather than the grid because it needs to be negative feedback.

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