Bench tool
Negative Feedback Resistor Calculator
Size the global feedback resistor for a target dB of feedback, taking your amp's open-loop gain into account — or check what an existing Rf/Rk pair actually delivers.
Solve for Rf
Rk is the PI (or input stage) tail/cathode resistor the feedback signal is injected across. Rf runs from the OT secondary tap back to the top of that resistor.
Open-loop gain is the gain from the feedback injection point to the speaker output with the loop disconnected — measure it directly if you can (disconnect Rf, drive the PI input, compare speaker-out to a reference), or estimate it from the stage gains in your schematic. A typical guitar power amp runs somewhere around 30–150 open-loop, and most designs land on 3–8 dB of feedback.
Check Existing Feedback
Already have Rf and Rk values (yours or from a schematic)? See how much feedback that combination actually delivers.
More feedback tightens bass response and lowers output impedance (higher damping factor) at the cost of headroom and can push a marginal design toward instability/oscillation at high frequencies if the loop isn't compensated — a small cap across Rf is the usual fix, not covered by this calculator.