B+
Supply
Node B - Screens
Node C - Phase inverter
Node D - Preamp 2
Node E - Preamp 1
B+ along the chain
| Resistor | Current | Drop | Power | Node V |
|---|---|---|---|---|
| R1 1 kΩ | 18.5 mA | 18.5 V | 342 mW | 421.5 V |
| R2 10 kΩ | 6.5 mA | 65.0 V | 423 mW | 356.5 V |
| R3 10 kΩ | 3.5 mA | 35.0 V | 123 mW | 321.5 V |
| R4 22 kΩ | 1.5 mA | 33.0 V | 49.5 mW | 288.5 V |
Engineering Notes
How it works
Valve amplifiers feed their stages from a chain of RC filters. Each resistor drops the voltage a little further and, with its capacitor, filters ripple and isolates stages from one another (preventing low-frequency oscillation called motorboating). The key detail: each resistor carries the current of its own node plus every node downstream.
I_Rn = Σ I_k (k ≥ n) V_n = V_(n−1) − R_n · I_Rn P_Rn = I_Rn² · R_n
Typical guitar amps run the power valve plates from node A (reservoir), screens from B (via a choke or ~1 kΩ), the phase inverter from C, and the preamp stages further down where the supply is cleanest. Changing any current upstream shifts every voltage below it, which is why a change in the screen supply can alter preamp voltages.