The handful of sums that come up over and over on the bench — each one live, each one loaded with a real amplifier value to start from. Change a number and the answer updates as you type.
Enter any two of voltage, current, resistance, and power; the other two fill in.
Resistance accepts shorthand — 100k, 1M, 4k7. All four are sizes, not polarities: a −55 V bias rail is entered here as 55.
Two resistors from a supply down to ground; the tap sits between them. R1 is the top (supply-side) resistor. A negative supply is entered as it is written — a −55 V bias rail divides to a negative tap — but a resistor is a size.
With the grid returned to ground, the cathode sits as far above ground as the grid sits below it. Give the target cathode current and that bias voltage. Either sign names the same 1.5 V: a datasheet letters the bias grid-to-cathode (−1.5), a meter on the cathode reads it the other way up (1.5).
Each amp's parts list on this site records the cathode resistor that was actually fitted; this is the design target before tube spread and real-world tolerance. Browse the circuits to see the values in context. To work the other way — start from a resistor and find the current and bias it produces in a real tube — use the load line explorer.
A coupling capacitor and the following stage's grid-leak resistor form a high-pass filter. Below the corner, bass rolls away.
f₋₃dB = 1 / (2π · R · C). Cap accepts 0.02, 0.02mfd, 22n, 470p. mfd is the tube-era word for µF. Unsure what the "−3 dB" in that name means? See the units & notation guide.
Primary and secondary impedances set the turns ratio, and the turns ratio squared is the impedance ratio. Enter any two.
The primary impedance a tube actually wants is a load-line question. Put the number into the load line explorer and watch what it does to the swing available at the plate.