Triode gain stage
One triode preamp stage, with the parts amp designers use to shape it. Switch in a small or partial cathode bypass, caps at the grid and plate, a treble peaker, a bright cap on a volume, or a cathode follower after the stage. Change any value and see the frequency response, the distortion, and where the distortion starts.
Swipe sideways for the whole drawing. Tap a part to jump to its value. The red numbers are DC voltages to ground.
Gain from the source to the point you picked, small-signal, every part in place, tube capacitances included. The grey curve is the stage with the tone parts off (the cathode bypass and load stay). A sweep shows standard values around yours, lightest for the lowest.
Output, two cycles
outputinput times the small-signal gain at this frequency
Harmonics
evenodd dB below the fundamental, 2nd to 10th
All values
Other parts
What the parts do
A triode stage’s gain comes from its plate load and from how much of the cathode resistor’s signal gets bypassed to ground. Everything else here shapes that gain across the frequency range.
A cap that cuts treble always works against a resistance: the grid shunt and plate-to-grid cap against the source and stopper, the plate cap against the plate resistor and the tube’s plate resistance, the next grid’s shunt against whatever feeds it. A resistor in series with any of them turns the roll-off into a shelf. A cap across a resistor in the signal path does the opposite: the treble peaker and bright cap let treble around it.
The bass is set by the coupling cap against the load after it, and by the cathode bypass. A small bypass cap restores full gain only above the frequency where its reactance drops below the cathode resistor. A resistor in series with it limits how much gain it restores.
Grid resistor. The source here has no resistance of its own, so the grid needs no resistor to ground. Switched in, it makes a divider with the source and stopper. 1 MΩ under 68 kΩ costs about half a dB.
Feedback to the grid. A resistor from the output back to the grid feeds signal back upside down, against what comes in. It works against the resistance in front of the grid, here the source and stopper. The gain falls toward the feedback resistor over that resistance, and distortion falls with it: with 1 MΩ and 68 kΩ a 12AX7 stage drops from about 60 to about 12. From the plate it needs a cap to keep DC off the grid, and the feedback fades below where that cap’s reactance passes the resistor. From after the coupling cap no cap is needed, and the coupling cap sits inside the loop.
Warnings. Red text flags grid current in the stage or in the follower, the tube nearly cut off at rest, or the solver not settling.
How this tool works. We solve the stage with the circuit bench’s engine and tube models: the operating point, then the small-signal response at 161 frequencies from 20 Hz to 20 kHz. The sine runs in time until the caps settle, and the last two cycles are drawn. The 1 % THD level comes from rerunning the sine at different levels until it hits 1 %. The next grid’s capacitance is a value you set. The next stage isn’t solved. To see the operating point on the plate curves, use the load line tool. We don’t model the supply sagging with the signal, or wiring capacitance.
Spot an error, or want a circuit added? Email info@swellamps.com. A schematic or a clear photo of one helps. Anything you send may be used to improve these models and our desktop app.
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