6SL7 load line

Reading the chart

Each grey curve is the plate current at one grid voltage as the plate voltage changes. The plate resistor allows only the currents on the DC load line, from the full supply at zero current up to the current with the tube as a short. With cathode bias the tube also has to sit on the cathode bias line, where the grid is below the cathode by the plate current times the cathode resistor. The operating point is where the two lines cross.

For audio, the coupling cap puts the load in parallel with the plate resistor, so the signal moves along the steeper AC load line through the operating point. An unbypassed cathode resistor adds to its resistance. Swinging up, the grid starts pulling current near 0 V. Swinging down, the plate current falls toward cutoff. Whichever comes first sets the clean headroom. Where the curves bunch up or spread out along the line, the stage distorts.

Warnings. Red flags the plate over its dissipation rating, grid current moving the bias, the tube nearly cut off at rest, or the solver not settling.

The 6SL7

An octal high-mu twin triode (mu about 70), found in amps from before the 9-pin tubes.

Model. 6SL7 (RCA): a Koren-equation model, fitted to the RCA plate curves (0 to −7 V grid, up to 2 mA) and its point 250 V/−2 V (2.3 mA), curves within about 3 V rms. Near 0 V on the grid, the GE, Sylvania and JJ charts need 10 to 27 V less on the plate for the same current and read 4 to 12 % more gain: the 6SL7 (GE, JJ) preset. It stands for the type, not any one tube. Real tubes spread 20 % and more.

Dissipation. 1 W per section.

Starting stage. 300 V supply, 100 kΩ plate resistor, 1.5 kΩ cathode resistor bypassed by 22 µF, 22 nF into 0 Ω, a 68 kΩ grid stopper, and 1 V peak at 1 kHz. It’s a common setup for the type, not a copy of any one amp.

How this tool works. The plate curves use the circuit bench’s tube models, Koren-equation fits to each type’s datasheet curves and published operating points. We solve the stage with the bench’s engine, every part in place, interelectrode capacitance and grid current included (a diode law, about 5 µA at 0 V). The sine runs 40 ms (at least 12 cycles) so the caps settle and any bias shift shows, and the last cycles are drawn. The headroom figures walk the AC load line with the caps as shorts and no grid current, the way you’d read a load line by hand, and the sine follows it at 1 kHz. At 40 Hz a small coupling cap stops being a short and the path opens into a loop. At 15 kHz the tube’s capacitance works against the grid stopper and the gain drops, though the path stays near the line. For what bypass and bright caps do to the response, use the gain stage tool. Supply sag, heater and hum effects, noise and the spread between real tubes aren’t modeled.

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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