Formula Used
Effective load: single ended load = primary load. Push pull per tube load = primary plate to plate load / 4.
Load slope: slope = -1000 / effective load. The result is shown in mA per volt.
Cutoff intercept: cutoff voltage = plate voltage + idle current × effective load.
Current intercept: zero volt current = idle current + plate voltage / effective load.
Plate dissipation: plate watts = plate voltage × idle plate current.
Screen dissipation: screen watts = screen voltage × screen current.
Clean peak swing: use the smaller value from swing to cutoff and swing to knee voltage.
Output power estimate: watts = peak swing² / (2 × effective load) × transformer efficiency.
Target idle current: target mA = max plate watts × target percent / plate voltage × 1000.
How to Use This Calculator
Choose the output stage mode first. Use push pull for a pair of tubes. Use single ended for one power tube.
Enter measured plate to cathode voltage. Then enter the idle plate current in milliamps.
Add screen voltage and screen current. These values help review screen dissipation.
Enter the transformer primary load. For push pull, use the full plate to plate rating.
Set the 6L6GB plate rating and target percentage. Then press the calculate button.
Read the result above the form. Use CSV or PDF export for amplifier notes.
Understanding 6L6GB Load Lines
A 6L6GB load line shows how plate voltage and plate current trade places through an output transformer. It turns tube data into a working design map. The line starts near the idle point. It crosses the voltage axis at cutoff. It crosses the current axis near saturation. These points help you judge swing, power, and stress before you build.
Why the Idle Point Matters
The idle point is the quiet operating point. It is set by plate voltage, bias, and plate current. A cooler point improves tube life. A hotter point can reduce crossover issues. The calculator compares idle plate dissipation with the selected rating. This makes the setting easier to review. It also estimates screen dissipation, because screen stress is common in guitar amplifiers.
Choosing the Load
The reflected load controls the slope. A higher load gives a flatter line. It usually lowers current swing. A lower load gives a steeper line. It can raise power, but stress also rises. For a push pull stage, each tube sees one quarter of the plate to plate primary impedance during the basic estimate. This page applies that conversion automatically when push pull mode is selected.
Power and Headroom
Available swing is limited by cutoff on one side and the knee region on the other. The smaller swing becomes the clean peak swing. From that value, the calculator estimates audio power through the chosen load. Real amplifiers also depend on transformer loss, supply sag, grid drive, and screen behavior. Use the result as a design guide, not as a warranty.
Reading the Result
A result is balanced. Dissipation stays within the chosen target. The cutoff voltage is reachable. The saturation current is not excessive. The recommended bias current gives another reference. It helps compare warm, moderate, and conservative setups before changing parts.
Practical Use
Start with values from a known 6L6GB circuit. Enter the real plate voltage measured from plate to cathode. Enter the idle plate current, not total cathode current, when possible. Add screen current separately. Then compare the safe current range, the load line intercepts, and the estimated power. Export the result for bench notes. Keep final checks on the actual amplifier with proper safety steps.
FAQs
What is a 6L6GB load line?
It is a straight design line drawn on tube curves. It shows the relationship between plate voltage, plate current, and load impedance.
Is this calculator for fixed bias or cathode bias?
It can help both styles. Fixed bias users can review grid bias. Cathode bias users can use the estimated cathode resistor as a guide.
Why does push pull divide the load by four?
In a basic push pull estimate, each tube sees one quarter of the full plate to plate primary impedance during its load line calculation.
What plate rating should I use?
Many designers use 19 watts for a 6L6GB. Check the actual tube data and choose the rating that matches your part.
Why is screen dissipation included?
The screen grid can overheat even when plate dissipation looks acceptable. Including screen watts gives a better safety review.
Is the output power exact?
No. It is an estimate. Real power depends on transformer losses, supply sag, drive level, tube condition, and waveform limits.
What is knee voltage?
Knee voltage is the low plate voltage region where the tube stops behaving linearly. It limits clean downward voltage swing.
Can I use this for 6L6GC tubes?
You can change the plate rating and values. However, 6L6GC ratings differ, so do not keep 6L6GB defaults blindly.