Paralleled
Tubes V2

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First use the pull down menu above the "Pre-load" button to pick a tube
and then click on the "Pre-load" button to load the tube data and a suggested load,
or just enter all the tube data and tube loading yourself.

Second, click on the "Calculate" button.
Clicking on the "Calculate" button fills the table and plots the plot.


When looking at the output power, remember that our ears hear volume logarithmically. For something to sound twice as loud requires ten times the power (10dB). A 20% difference in power is 0.8dB. That amount isn't a change in volume, but if that power difference is located in one frequency range, it can be a difference in "tone." Keep an eye on the DF (Damping Factor), for that impacts the "tone" or  sound of bass frequencies and impacts how the speaker performs at its crossover's impedance peaks and dips.

The calculator outputs three different bias points for a given load impedance.


On the bottom left, there is a suggested "RDH (Radiotron Designer's Handbook)" Best Load" resistance that will get you close to the maximum output power for the group of tubes being used. I prefer to use something 33-50% higher than this in order to deal with 8 ohm speakers being closer to 6 ohms in real life (i.e. use a 6800 to 7500 ohm transformer instead of the recommended 5000 ohm transformer.) The RDH best load resistance value will change if you change the number of tubes in parallel. To see the new number, you'll have to hit "CALCULATE" again. The "pre-loaded" values automatically display the RDH best load, but if the plate dissipation or number of tubes in parallel is changed, the total load resistance on the bank of tubes will not automatically update. If the recommended RDH value is negative, lower Ib0.  You may also need to adjust Ebmin if you don't like the results.

The DF (Damping Factor) is primary side damping factor that assumes that the output transformer is wound with super conducting (zero ohm) wire on the primary and secondary windings. We can't buy this part yet. As an approximation, when the primary resistance of the transformer is 1/4 the effective plate resistance of the tube, you are at the point of diminishing returns. The effective plate resistance is the Rplate for one tube / the total number of tubes in parallel / any reduction in plate resistance from primary side feedback. 

Reminders:

• The left sloped portion of the purple "Tube 0V Grid" line shows the curve fit for plate's V and I. The curve fit for the "Tube 0V Grid" line must have high accuracy above the Ib1 current, i.e. the dc bias point, for the equations to work. Don't worry about fitting data points for the curve down by the Ib0 current, your curve fit software will help you figure out what Ebmin should be. The "Tube 0V Grid" curve is the plate voltage and current with Vgrid = 0V which defines the tube's plate resistance at this grid voltage.

• After the turquoise colored "Load Line" curve hits the "Tube 0V Grid" line, Ib1 and Eb1 are then shown then on the purple "Tube 0V Grid" plot. 

• Maximum output power occurs when the current swing from where the "Load Line" intersects the "Vgrid=0V line distortion point" to the "Ib1" bias point is equal to the current swing from "Ib1" bias point to the "Ib0" distortion point. This results in having the same output swing before distorting whether the current is going up or down from the DC bias point.

• The "Load Line" should be below the brown "Tube's Power Rating" at all voltages to avoid stressing the tube.

• The plots are calculated with finite voltage steps that change depending on the tube and the bias point. This can cause the plots to not perfectly match the data in the table.