Load Line question

[Groover]

What does a load line show? How do I calculate it?

Please define all terms. (for example Rk = cathode resistor)

Thanks.

[Joey Voltage]

What does a load line show? How do I calculate it?

Please define all terms. (for example Rk = cathode resistor)

Thanks.

I could take a lot of time, and write out a long explaination, but I'm not going to, instead I'm going to pass the buck off here:
The Valve Wizard

My theory, is why do the same work twice when Somebody has already done it, and did it well!

[Old Tele man]

...which?

A) load line for single-ended (SE) amp
B) load line for Class-A pushpull (PP) amp
C) load line for Class-AB pushpul (PP) amp

[Groover]

I have ordered Merlin's book, and will be reading it closely on my trip April 3rd - 12th. Maybe after that I will be a lot more edjumacated!

OK, I have just started reading Chapter 1 online, and it is exactly what I need right now! Thanks

[Wilder Amplification]

Here's one I drew for an EL34 running at a plate voltage of 500V, screen voltage of 250V, and a 10K Zp-p (Zp-p = Impedance from one plate to the other, or "plate to plate"). It shows which is the A line, which is the B line, and where the transition from Class A to Class B happens, which is where the tube is dissipation the most instantaneous power. According to the lines, the load and the bias are set such that the tube never exceeds its max Wa rating (the downward curve you see on the graph), which is the "textboot perfect" design practice.

If you look at this load line you will see that it is biased to 35mA @ 500VDC on the plate. This translates to -

500V x 0.035mA = 17.5 Wa (Watts anode dissipation)

Max dissipation for an EL34 is 25 watts. To find the % dissipation that puts us at -

17.5 Watts/25 Watts = 0.7 or 70% Dissipation

Now you can see where the "70% rule" comes from.



The dot labeled "Bias" is your signal. It is labeled "Bias" because with no input signal, that's where plate current stays. When the tube gets hit with signal, this pushes that dot up the load line as more plate/load current is drawn from the supply and plate voltage drops (plate voltage and plate current are out of phase with each other). As the signal swings positive/negative, the dot moves up and down the load line, first on the A line, then it transitions to the B line at the transition point labeled "AB" on the graph.

So as you can see from the graph, the actual output power is not seen by the tube. Only the load sees it (i.e. the OT primary). The tubes just control load current and in the above example, will never see more than 25 watts at any point in the swing. If you think about it, it's basically a simple adjustable voltage divider (most things in electronics are) that's voltage controlled by an input signal.

In this example, the Class AB transition is happening at mid-swing of the signal. This is also where max dissipation is occuring as well, with the least amount of dissipation happening at the signal peaks.

However, in Class AB you can get away with running a lower load and allowing the tube to cross the "max Wa line" for a portion of the swing (Wa = Watts anode dissipation) since the tube gets driven into cutoff for about 30% of the total signal swing and is theoretically dissipating 0 watts when this happens (matter of fact this is exactly how most guitar amps are ran). This makes it so that the AVERAGE power never exceeds the max dissipation rating of the tube (i.e. the total average power over the full duration of 1 signal swing).

When you raise the screen voltage to guitar amp levels, this will shift the negative grid voltage curves up the graph the more you raise it. In order to know by how much they raise up the graph you have to look at either the screen transfer characteristics or use a curve tracer to find it. The trick with that though is to use a screen resistor that allows the screen voltage to sag enough at full power to pull the curves down such that the Class B load line will cross the zero volt grid curve at or just below the "knee" of the curve.

Of course this chart is for one tube in a push-pull pair. The composite load line for the other tube would be an "inverse mirrored" version of this one since the tubes operate out of phase with each other.

[tubeswell]

I have ordered Merlin's book, and will be reading it closely on my trip April 3rd - 12th. Maybe after that I will be a lot more edjumacated!

OK, I have just started reading Chapter 1 online, and it is exactly what I need right now! Thanks

Bear in mind that what you are reading in Ch1 is about DC load lines. (You need to get the book for Ch2 which covers AC load line)

[JoeM]

There's some good articles here by Norman Crowhurst on load lines:

audioXpress - Track Listings - Audio Classroom Series

[Wilder Amplification]

In the case of preamp tubes, I will also add that load lines are VERY useful for showing what your plate voltage will end up being for a given cathode/grid voltage (grid voltage relative to the cathode = relative to ground cathode voltage but reverse polarity), showing the gain of the stage (i.e. the amount of expected output voltage swing for a given input voltage at the grid) as well as the symmetry of the stage's output at maximum swing. It will also show you what your expected idle plate voltage will be given a known power supply node voltage and grid voltage relative to the cathode.