Load Line Exercise...How to ?

[ms660]

Just an exercise, hypothetical schematic, triode amplifier and a pentode amplifier, both valve types are known, both have resistors for their anode loads, both use cathode bias, resistor values are known as is the HT supply voltage, no other voltages given on the schematic.

Need to find the operating point as designed for both valves, done the triode using anode and cathode load lines.

Can't get anything to tally when trying to do the pentode...why?

Lawrence.

[ukvrshaun]

Usually the Anode Current v. Anode Voltage plot for a Pentode is for a particular Vg2 voltage. For other values of Vg2, the Vg1 lines will be either stretched or concertinaed in the vertical direction. You can guess the appropriate Vg1 lines for a particular Vg2 using the Anode Current v. Control Grid (Vg1) plot (Vg2 is the parameter).

See this article which explains how to do that:
http://valvewizard.co.uk/pentode.html

Then for the design intended value of Vg2, the operating point can be guessed from the load line, as per the triode method. It will probably require iteration to get to the design values.

You could also do an LTspice simulation using the resistor and HT values you have. The 'correct' voltages and currents will 'fall out' of that simulation and will thus given the pentode operating point.

Shaun

[ms660]

Looked at that, can't get a cathode load line to tally up....Why?

Lawrence.

[ukvrshaun]

Should work, assuming the data sheet is correct and the values on the schematic are correct! The Vg1 lines do move up or down quite a lot for different values of Vg2.

I can probably LTspice it for you if you let me know the details...

[Diabolical Artificer]

What pentode Lawrence? One other thing occurs somewhere at the back of my noggin, when drawing a cathode LL the mu of the valve has to taken into account. Also that you have to include the screen grid current in your calculations. These effect the slope of the line. Have you looked at Radio Designers?

Andy.

[ms660]

Originally Posted by ukvrshaun

Should work, assuming the data sheet is correct and the values on the schematic are correct! The Vg1 lines do move up or down quite a lot for different values of Vg2.

I can probably LTspice it for you if you let me know the details...

I still can't get it to work for a pentode...intersection of anode and cathode load line...can't get the cathode load line to work out.

Eg:

ECC83, Ia/Va characteristics, I can do the anode load line for a given value of anode load resistor and a cathode load line for a given value of cathode resistor, where the two intersect gives the operating point, no problem.

EF86, Ia/Va characteristics, I can do the anode load line for a given value of anode load resistance, cathode load line is no go, nothing makes sense, eg: in your linked example (valve wizard) try marking the point on the curves for the 680 ohm cathode resistor on the -ve 0.5v curve then the -ve 2.5v curve as the reference points for drawing a cathode load line, I can't get it to work so what's going on? It works for the triode example, everything fits no problem.

Maybe my method is wrong?

Lawrence.

[ms660]

Got it..wood for the trees...constant current.

But.....still, how can a pentode cathode load line be plotted using the manufactures valve curves?

Lawrence.

[ukvrshaun]

Different Vg1 lines for different Vg2 (300V, 250V, 200V) for a KT88 pentode with same load line.

Different Vg operating points (-26V, -18V, -12V) for the same anode voltage (300V)...