• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

drawing a loadline from schematics

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Something is really annoying/confusing me. I am trying to understand a couple of SET schematics. One is the 6V6HiFi from Boozhound Labs. I am trying to work backwards from the schematics. It should be quite trivial shouldn't it?

Take the OPT of 5K and plot a 5K loadline on the 6V6 datasheet. But what value of B+ to use? The schematic shows 306V at the plate. But doesn't the 5K of the OPT cause a voltage drop? If so, then at 36mA, the voltage drop would be 180V, so isn't the actual B+ at 306+180V = 486V? If this is the case, then I can plot the schematic's 18V cathode bias at 36mA.

If this is the case, then doesn't the PSU need to be designed to produce 486V when loaded - measured at the PSU, prior to the Plate resistor / OPT?

As I attempt to understand these designs, the clouds of ignorance are slowly dissipating.

As stated 5K is AC impedance, not DC resistance. When plotting transformer or inductor loaded stages it's confusing at first because the AC signal actually swings higher than B+(by a lot). Plotting it works like the following.

1)Draw a reference load line: Take the B+ and divide by 5k(transformers AC impedance) and draw the line. This line IS NOT the actual loadline, but is just a reference for the slope of the actual load line to come.

2) Find your DC operating point: Take B+ then subtract voltage loss from transformer DC resistance and cathode resistor (if it is using one). Place a dot at the operating point.

3) Draw actual load line: Draw a line that parallels the reference load line, but do it so that the line runs on top of the dot you placed (DC operating point).

Steve Bench has a good illustration about loadlines on his website.
Look under "Technical reports"
OK, I got it! I actually get a voltage drop of around 3V for the DC resistance of the OPT, so it is negligible and lost in the relatively crude plotting of loadlines and probably also when manufacturing variations are accounted for.

I noted from S Bench's site that Ia must not exceed the max listed in the tube specs. I cannot find where it actually says Ia, although I guess that they mean this when they say "zero-signal plate current" ???

If this is the case, the loadline that I plotted exceeds this by 8mA, assuming that I drive the tube with 30V p-p. Of course, this will depend upon the operating conditions of the driver.

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