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Load Line Assistance Needed

This is my first post on this forum. Fitting for a tube noob, lol! 

For my first question/problem, I need to be pointed to clear and concise info on how to plot a loadline for a tube. In particular, I have 12AX7, 12AU7, 12AZ7, 6K6, and 6V6 tubes around some of which I plan on building a guitar amp. I've been to several websites and done Google searches, but haven't found anything that truly cleared up the mud with regards to plotting load lines. My first BIG question is plate voltage. Do you use the unloaded B+, or measure it under a DC tube load?

Thanks in advance for any helpful replies!
Hi dtbradio

Welcome to the forum.

First, a caveat: Designing guitar amps is not rocket science although there is math that we use to do certain things and one can take that to whatever extent pleases yourself. There is no "right" or "wrong" way to do things for the most part, and with tubes there are no wrong circuit values particularly in the small-signal circuits. For output stages, you need to be a bit more careful as there are higher currents and the highest voltages in the amp present. There are no "magic"numbers. Whatever sounds good to you without smoke erupting is okay.

In general, you rarely need load lines to design a preamp stage as most tubes have look-up tables with a plethora of values, giving the performance at different common supply voltages. Load lines are more useful for output stage design.

To draw a load line you need the plate curves for the tube in question. Each curve is at a fixed grid-to-cathode voltage and curves up from the bottom right and heads towards the top of the graph, illustrating how much current the tube will conduct as plate voltage is swept from zero.. Multiple tests provide a family of curves for the tube, and this is what you see in data sheets.

The load line is drawn with a straight-edge hinged at the supply voltage along the x-axis and going up to the maximum current the load would allow were the entire supply across it. The slope of the load line is the resistance of the load and is straight because we are assuming a resistive load. Even were the load inductive, you would do the "first approximation" as if it were resistive.

The load line will be at an angle to all the plate curves and there will be a point at higher than half the B+ across the load that will be the optimal idle point for symmetric plate signal swing. The grid voltage of the curve near that intersection is used to determine Rk. First, you see what the voltage is across the load and use this value and Vg to calculate Rk. Setting the plate voltage to Va/2 or a bit above will provide the best sustain and highest headroom, as TUT shows.

Note that many tubes still have data listed according to an outdated guideline that took into account the manufacturing variation AND a guess as to the variations in the user's circuits. There is alos often an assumption that an inductive load will be used. Therefore, you see 12AX7 listed at 300Va max even though it has an arc-over voltage rating >540V. Similarly, any power tube has to sustain over twice the listed voltage to work properly in the application circuits. For example,6V6 has a low data sheet rating of 350Va but is used in TV circuits with 1500V pulses. 6K6 is similarly rated at 315Va and 1200V pulses. To handle these conditions reliably, both tubes have to handle 100V or more over these values.

Assuming you are building an integrated amp, the PA is designed first so that the PSU and output stage are matched. From there, the maximum possible plate voltage is known and you use a standard proportional divider style supply for the preceding stages. The successive RCs provide quieter voltage to the stages as you move towards the input stage. The RCs are selected a bit arbitrarily at first, simply to provide this progressive filtering effect, and later you can tweak values to make changes to gain stage voltages as suits the tone you want. Cs are typically 10uF minimum, with 22uF being the most common.

As a stand-alone preamp, B+ may be determined primarily by what PTs are available to you at reasonable cost. Generally, a tube preamp sounds best at 160V to 400V. Higher voltages tend towards a brighter tone with greater headroom for clean sounds and easier sustain for distortion sounds. However, high voltages require more careful layout to avoid oscillation. Lower voltages tend towards a smoother sound for both clean. ANY voltage environment can be used to achieve ANY tone provided you are willing to think outside the limited framework of "what Fender did" or "what Marshall did".

TUT discusses gain stages in a general fashion, with tips for improving tone. TUT2 has a brief look at load lines and provides the look-up tables for common tube types to give a quick start to your project. TUT5 expands on preamp design using traditional equations, but only goes so far as reality is often quite different than mathematical prediction.

TUT2 delves into PA design, but TUT5 provides an easy precis of the process which generally is quicker and more to the point. TUT6 goes back into deeper power amp design, using both traditional and our alternate methods.

TUT3 should be the first TUT-series volume you buy if you want to build an amp, as it shows the correct way to lay out and wire everything for lowest-noise and best note articulation, using a universally-applicable approach that can be applied to all technologies and all construction methods.

Have fun
very HELPFUL and detailed reply, thank you VERY much!!!! I at least have the mud out of one eye now, lol!

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