Quote:
Originally Posted by mpegjohn
When the Mullard datasheet for an EL34 says Ra-a 6600 Ohms is that the ac resistance? So an output transformer with a 6600 ohm primary will halve this then. Or is this value what the load resistance should be, and you need to look at the graphs to get the ac anode resistance.
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John, Ra-a is the speaker load as seen by the output stage valve anodes operating in push-pull arrangement. The speaker impedance is transformed by the OT to a plate-to-plate impedance that the valves pull their operating current through. The output transformer is usually defined by its PP primary impedance, and its speaker impedance - as that is just a ratio determined by the turns ratio of the transformer, the designer can use it in any way they want, and the variation in speaker impedance means you have to be mindful not to pedantically assume that only that constant impedance has to be designed for.
A PP loadline refers to just one valve in a push-pull configuration, so the mode of operation dictates the slope of the line being overlaid on a valves plate characteristics. As indicated, many books like RDH4 go through the process and understanding. Modern links sometimes go through the design process with actual voltage and current levels like:
https://robrobinette.com/Drawing_Tube_Load_Lines.htm
Note that loadline design typically doesn't concern itself with the inherent anode resistance parameter Ra, as the design process starts with the assumption of a target Raa. Some datasheets just show a single Raa value in amongst tables of operating valves - that Raa value is just a recommended value (a benchmark value that is known to be a good compromise). Some datasheets show characteristic plots of the influence in choosing a particular value of Raa for a PP output stage - often the distortion and max output power curves of the stage are plotted, and they often show that the typical Raa value used for that valve aligns with the output stage achieving its maximum capable output power, and having near its minimum distortion level.