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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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6.6k is a recommended load impedance, for a particular bias point. The actual anode impedance of an EL34 will be much higher, but this does not lead to a loss of signal - you cannot peer inside a Thevenin equivalent circuit and make deductions about losses in there. Most datasheets will give the anode impedance for a particular bias point too, but this will not necessarily be the same bias point as for recommended load. The graphs will tell you.