Absolutely!!mat wrote: Is it normal the bias 'voltage' to go from 40mV to 220mV when feeding the amp with 1khz sine wave ?
Here's what bias is mat: When a tube sits there with 450 volts on the plate there's no place for the voltage and it's current to go. It just sits there. If we put a wiggle signal on the grid some electrons may flow from the grid to the plate but not much. Remember that all the current used by a tube flows from the plate to the cathode. The reason for this is that the cathode is made negative with respect to the plate because it's grounded. In the case of self bias it's grounded just a little above DC ground with the cathode resistor and AC grounded for a certain band of frequencies by the bypass cap, thus making it just a little positive of ground. Or in your case with a fixed (variable) bias supply from a tap off the main DC supply connected directly to the grid of the tube. In any case we have to make the grid negative with respect to the cathode so that when we supply the the wiggles to the grid it will act as a variable gate to let electrons flow from the cathode to the plate. If the grid were positive with respect to the cathode, it would stop all electron flow through the tube. (and also burn up, but that's not important right now) By using the variable bias knob you set grid just negative enough that the output tubes will have a certain predetermined amount of current pass from the cathode past the grid to the plate with no signal at all. This is called bias. It is used to put the tube in a conductive state where it is in a straight part of the conducting curve. Take a look at the plate characteristic curves in the tube manual.
So anyway, all of the current in a tube flows from plate to cathode. When you bias at say 68ma on a pair of EL-84s you are putting the tube on that conducting line in it's straightest part.
Now when you apply a signal to the grid, it swings (as AC waveforms do) from more positive and more negative FROM IT'S CENTER. When this happens there is a voltage drop across the load resistor, or in the case of an OT current flows first in one direction and then the other through the windings of the primary of the OT. On the other side of the OT core is the secondary winding. It's attached to a speaker or dummy load, where the power is consumed as noise or heat.
Now. A pair of 6L6GC tubes, running at max plate dissipation will deliver 210 MA or so to the OT and across to the load. That's why the bias points go to 200ma. You are simply measuring the NORMAL operation of the tube. Capeesh?
I've printed and read loads of info about tube amps but somehow it just doesnt to seem to stick to my brains 
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