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Last night I put together the RC-coupled stage above with 400V B+ voltage. It works way better than expected. With roughly 2V RMS input I get 30V RMS output with (as far as I can see) no distorsion. But there is something that puzzles me. I took the output and fed it to a scope a friend of mine let me borrow. But when I connect the output from the cd-player to channel 2 on the scope, I noticed that the signal is inverted, or maybe out of phase with 180 degrees. Is this normal???
Mikael Abdellah said:
Yes. A common cathode stage will invert phase.
A tube is a tranconductance device, so the current flowing through it is proportional to the voltage between the grid and cathode. With the circuit setup the same as the resistive loaded one, with a B+ of 400V, there is approx 4mA flowing through the tube, and -4V grid to cathode. The anode is therefore at about 212V.
If you apply an input signal of a 2V peak to peak sinewave, as the voltage increases up to the +1V peak, the current flowing in the tube will increase to 4.5mA and the Vp will go down to 188.5V. Remember Ohms law: an increase in current through a resistor means the resistor has more voltage across it, so there is less voltage across the tube. So as the input voltage increases, ie, becomes more positive, the output voltage at the anode goes down in proportion, so the phases invert in this type of stage.
Same basic theory applies to FETs, and similarly (but a bit different) to transistors.
Cheers
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