Exact values for both channels? So what voltage do you now have over each 4k75?
Iko I don't have connected the 100R before the OPT, do you want that I connect to see if the voltage drop is 0,5-01V? also I don't have connect the 100nF input cap, is necessary?
Right channel
137V plate
0V across grid stopper
9,86V cathode positive to ground
over 4k75 64,4V
Left
125V plate
0V across grid stopper
9,47V cathode positive to ground
over 4k75 73,4V
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It helps you measure/monitor the current through the tube very easily. While you're debugging your project, I recommend that you do it, just to make sure you have the right bias on the tube.
Ok, we're at 13-15 mA so there is something about the bias. It should be 5,5 mA at 135 V B+ when grid is 10 V below cathode. Try go down to 3k3 istead of 4k75
And, just to be sure, measure voltage between grid pin and cathode pin (the one with the 10R on)
And, just to be sure, measure voltage between grid pin and cathode pin (the one with the 10R on)
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When you come back, could you draw a picture like I did, exactly with the circuit that you have, and with all voltages that you measure on it? One channel only.