The 5763: Ia = Ik = 50 mA. So it is conducting hard: it is 'bottomed'.
LH 12AU7: grid @ -143 v.; anode @ + 140 v. Ia = 2.5 mA. But with grid @ -143 v., valve should be hard 'off': something not right there. Suspect possible defective valve.
RH 12AU7: grid and cathode at same potential. So no -ve bias on grid. Therefore valve is hard on. Hence anode @ - 143 v.
But where is the - 150 v. @ grid coming from? It isn't coming from V508, etc., since as we go from V508 to grid of valve in question, voltages become increasingly -ve. Suspect this valve has a grid / cathode internal short.
Conclusion. Try a replacement 12AU7 - and then let me know what happens.
HTH,
Al.
LH 12AU7: grid @ -143 v.; anode @ + 140 v. Ia = 2.5 mA. But with grid @ -143 v., valve should be hard 'off': something not right there. Suspect possible defective valve.
RH 12AU7: grid and cathode at same potential. So no -ve bias on grid. Therefore valve is hard on. Hence anode @ - 143 v.
But where is the - 150 v. @ grid coming from? It isn't coming from V508, etc., since as we go from V508 to grid of valve in question, voltages become increasingly -ve. Suspect this valve has a grid / cathode internal short.
Conclusion. Try a replacement 12AU7 - and then let me know what happens.
HTH,
Al.
