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10-09-2019, 08:39 PM
Hi liquidair
That is an interesting set of observations.
First, the currents in a push-pull stage will look half-wave rectified, but add together in the OT as a complete sine wave, so that part is normal. FW rectified on one side would be odd as that would mean that it is not just flyback energy being expended in the 'off' tubes, but maybe you meant FW rectified considering both halves of the circuit?
Ideally, the idling tubes should appear like current sources, although being tubes, they will be far from ideal CSs. Still, I would think that the tube's conduction is essentially set by the bias condition and it should only conduct the idle current despite the change of voltage on the plate, just as the plate curves show. There might be something amiss in the layout or signal leakage around the pots? Do you see/hear this effect regardless of the bias method?
I've built several 4-tube output stages with signal level controls in place and do not recall hearing the distortion you cite, using mixes of 6L6, 6V6 and EL-34. To me the 34s always sound a little distorted.
Regarding the relay coil snubbing. The zener is used across whatever CONTROLS the coil. Say the coil is switched by a mechanical switch. The zener would be placed across that switch. With the switch open, both ends of the coil are at +12V. Close the switch and you see one end of the coil go to zero volts as expected. Open the switch now and see the end of the coil fly up above the supply but be clamped at the zener voltage minus the supply voltage, then return to the supply voltage.
That is an interesting set of observations.
First, the currents in a push-pull stage will look half-wave rectified, but add together in the OT as a complete sine wave, so that part is normal. FW rectified on one side would be odd as that would mean that it is not just flyback energy being expended in the 'off' tubes, but maybe you meant FW rectified considering both halves of the circuit?
Ideally, the idling tubes should appear like current sources, although being tubes, they will be far from ideal CSs. Still, I would think that the tube's conduction is essentially set by the bias condition and it should only conduct the idle current despite the change of voltage on the plate, just as the plate curves show. There might be something amiss in the layout or signal leakage around the pots? Do you see/hear this effect regardless of the bias method?
I've built several 4-tube output stages with signal level controls in place and do not recall hearing the distortion you cite, using mixes of 6L6, 6V6 and EL-34. To me the 34s always sound a little distorted.
Regarding the relay coil snubbing. The zener is used across whatever CONTROLS the coil. Say the coil is switched by a mechanical switch. The zener would be placed across that switch. With the switch open, both ends of the coil are at +12V. Close the switch and you see one end of the coil go to zero volts as expected. Open the switch now and see the end of the coil fly up above the supply but be clamped at the zener voltage minus the supply voltage, then return to the supply voltage.
