Tweet
It been about 10 days and nobody commented on your scope photo's so I'll give it a shot. The top one looks like the line frequency (50Hz?). The wave shows distortion and normally you see some coming off of a transformer winding but I would expect less than what is shown unless you are using a really old overloaded transformer. The source for hum like this is usually the heater supply either coupling thru layout to a signal wire, an ungrounded center tap or a bad tube.
The middle trace looks like the noise on the high voltage center tap. Those little lumps are the charging currents to the filter caps. Note the frequency is twice the line frequency. Look at the sawtooth (3rd trace). When the voltage is rising, current is flowing thru the rectifier and charging up the filter cap. When the voltage is falling, the cap is discharging and no current is flowing in the rectifier. I've drawn red lines to show how these two signal should line up.
Poor grounding is usually how the little lumps on the HV center tap get into the signal path. It's hard to say what is the best way to get rid of the little lumps and many people will disagree. I think the problem starts when you ground the HV CT to a transformer bolt. I prefer to run the HV CT directly to the ground side of the first filter, then from the same point, run wires to whatever else is grounded. Since you are using a zener to drop the B+, and you just want to ground the zener to the chassis, we have a problem. I have edited a copy of the schematic to show how I would wire the ground.
The middle trace looks like the noise on the high voltage center tap. Those little lumps are the charging currents to the filter caps. Note the frequency is twice the line frequency. Look at the sawtooth (3rd trace). When the voltage is rising, current is flowing thru the rectifier and charging up the filter cap. When the voltage is falling, the cap is discharging and no current is flowing in the rectifier. I've drawn red lines to show how these two signal should line up.
Poor grounding is usually how the little lumps on the HV center tap get into the signal path. It's hard to say what is the best way to get rid of the little lumps and many people will disagree. I think the problem starts when you ground the HV CT to a transformer bolt. I prefer to run the HV CT directly to the ground side of the first filter, then from the same point, run wires to whatever else is grounded. Since you are using a zener to drop the B+, and you just want to ground the zener to the chassis, we have a problem. I have edited a copy of the schematic to show how I would wire the ground.
Attached Files
Comment