I’m now back in business. Building a new 4P1L PSE output stage so will reuse the 300B/4P1L Flexible PSU. I never managed to post an update on the troubleshooting I had to do to get this HT PSU to its optimal state.
The output voltage was lower than expected and the 50Hz component extremely high. Something was wrong. So I traced the issue down. I found a bad solder in one of the rectifier’s cathode. The supply was operating in half-wave mode.
The supply is choke-input with 6AU7 rectifiers (hybrid bridge with FRED rectifiers). The transformer is custom made and has multi-taps for 300-400 and 500V. The tuning capacitor for the choke input is 470μF, then choke is 2.5H into 50uF oil cap. The filtering stage per channel is 20H + 100μF Oil caps;
Here’s a test of the supply at 330V/60mA per channel. It’s very rewarding now to see no 50Hz component and that the ripple at 100Hz is just 4mV (ignore the mA typo on the image):
