I’m recapping a small preamp and want to replace the selenium rectifier. This is on an analog chime system that has small pickups on little chime bars that are struck by solenoids. The preamp is built around a 12AX7 tube. I finished the recap and verified all the resistors are in tolerance.
The transformer AC secondary is 120v and the rectifier is configured as a voltage doubler - so there are two rectifiers. The output voltage was 240VDC before changing out the seleniums. This yielded 106VDC on the plate of the first state and 148VDC on the plate of the second stage.
With 50 ohm 5W resistors in series with the diodes the output voltage was 322VDC. So I changed to 200 ohms and the output only dropped to 318VDC with plate voltages of 138 and 190 on first and second stages. I then went to 300 ohms and the voltages only dropped another volt.
Any suggestions? Everything I’ve read suggests that the series resistance shouldn’t have to be so large and it doesn’t look like further increases are going to help much to bring down the voltages to previous levels.
The transformer AC secondary is 120v and the rectifier is configured as a voltage doubler - so there are two rectifiers. The output voltage was 240VDC before changing out the seleniums. This yielded 106VDC on the plate of the first state and 148VDC on the plate of the second stage.
With 50 ohm 5W resistors in series with the diodes the output voltage was 322VDC. So I changed to 200 ohms and the output only dropped to 318VDC with plate voltages of 138 and 190 on first and second stages. I then went to 300 ohms and the voltages only dropped another volt.
Any suggestions? Everything I’ve read suggests that the series resistance shouldn’t have to be so large and it doesn’t look like further increases are going to help much to bring down the voltages to previous levels.