Dropping 50V on psu

[eeyore]

Hi, need help in dripping 50V on my tube PSU. It is bridge with damper diodes followed by RCLC. the R is 50R. Output voltage is 556V on PSU Designer. Transformer is 420VCT, and has been custom wound and don't want to change. Loading is 36mA for a 5K tube transformer with a 45 tube.

Best way to drop 50V whilst keeping the impedance low. I tried changing the RC to around 470R and that did it, but alternatives?

[bob91343]

You could use a zener diode. More complex methods are possible but it depends on how important this is.

[eeyore]

Would like to avoid transistors, as I have used a design where there is no silicon.

I tried adding and extra LC stage with a 15H/411R and also reducing the first stage RC filter to 2.2uF and that seem to do the trick.

Does reducing the first or input cap to a lower value have any major implications.

[bob91343]

No harm except you may have more ripple in the output. In fact, it's easier on the rectifier with lower peak energy during conduction.

[eeyore]

Yeah that is what I was reading, so with a 2.2uF cap, there certainly a lot of choices as well. Was thinking of a Russian PIO K75 or a Mundorf Supreme or Silver/Oil. I am using ASC 50uF after that, any recommendations?

[AndrewT]

Hi,
with an excess of voltage you can use resistors rather than expensive inductors in the filtering.

I would be tempted to simply add an extra RC filter and drop sufficient voltage across the two series R.
The C supplying current to the amplifiers stages should preferably be good audio quality. Film or better film or best film maybe even foil rather than metallised.
The first C or any not supplying amplifying stages can be any HV electrolytic or preferably cheap film cap.
Use a film cap across the primary and across the secondary for some RF attenuation.

[DF96]

As you only want to drop 10% reducing the first (reservoir) capacitor will do it. You might need more smoothing after it. The peak charging pulses will be smaller but longer, so potentially less problems with induction into signal circuits.

This trick can't always be used for a bigger reduction, as electrolytics don't like being continually charged and disharged - you need special ones for that.