# Why does a cap in front of a choke input suply reduce rectifier current? How to calculate optimal value?

As I was designing the power supply for my tube amp, I was trying to reduce the negative transients of the choke input supply, I tried to add a capacitor in front of it (C1 in the schematic).

It reduced the transients, but also reduced the peak current of the rectifier. I tried experimenting with different values (in the Duncan's PSU designer) and it looks like 1uF is better than other common values.

Rectifier current with the cap 1nF (effectively no cap):

Rectifier current with 1uF:

Rectifier current with 2uF:

I guess the capacitor is compensating for the choke current (power factor?). Anyone knows the proper theory why does this happen? Also, does anyone know a formula that can be used to calculate the optimal capacitance?

I tried this with Multisim too and saw similar results.

• What do you meanly "negative transient"? I can't see any. Also although the capacitor is reducing the peak at the middle of each cycle it increases the positive leading edge transient. It would be easier to see if you just showed 1 or 2 cycles rather than ~10. A choke input filter is often recommended because it is more gentle on the rectifier tube. – Kevin White Jun 15 '15 at 1:57
• @KevinWhite I meant the PIV across the tube when this power supply is with lower load (which would happen in case of a failure of one of the output tubes or if the rectifier warmed up faster). The cap increases the positive leading edge transient but reduces the peak (no cap - 0.55A, 1uf - 0.45A). – Pentium100 Jun 15 '15 at 2:56
• Those graphs tell you nothing. To determine that something has reduced a full peak-to-peak view is needed. – Andy aka Jun 15 '15 at 7:42
• @Andyaka, well this is a current trough one rectifier, so it is either zero or what is shown in the graph. – Pentium100 Jun 15 '15 at 10:08
• I would like to know what RMS current you get in these situations, I think it would be mostly the same for all cases. – akaltar Jun 26 '15 at 15:56