It does it immediately upon power up...standby switch is still in the standby position. This is a homebrew 50W NMV Marshall (built around 1999) and I did it a little differently. The first filter node (which is a 50x50 LCR can, both + lugs connected together) is straight off the diodes...then from there it goes to the HT fuse then to the SB switch. When I originally did it...it went from the diodes to the HT fuse and then to the filter then to the SB switch but it blew the HT fuse everytime. Now that the HT fuse is after the LCR can it doesn't blow the HT fuse. The mains fuse only does this maybe 1x out of every 10x power ups...if that. Also, I just happened to have these massive 25A FREDs around and that's what I used for the recifier in this amp. Do the smaller diodes we normally use 'limit' the inrush current at all? Thanks for any help/suggestions.