Smoothing cap question

[Steve Blackdog]

Evening all,

My brain seems to be on strike.

Please can you help me with the capacitance of smoothing caps for my power supply for a guitar preamp I am building (12ax7x 2)

The power transformer has a heater secondary, which I am ok with. However, my brain is letting me down with regards to smoothing the HT supply.

The transformer starts with 93vac, which I am rectifying and doubling with 2x 1N4007 diodes and 2x 1uf 400v caps, which gives 267vdc / 2Amps.

I am in the UK so my power supply frequency is 50Hz at the wall.

I want to keep my B+ as high as possible, so my plan is to use two relatively low impedance resistors (2 or 3W) and two electrolytic capacitors to smooth out the ripple.

For some reason my calculations suggest either 68 or 82uF/400v caps. Could that be correct?

What about resistors? Can I go much lower than 1k ohm? Could I substitute the first resistor for 220kH inductor?

Any help unblocking my brain will be very much appreciated!

Many thanks

steve

[loudthud]

Does this look like what you are trying to do ? It seems like the 1uF caps in the doubler are holding you back. Ripple is low but B+ is low and it takes 25 to charge up.

[Steve Blackdog]

Thanks, loudthud

if I went for something along these lines (see schematic from Bristolwatch with 93vac rather than 120vac) with 220uf caps on the doubler, or a halfway house, say 100uf, then 50-100uf smoothing caps would probably work. Does that make sense to you?



cheers

steve

[loudthud]

120VAC gives you lots of options. You could limit all the caps to 47uF, and drop some Voltage in the filtering chain (resistors 1K to 10K) to get ripple really low.

Note ripple is 1.47mV !