What size PSU caps for bass preamp?
Posted:
Mon May 19, 2008 4:12 pmby Newbietube
Hi I'm in the process of planning a tube preamp for my bass guitar. However as I've taken the design from a schematic that is for an entire amp I'm uncertain what value of capacitor to use in the psu. I've heard that too much can result in too much bass. The preamp will be running two 12ax7's and my proposed voltage( after rectifying) is 350v.
Cheers for looking.
Cheers Just
Posted:
Mon May 19, 2008 4:47 pmby Geek
Hi,
Assuming you're using a C-L-C filter....
First cap is determined by your rectifier. If you're using a tube rectifier, it will be 10, 22 or 47uF maximum, depending on the tube. If you use a solid state rectifier, 47 or 100uF is OK. Don't forget to bridge SS diodes with a disc cap to keep the fuzz out.
The choke minimum value can be determined by this rule of thumb:
L (henries) = Volts output / current draw in mA
So if you want 350V and let's say the whole thing draws 10mA for four 12AX7 stages (two tubes) + bleeder, that's 350/10 = 35H. Hammond 157G is close enough.
Last capacitor can be another 47uF.
Or you could overshoot the rectified voltage by 50 to 100V with a different transformer and use a C-R-C filter.
Lastly, a regulated supply can be chosen. Probably the best bet here for a preamp.
Cheers!