Bridge Recto

[kurtlives]

Have a tube project that use a 24V centre tapped transformer. I am using a bridge rectifier to convert the AC to DC.

Should bridge rectifiers be used with a centre tapped transformer? Could this be why I have hum in my PSU?

Thanks...

[brett]

Hi
there are many causes of hum, but a centre-tapped secondary isn't like;y to be one of them.
Sometimes, a cheap transformer will physically vibrate and gives mechanical hum.  My JTM60 Marshall does this.

How much filtering are you doing?  For a power section, you'll want at least two 1000uF caps in parallel per amp of current drawn.  For preamps, you'll want a regulated supply.  That's at least 100uF of electrolytic filtering, a regulating IC, then a few uF in parallel with 0.1uF of metal film filtering.

Use a DC-blocking cap (ie any cap) in series with your DMM to check how much AC is coming out.  1V is probably ok for a power section, and 0.01V for a pre-amp.   
cheers

[kurtlives]

This is a two 12AU7 pre-amp. Starved plates more or less...

The heaters have their own separate voltage source.

Right now I have 500uF of filtering, the unit dosent draw much current.

So how do I measure the AC. Have a cap in series with my DMM..I then set my DMM to what setting and where do I measure?

[brett]

Hi
I suggest that you measure the AC voltage between B+ and ground.  Don't forget that any half-wave rectifier need twice as much filtering as a full bridge rectifier.  Are you half wave rectifying one of the secondaries and grounding the other?

Also, there is a fair chance that hum is comming from the heater supply.  In my experience it is always best, and fairly simple, to use an LM7812 to supply DC 12 V to pre-amp heaters.

How are you connecting the ground between the two power supplies?  If using a bus rather than a star ground system, it is important not to connect a large ground current system (e.g. power supply, heaters) upstream of a sensitive small current system (e.g. preamp).  Star grounding with FAT wire solves those problems.

You might want to look at the Real McTube power supply.  It uses the heaters as a voltage divider to obtain a virtual ground.  You could connect your centre tap to that, which would give a nice, solid ground.  Then bridge rectify, filter, regulate and filter after that (470uF, LM7824/7830, 22uF ?).
cheers