Using PSU Designer to calculate DC filament supply

[Yvesm]

Originally posted by astouffer
How can DC be noiser than AC?

Huge current peaks in transfos, diodes and caps produces buzz (rather than hum).
Wrong ground returns for thoose peaks could be terrible !

Except in lo level ckts or some DHT, try to stay in AC.

Yves.

[porkchop61]

Originally posted by dhaen


Hum is easily managed by twisted wiring and centre-tapped (or resistively divided) windings, referenced to ground.

I had a question on this. I'm using a PT without a CT for the 6.3v for the first time. The schematic calls for using (2) 50 ohm resistors for the "artificial center tap". Is there any specific reason for using theses values for the resistors? The main reason is that 51 ohms is a common value. I know they do make 50 ohm rheostats for hum balance, but I not to use one.

Thank you.
Glenn

[dhaen]

Glenn,

50 ohms is just a rough value. I've used 100 ohm, and various other values. It's just got to be low enough to tie hum pickup to ground, without dissapating too much heat.
Doing this, or grounding a centre tap, makes the heater wiring into a balanced feed which radiates next to no common mode hum, and the twisting cancels differential hum. Magic..

[porkchop61]

Thanks for the explanation.
Come to think of it, I've also seen 100 ohm resistors on a few guitar amp schematics.

Like I said before, I've never had a hum problem on any of my DIY stuff, but there's always a first time

Glenn

[aletheian]

100 ohm is the universal standard in thousands of guitar amps f they don't have center taps. It is also customary in guit fiddle amps to tie the junction of those resistors to the output tube cathode bias resistors for a slightly elevated reference... although I usually like to go to about 25% of the supply voltage on my reference from a voltage divider from the B+, decoupled with a small capacitor. Never had any hum problems that way. i have used DC in the past and gotten low level buzz, like you fellows pointed out, so I do all AC now.