9 VDC power supply from 6,3 VAC

[larsvictor]

Hello,
I tried to add a 9 VDC power supply circuit in my little champ style amp for effectpedals. It´s a voltage doubler followed by a LM7809 voltage regulator. When I engage the effect unit there is only a loud hum and no effect. I assume this is an issue of wrong grounding. The 6,3 VAC taps have a 100R resistor to ground whereas the LM7809 is directly grounded to the chassis.

Could this be fixed by an additional coupling cap ?
Thanks
Lars

[pdf64]

I suggest you remove the 100 ohm resistors from the winding to 0V.
It's not valid to reference both the ac and dc circuits to 0V in such an arrangement.

[Leo_Gnardo]

I suggest you remove the 100 ohm resistors from the winding to 0V.
It's not valid to reference both the ac and dc circuits to 0V in such an arrangement.

What pdf64 said ^^^.

Another solution:

It may be a good idea to use a separate 6.3V transformer for your pedal supply, then you can 1) leave your amp filament circuit as-is and B) not suffer any switching noise (buzz, hum) from the pedal supply rectifiers, showing up in your audio.

There's plenty of room under the chassis for another small transformer.

[bal704]

I'd try one of the Meanwell 12V transformers. Your effects will run off 12V just fine, so no need to step it down to 9V.

[JMFahey]

Of course, once you remove filament winding center tap you will have strong hum coupled into preamp tubes cathodes.

If not, your power supply ground is not ground at all but floats on top of 3.15VAC, in any case a lose-lose situation.
See it for yourself: the last filter capacitor does not and can not go to ground, but to a 3.15V winding.

So you need a separate winding or a full extra transformer or power supply.