Tubescreamer + Switchmode question

[markusw]

Hi all,

I have built a tubescreamer according to the GGG PCB layout. With a 9V battery everything is fine. But when it is connected to a Switchmode power supply I get a rather high hum, much higher than 50-60 Hz (actually like a beeeep).

To get rid of this hum I added a 100 Ohm R to the cathode of the power jack and a 100 µF cap (I did not have a 1000 µF cap) between the cathode and the anode as suggested by Craig Anderton. Great, the hum was gone.

BUT: the hum was also gone when I just added the 100 Ohm R.

So my question: how is this resistor able to block the hum/beep??? :?:  :?:  :?:

Markus