Background Hum/Breeze/pink/white-noise

[Paul Birkeland]

The 22uF at 400V is OK.

Start by connecting the negative end to terminal 1, and the positive end to terminal 2, then run the amp. 

[Viktor]

I checked using the shunt capacitor. On terminal 1 and 2 I thought that some of the higher frequency "breezing" disappeared using the capacitor. There was still a very low hum which may have been a low 50hz tone. It was hard to determine.
Then I checked using the same cap at terminal 4 and 5 and the same thing was noticed but in the left channel.  What does this tell us? As I understand the capacitor removes the higher frequencies which makes sense but what new information has been learned here?

As a side note I also poked around the inside of the amp, the chop-stick test, and could not produce any noise by poking there.

[Paul Birkeland]

On terminal 1 and 2 I thought that some of the higher frequency "breezing" disappeared using the capacitor.

Is it gone, or still about the same?

[Viktor]

I would say that it almost disappeared. I talked to a friend who is experienced in DIY audio and he was surprised there weren't any capacitors placed there by default? He also suggested adding a grid-stopper before the first tube to reduce any small capacitances inside the tube.

[Paul Birkeland]

I talked to a friend who is experienced in DIY audio and he was surprised there weren't any capacitors placed there by default? He also suggested adding a grid-stopper before the first tube to reduce any small capacitances inside the tube.

I'd recommend listening to the amp with the capacitor installed, then giving your friend a hard time.  (There will be nearly no sound out of that channel)  Solid state equipment and some push-pull tube amps will have a very small cap to ground from the plates, but this is to prevent oscillations well above what we can hear. 

Grid stoppers don't reduce the capacitances inside the tube, but they do help with oscillations concomitant with high Gm tubes.  (The 12AU7 is low Gm)

[Viktor]

But if you use a capacitor with lower capacitance on the nF scale you could cut out frequencies above for example 50kHz. With the one I tried (22uF) with any frequencies above 45Hz would be cut away if I understand correctly?
This was done by calculating the time constant based on the resistance of 22kOhms.

[Paul Birkeland]

If you're hearing the noise, and you cut it out with a capacitor, you will also be cutting out the signal at those frequencies. 

[Viktor]

Of course but I think it is safe to say my hearing does not go to 50kHz... 

Anyway I'll try it. Otherwise what do you suggest doing about the background noise?

[Paul Birkeland]

You may consider contacting a tube dealer regarding an ultra low noise 12AU7.