Does a cathode bypass capacitor reduce hum?

[ThorstenL]

Konnichiwa,

Imagine a cathode follower with an cathode bypass! Does not work.

NO! You are not quite getting it. In the cathode follower you would have to go based on the ANODE not the CATHODE.

In the "ANODE FOLLOWER" (or common cathode) stage the cathode needs to be bypassed (or not if you wish to introduce current feedback).

In the "CATHODE FOLLOWER" (or common anode) stage the anode needs bypassing (or not if you wish to introduce current feedback). .

And yes, in the common Grid stage the grid needs bypassing (or not).

Do cathode followers produce hum, because there is no bypass? Absolutely not!

Absolutely yes, if there is a suitabel hum signal injected into the Anode and if the anode is not returned to ground through an impedance that is low compared to the anode impedance at thehum frequency - guess what, the cathode follower will hum.

To use a cathode bypass cap is a question of local current feedback, influences the gain, impedance and distortion.

Among other things.

It is definitely not a question of hum or not.

Nope, this MUST read:

"It is definitely not just a question of hum or not."

Sayonara

 

[Sch3mat1c]

The thing you people are all forgetting is the simple, bold faced electrical truth.

If you have no input and measure plate voltage with resepect to the cathode return (thus negating ground loop style errors), with a certain ripple on the plate supply and a given amount of hum induced (electrically and magnetically) by the heater supply, you WILL see an improvement in hum rejection with a bypassed cathode. Both induced hum from the heaters (noise and 50/60Hz artifacts) and ripple from the plate supply are reduced when the cathode is bypassed.

If you fail to realize this, well, how about this -- I have some special new aluminum interconnects for you to try. I'm told they sound shiny and give a lightweight soundstage.

Tim

 

[MIKET]

Some Sanity PLEASE

Using a multimeter or scope with no input into the tube do you maesure an AC signal. Is it on the plate supply or is it on the cathode?
If it is on the cathode a bypass cap will help but I much prefer designs that don't need it.
If ripple is only on the plate you need better filtering.
If your gounding is not up to snuff you may be dealing with a ground loop.

 

[Sch3mat1c]

Dude, the WHOLE POINT of the argument was:

You have an amplifier stage.

You measure hum on the OUTPUT without cap

You measure hum on the output WITH cap

The difference proves (yes) or disproves (no) the notion that a cathode bypass capacitor reduces hum.

Why is because a voltage appears on the cathode, yes, but that has nothing to do with the answer unless you want to prove it with algebra. Anything else (heater noise, etc.) is just icing on the cake.

Tim