How to tell if coupling caps are conductive?

[frank57]

Is there a way to tell if coupling caps might be the source of hum?

[jhadhar65]

Do they know the tune?

The main issue with coupling caps is DC leakage, which is why they're there for the most part.  Tone shaping is another reason, but neither consideration 'should' contribute hum.  Lead dress, on the other hand, is very culpable.

[billcreller]

If any B+ is leaking through from the previous stage to the next grid it will hum. I think you could try checking for that on the grid. (?)

[Fresh_Start]

If any B+ is leaking through from the previous stage to the next grid it will hum. I think you could try checking for that on the grid. (?)

Scratchy control pots may be another hint that there's DC where it shouldn't be.

Chip

[EL34]

Coupling caps can leak DC
The only way to get an accurate reading is to lift up the cap leg on the non high voltage side and then measure to see if there is any Dc leaking through from the high voltage side.

If you try and measure the cap with both legs soldered down, the reading will be off
The side you are trying to read is usually connected to ground via a resistor or pot.
This will not give you a proper reading.

You may read less than a volt with the cap in circuit and 10 volts with one leg lifted up

[Fresh_Start]

Thanks for that tip Doug!

Chip

[eleventeen]

THIS is an instance where a scope is very nice to have.