Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: frank57 on September 09, 2010, 01:25:28 pm

Title: How to tell if coupling caps are conductive?
Post by: frank57 on September 09, 2010, 01:25:28 pm

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

Title: Re: How to tell if coupling caps are conductive?
Post by: jhadhar65 on September 09, 2010, 10:44:53 pm

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.

Title: Re: How to tell if coupling caps are conductive?
Post by: billcreller on September 10, 2010, 04:21:50 am

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. (?)

Title: Re: How to tell if coupling caps are conductive?
Post by: Fresh_Start on September 10, 2010, 09:25:52 am

Quote from: billcreller on September 10, 2010, 04:21:50 am

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

Title: Re: How to tell if coupling caps are conductive?
Post by: EL34 on September 10, 2010, 02:53:59 pm

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

Title: Re: How to tell if coupling caps are conductive?
Post by: Fresh_Start on September 10, 2010, 08:44:49 pm

Thanks for that tip Doug!

Chip

Title: Re: How to tell if coupling caps are conductive?
Post by: eleventeen on September 10, 2010, 08:54:25 pm

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