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Symptoms of leaky capacitors?

6/26/2005 3:47 PM
Carlo Pipitone
Symptoms of leaky capacitors?
Hello guys.  
This is time for dumb questions! ;)  
What are the symptoms of a leaky capacitor?  
I mean, when should I check a coupling cap for leakage? How does a leaky cap sound?  
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6/26/2005 8:15 PM
Wild Bill

Carlo, if a capacitor goes leaky severe things happen. You will have bigger worries than a change in tone.  
If the capacitor is a filter in the power supply it will leak current. Hopefully before it gets really bad it will blow fuses.  
If it is being used as a coupler from one stage to another and starts to lead it will allow plate voltage from the previous stage to hit the following tube's grid, causing it to turn full on and try to burn itself up!  
These two examples are probably 99% of the situations in an amp circuit.  
So don't worry about it. No need to go checking. If you get leaking caps you will know for sure! :(  
---Wild Bill
6/28/2005 5:04 AM

Hi Carlo  
No dumb questions, just another dumb answer, LOL (just kidding!).  
Scratchy controls and red glowing outputtubes are very common results of leaky caps.  
Regarding the sound of leaky caps: I don't care :D an amp has to be functioning good.  
In the pre-amp you can check for dc-voltages after coupling- and tonestack-caps.  
6/28/2005 5:19 AM
Carlo Pipitone

Bill and Chris,  
thanks for replying.  
I didn't think that scratching noises in a control pot can be due to a leaky cap. I can imagine that it's a different scratch than the usual "dirt noise", maybe a sort of "electric scratch"...  
6/28/2005 5:37 AM

Hi Carlo  
You can compare it to a 5k-lin-presence-circuit in a Marshall (altough that one is designed that way).  
Dc-voltage on a pot can be heard if afjusting that pot, it's sometimes hard to recognize the kind of scratch but if cleaning doesn't help and the pot isn't worn-out most of the time the problem is due to leaky caps.  
Have a nice day!  
6/28/2005 11:56 AM
Any leaking cap lets the bias on the following stage be wrong.  
For 12AX7 stages run with a grounded grid and an elevated source in the common self bias circuit, if the grid is more positive than about 2mV, the previous stage's coupling cap is leaking.  
To check a coupling cap for leakage, pull up the low voltage end, leaving the high voltage end connected to the previous plate. Measure the free end voltage to ground across a 1M resistor. If the voltage is over 2mv, the cap is leaking, and should be replaced.  
Leaky caps sound like a capcitor with a resistor in parallel with them. That is, there is no way to tell other than whatever effect the bias shift has on following stages. This is not one you can debug by ear.
6/28/2005 5:18 PM
Dr. Photon

Leaky coupling caps will misbias the following stages, leaded to bad sounding preamp stages, and worse, melting output stages. if they lead to a tone stack, gain control, or other stage that invlves pots and/or switches, they'll add some "snap Crackle, and POP" to the amp. I've found that amps with leaky coupling caps would have extremely scratchy pots (after cleaning), and the amp would making popping noises, scratchy noises, and otherwise misbehave. To check, measure the voltage from the grid of the next preamp tube to ground, should be around zero. Do the same on the top of the tonestack and gain controls. You can also measure to ground if you have a cathode biased output stage; with fixed bias, you would have to measure accross the bias resistors. In every case that I've dealt with, the amp sounded much MUCH better after replacing the caps (guitar AND especially HiFi).

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