2 questions . How do i test decoupling caps and my output for wattage

[blindboybenton]

Hi

First day on here and second post already

Im trying to learn about valve amps, I have a question about decoupling caps
I hear engineers talking on here about changing old decoupling caps. My question is " How does a faulty/leaky Decoupling cap affect the sound? and how can i test them. I have a scope and signal gen

Last question

I would like to test my amps to see what wattage they produce. Ive got some 4ohm and 8ohm 100watt resistors ordered for the dummy load
Whats the best way of testing the amp or any other amp i have ?

Many thanks
bassman1965

[Tom Phillips]

...How does a faulty/leaky Decoupling cap affect the sound? and how can i test them...

If a coupling cap is leaking DC then that DC can upset the normal bias on the next stage and prevent it from operating as designed. It could sound distorted, weak or even damage the circuit in extrema cases. See the thread at http://music-electronics-forum.com/t33420/ for a discussion about how to test the caps.
Cheers,
Tom

[Joe L]

Welcome to the forum!

Electrolytic caps loose the ability to filter so one of the signs they are getting bad is increased hum from decreased capacitance. Their internal resistance ESR goes up so when called upon to deliver large currents, they won't deliver which causes the amp to lack "punch" and low end.

You need a capacitance meter to measure the value but leakage can be approximately measured with just a voltmeter. To measure leakage, put a 1K resistor in series between the (-) of the electrolytic and its ground connection. Set your meter to millivolts, turn on the amp and give it a moment to warm up, measure the voltage drop across the 1k resistor. If it is still dropping, wait for it to bottom out. This reading will actually be a direct readout in uAmps (see Ohms Law). Use the chart on page 22 of this link to determine if the cap is leaking too much.

ESR is a bit harder to measure but substituting a good capacitor in parallel with the one under test will let you hear if it is making a difference sound wise.

To measure power, connect a proper impedance dummy load and inject a 100mV 400 Hz tone into the input jack. Connect your scope to the speaker output. Turn the Master to full and slowly bring up the preamp volume until the waveform just hits clipping then back off just until it isn't clipping. If you have a true RMS voltmeter, now would be the time to use it. Otherwise, measure the Pk-Pk voltage on the scope. Don't keep the tone on for very long or you could overheat things.

Watts = E²/R = (P-P/2.828)²/R

example:

P-P = 30v R = 8 ohms

Watts = (30/2.828)² / 8 = 14 watts rms

I think my math is correct. I'm sure someone will correct me if I'm wrong.

[Tom Phillips]

Opps. I answered about "coupling" caps rather than "decoupling" caps. That recent discussion just popped into my head.
Hopefully, you will find that information useful too.
Cheers,
Tom

[Joe L]

Opps. I answered about "coupling" caps rather than "decoupling" caps. That recent discussion just popped into my head.
Hopefully, you will find that information useful too.
Cheers,
Tom

I almost did the same thing Tom.

[Jazz P Bass]

It helps to be able to identify which caps are filtering the raw rectifier voltage & which ones are actually decoupling the signal from the supply.

[blindboybenton]

Hi Jazz bass

Well lets say the " decoupling the signal from supply " I hear engineers saying that they changed them. My question was how can i test them to say they are faulty/leaky
If they are faulty, how will that effect the sound

cheers

bassman1965

[Joe L]

Well lets say the " decoupling the signal from supply " I hear engineers saying that they changed them. My question was how can i test them to say they are faulty/leaky
If they are faulty, how will that effect the sound

Hmmm... asked and answered.

[Jazz P Bass]

Hi Jazz bass

Well lets say the " decoupling the signal from supply " I hear engineers saying that they changed them. My question was how can i test them to say they are faulty/leaky
If they are faulty, how will that effect the sound

cheers

bassman1965

If the first stage decoupling cap is bad, you can actually measure the Vac ripple.
As to the sound , it sounds bad because the signal rides on the voltage (it is not decoupled)

[Enzo]

It is one thing to be analytical, it is another to be practical. If you want to know if the decoupling/filter caps are not doing their job, take another cap, power down, connect the new cap in parallel with the suspect, and power back up. Now, does having the new cap clipped in make the problem disappear?

In other words, instead of exhaustive testing of the part, we use simple substitution to see if it solves the issue. If it makes no difference, the cap is probably not the problem. If it does make a difference, then the cap we suspected must be bad.

[blindboybenton]

Hello

Many thanks to you all. Had time to read all the posts and replies. Now i think i know what sort of faults a bad decoupling cap can cause and how to test

I did also post another tread yesterday can any of you fine engineers help me on this one.

http://music-electronics-forum.com/t33574/

[blindboybenton]

Sorry joe

On reading your earlier post i can see you answered my question

many thanks

bassman1965