Capacitors.
The other day I worked on a tube audio amp that was very noisy. The amp was just rebuilt and the guy did a fantastic job. But he could not figure out was the noise was coming from so he brought it to me.
TECH TIME…..
We all know that when we install electrolytic capacitors in a circuit we have a polarity to follow. You defiantly do not want to install them backwards or the magic smoke will escape.
So how about non-polarized capacitors? Surly it does not matter which way we install them.
Or does it?
As we know most capacitors are nothing more than two plates with some type of dielectric separating the plates. Sometimes these are rolled up or sandwiched together. When looking at old tube radios you will find a lot of wax type capacitors. Some have a mark on one end but are non-polarized. Most of the time they marked the caps to show a band on the ground end. Other times the caps were switched around and the printing was put on the wrong side. So you cannot always go by the banded end of caps.
INSERT CAP PICTURE HERE
In the tube circuit below we can see several capacitors that are used. We have caps going from low impedance to high impedance circuits. We also have DC blocking. All caps block DC…
Now look at it this way. These caps are non-polarized cylinder type caps. One lead is connected to the outer foil while the other lead is connected to the inner foil. If the caps are installed the wrong way in this low to high Z circuit several things can happen.
Look at the circuit below. I have labeled the caps 1 2 3. Cap 1 is an input. Imagine the ground side of the cap is on the left. This means the left side shields the whole cap and no voltage of signals from the grid of the tube are exposed on the capacitor. Other than the lead from the cap to the grid which we would keep as short as possible, this also keeps the voltage of the grid from traveling all the way to the other side of the cap.
Same with cap 2. The trick here is to keep noise off the circuit. Plus we would not want voltage traveling that close to ground. If the cap has a hole in the outer jacket you can get a nasty burn.
In cap 3 we are taking a low Z and feeding it into a high Z. Again proper placement of the capacitor helps to get the noise out.
INSERT PICK HERE OF TUBE CIRCUIT
So how do we determine the correct polarity of our capacitors when they are not marked from the factory?
It is really quite simple. We will use a scope set on the lowest setting and use a 1:1 probe and our body….
The pic below shows how to connect the probe and how to hold the capacitor. We are using our fingers as a signal source to couple into the cap.
INSERT HOLDING CAP HERE
We will now look at the signal on the scope. Notice the wave form. The scope is set to 2ms per division and we are taking 2 divisions.
INSERT CAPTEST1
Now we switch the leads on the capacitor. Notice when we look at the scope that we see a very small reading. This is because the scope center probe is shielded from the signal out fingers produce. So the scope shows a very small waveform.
INSERT CAPTEST2
So looking at the way the scope probe is connected we would mark the ground end of the cap so we can remember the correct way to install it.
INSERT MARKED CAP HERE
This works with all type of capacitors. Regardless of what type of circuit it is. It was just easier to show you this using a tube circuit. This is especially helpful when building transistor amplifiers. Even silver dip caps need to be installed correctly.
I know there are not many folks here to teach but perhaps someone did not know this….
