So the bargain amplifier you picked up at the local "invitation only" garage sale seems to have it's own idea as to what sounds you are entitled to hear. It has more snaps, crackles, and pops than a bowl of cereal on a Sunday morning, and a constant, annoying hum that could be used as an effective torture device on prisoners of war. What do you do? Well, if you read this column last month, you will have already addressed the most temperamental part of a tube amp, the tubes, and will have replaced any that were worn out, and had the bias set correctly. To make any further changes however, you will need to go to work inside the amp. Now you are in dangerous territory.

     To those of you who are not familiar with working on tube amps, this is dangerous equipment even if unplugged as they can store lethal amounts of voltage. Actual work should be left to qualified persons who know and understand this kind of equipment and the proper safety precautions needed. The material in this article is provided for informational purposes only and should not be considered a license to go poking around your amp.

     The first thing to do is to make the amp safe to work on. This is accomplished by finding and draining the filter capacitors which is where the voltage is stored. Unplug the amp, remove the chassis from the cabinet and place it on the workbench with the tubes sticking up in the air and away from you. You can leave the standby switch on. Using the same Fender Deluxe Reverb from last month as our example amplifier, locate the filter capacitor cover. This is a 4-inch by 6-inch rectangular tray bolted to the underside of the chassis. Before you remove the cover and the four screws that hold it in place, remember why it is there - for your protection! With this cover removed, the only thing preventing you from being shocked is your awareness. Don't get careless! With that said, remove the cover. There should be five large, cylindrical objects soldered in place. These are your filter capacitors and are used to filter out the hum that is produced by the AC from your outlet that has been "rectified" or converted to DC. When they go bad they can cause your amp to lose power, blow fuses and cause you to go crazy by creating an obnoxious hum that slowly removes your will to live.

     If you look at the filter caps (look, don't touch) you will notice that there is a lead coming out of each end. Since these caps are polarized, one lead will be positive and the other negative. If you have the chassis on the bench as per the earlier instructions, you should be facing the positive end of the filter caps. This is where we will be discharging the caps.

     There are a couple of different ways to effectively drain the electrical charge from the filter caps. Since we already have the them exposed, we will use the simplest method. Get a long-handled screwdriver with an insulated handle. Touch the tip of the screwdriver to the positive lead of one of the caps. Lower the handle of the screwdriver until the shaft touches the chassis. Do Not Let Any Part Of Your Body Touch The Amp While You Are Doing This! If you hear a zapping noise, that is the sound of the capacitor discharging. If you hear nothing, then the capacitor is already discharged. Repeat the same procedure for each filter capacitor. Just because the first capacitor was discharged does not mean they all are. Treat this part of the amp with the utmost respect.

     Most of the original filter caps that Fender used, looked like brown cardboard. Of course, there are exceptions. If your amp has the original caps, chances are that they aren't in very good shape. An obvious sign that they are deteriorating is if a bubble has formed on the end. Sometimes it is subtle, sometimes quite obvious, other times it has actually burst open and some white stuff has leaked out. The cap can be bad however without these external signs. If you do have a bad cap, it is best to replace them as a set. Since a set of filter caps will last approximately ten years, you won't have to worry about this part of the amp for a long time.

     Removing and replacing the caps are quite easy. Just unsolder each lead on the cap and remove them from the amp. If the lead won't come free, snip it off with some wire cutters. When you put your new caps in, you will want to take note of a few things. First, the caps have a value listed in micro farads, and a voltage rating. Always try and replace the cap with one of comparable value and voltage rating. The value can vary somewhat, usually without a noticeable difference. For instance, the Deluxe Reverb came with 16 mfd. caps rated at 450 volts. Considering the original caps were manufactured with a 20% tolerance and that over time they drift even further, you can end up with a capacitor that is way off specification. At times we have substituted 20 mfd. caps when the 16 mfd. caps were unavailable and found that they worked fine with no adverse effects. However, you would never want to go to a lower voltage rating such as 300 volts as you would eventually destroy the cap, and possibly other parts of the amp as well. It would be perfectly acceptable to go to a higher voltage cap. Secondly, remember these caps are polarized and must be reinstalled the correct way with the positive and negative leads soldered to the correct location. In the case of the Deluxe, this is pretty easy since all the caps are going in the same direction. Most other Fenders, however, are not set up this way.

     The more common filter cap arrangement as used in the Twin Reverb, Showman, Bandmaster, Super, etc., is to have the first 2 caps (usually 70 mfd.) wired in series and then followed by three 20 mfd. caps. The tricky part is that because of the series wiring, the first cap is wired backwards. That means that on the first cap, the negative lead is towards you and the remaining caps have the positive lead towards you. The advantage with wiring caps in series is that you double their voltage handling capacity. Two caps rated for 350 volts and wired in series are now capable of handling 700 volts. This makes the power supply much more capable of handling the initial voltage from the rectifier when the amp is first fired up. A word of caution - don't try and get tricky and install this configuration in your Deluxe. It won't work without rewiring the power supply.

     Ok., so your new filter caps have been meticulously installed with careful consideration as to the polarity, value, and voltage rating, and your amp has been re-assembled and is ready for a test run. What can you expect?

     Your amp should be quieter, tighter, more reliable, and maybe even a little punchier. Most noticeably, the obnoxious hum will be gone or greatly reduced. Of course, those seasoned musicians with hearing that shames most dogs can always pick out the faintest of unwanted noise, but we'll take care of that in our final stage of electronic polishing.