What does that little capacitor soldered to the volume pot in my amp do? And why are some of those little caps hooked up to a switch? Let’s get started by looking at what these little caps do, why they are there and how you can swap them with different values to help tweak your amp the way you like it.

The little capacitor that is soldered to the hot lug and wiper lug of your amp’s volume control is called a “bright cap” or treble bypass capacitor. That little capacitor helps your amp retain its bright clear treble response when the amp is played at a lower volume. The human ear is not as sensitive to treble frequencies at lower volumes and the bright cap allows treble frequencies to flow through the cap. As you turn up the volume control the bright cap has less and less effect and with the volume control on “10″ it has no effect at all.

If you look at P1 on the layout you will see C1 with a value of 47pF. This cap is connected to the hot lug which is the lug on the right and the wiper lug, the lug in the middle. This value can be altered to adjust the amount of treble that is allowed to flow at lower volumes. We will get into that a little later in the article.

If you look at P2 on the layout you will notice that the bright cap is connected to a switch which is connected to the hot lug and wiper lug of the potentiometer, P2. This switch removes or engages the bright cap into the circuit such as on a Fender blackface or silverface amp. It is the same as having a bright cap connected directly to the potentiometer as in the P1 example but it is on a switch.

Click on layout to enlarge.

Both of the bright caps in the P1 and P2 example can be varied to tweak the amount of treble to your personal taste. This is a very easy mod to do on your amp. You just need a soldering iron, needle nose pliers and some small value capacitors. If you decide to experiment always make sure your amp is unplugged from the wall outlet and it is a good idea to discharge the filter caps. You can discharge them by having the amp unplugged from the wall and switching the power and standby switches to the on position. Make sure you have a speaker hooked up to the amp. After about 5 minutes or so the caps should be discharged. At this point you can remove the bright cap and start experimenting. The smaller values will have less effect than larger values. Small values are in the 47pF range and have little effect. Larger values in the 100pf-180pF range will affect the upper treble frequencies. And even larger values in the 250pF-500pF range will affect a wider band of frequencies making the amp sound louder and have the lower treble/upper mids pronounced. While doing your experiments with bright cap values always solder the bright cap to the potentiometer or switch. Long alligator clip leads are very noisy and will sound terrible making the changes difficult to discern.

The advantage of tweaking these bright cap values comes in when you own a wide variety of guitars. For example you may not need a bright cap at all with a Strat or Tele but may need a larger value to brighten up your Les Paul or SG with full size humbuckers. Having the bright cap on a switch or a second channel of an amp with a bright cap as on many Fender tweed amps like the 5F6-A Bassman can accommodate different guitars.

Trying capacitors made of different materials will affect the sound as well. Ceramic disc caps have a slight grainy sort of vintage sound to them. Silver mica caps have more of a clear edge to them. Poly film caps have a smoother sound that has a softer edge without the graininess. These are subtle differences. Try all of them and then decide. That’s part of the fun of tweaking! Now you can tailor your bright cap values to suit your taste and style and accommodate a variety of guitars.

WARNING: IF YOU DO NOT HAVE ANY EXPERIENCE WORKING ON TUBE AMPS, LEAVE THIS TO SOMEONE WHO DOES!!! THE WRONG MOVE IN A TUBE AMP, EVEN THIS LITTLE GUY, CAN BE PHYSICALLY PAINFUL, DAMAGING, OR EVEN FATAL!

Disclaimer: 300Guitars.com assumes no responsibility for any accidents occurring as a result of performing this mod.




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