Filter Caps/Cans- Important ? You Bet! Pt.#1

[toneman]

With tube/valve amps, we all know the value of having good tubes/valves in it as they can make your amp sound great or make it sound awful.
However, I don't think as much importance is placed on replacing filter caps/cans and cathode bypass caps. These play a vital role in your amp too!
What they do: They remove most of the 60 & 120 cycle hum and filter different sections of your amps' circuit so it's quiet.
What they're made of: Most of these use a mylar & paper compound impregnated with an Electrolyte solution (the same stuff that's in your car battery!) We all know that car batteries last anywhere from 3-5 years at best(depending on how much we paid for it..).
They operate at much higher current draws than your amp does.
Filter caps or cans are made to last between 7-10 years. In fact most can manufactures tell you not to use them if they are over 3-4 years old if they haven't been used yet. Why? Well, they dry out and crystalize inside much like the white stuff you see on your car battery terminals.
Once these dry out they are almost useless. They result in floppy bass, ghost notes, double notes and will hum in general and make your amp sound very loose.
Some cans/caps can be reformed( a process in which the can or cap is hooked up into a voltage applied circuit and the voltages brought up real slow to re-energize the electrolytes). Unless you know what you're doing is best left to a professional or even easier, buy new filters. In my shop I buy filters in quanities of no more than 25 at a time for larger values(like dual 16uf, 32uf & 50uf cans) I don't want cans sitting around here for more than 2-3 months before they are used. By buying in small quantites I'm assured of buying fresher cans from the manufacture. Most caps have a date code on them. This is so we'll know how old they are and when to replace them.
Anytime a customer brings an older amp here for service I always ask if they have had the cans or caps replaced anytime in recent years. If they haven't been replaced then I certainly make a strong suggestion to do so while it's here. Not that I'm trying to talk you into spending more money but it's worth the cost to make your amp sound as good as it can and save you trouble down the road.
If you want your amp to sound tight and punchy and as well quiet then replacing cans/caps is a needed item.
O.K. a bit off topic but not.. what do cathode caps do? They help set the gain in the preamp stages and if you have a Class A type output section then there's one there as well. Most cathodes on 12AX7's are pins 3 & 8 on the tube. You will mainly see a 25uf @ 25v cap there in parallel with a resistor(anywhere from 820 ohm to 2.7K). If you have some experience soldering and with electronics they try removing one end of the cap and see what happens to your gain..goes way down!