Filter caps and Cathode Bypass capacitors are big electrolytic (a type of large value capacitor) caps in your amp.  The reason they need to be replaced is because, due to the way they are manufactured, they use a liquid that will dry up over time. 

Typical service life for an electrolytic cap is 8-10 years.  Sometimes with regular use they will last longer.  Without regular use they will die more quickly!  If the amp has been sitting around a while, or you bought it used and don’t know how it was treated, its usually a safe assumption that the electrolytic caps are suspect.  I frequently see amps from the 60s, 70s, and 80s with the original caps.  These amps will always benefit from replacement of the electrolytics.


The amp works, it makes sound, why do I need this done?
To explain that, it’s important to have an understanding of what these caps do. Filter caps (including the bias filter):  These serve two purposes.  First, they smooth out the power supply to your amp, removing unwanted hum and noise.  If they are not working correctly, the amp will hum and buzz, sometimes just enough to be annoying, sometimes very loudly.   Second, they serve as a source of voltage for transient signal spikes (strumming a chord, picking a note) and give your amp the punch and dynamics it is supposed to have.  Power transformers in guitar amplifiers aren’t nearly strong enough to handle transient power demands, it would be too expensive to build them that way.  The filter caps store the energy your amp needs and provide it when necessary.  If they aren’t working right, your amp looses punch and dynamics, and doesn’t sound nearly as good as it is capable of sounding. 

Cathode bypass caps are part of your pre-amp circuit and add gain (clean volume, not just ‘distortion’ type gain) and frequency response to your amplifier.  If they aren’t working properly, your amp will lose low end response and volume. 

If your amp is a couple decades old or more and has never had this service done, it needs it.  Everyone is reluctant because it costs a bit to have it done (somewhere in the range of the cost of a set of new power tubes or a full set of tubes, for reference).  I have never had a customer yet who couldn’t tell the difference in tone and responsiveness from this service.  If it’s between 10 and 20 years old, it may need to have it done.  It’s a good insurance policy, because if a cap does blow it can take more expensive parts (including transformers) with it. 

I don’t recommend you do anything I don’t do for my own amps just to try to ‘generate business’.  When I buy a used amp, it always gets this treatment. 

Why spend money on a vintage amp just to have it not live up to its potential due to worn out parts?

Amps younger than about 10 years shouldn’t need this done, unless you are doing it as a tonal mod and changing cap values. 

The other thing to watch out for (and I see it all the time on amps I work on that other people have messed with before) is that you get a COMPLETE job done.   Just replacing the big power filters isn’t a complete job!  The bias filter should be replaced to, and the cathode bypass caps as well. 


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