We’d like to make a quick statement about the term “cap job” and what it really means!
Performing a “cap job” should NOT mean indiscriminately replacing ALL of the capacitors in your amp!! We have seen way too many vintage tweed & blackface Fenders come through my shop that have suffered this terrible fate.
First of all there are basically TWO DIFFERENT types of capacitors in your vintage Fender amp.
The first are the electrolytic caps, which perform two different duties in your vintage Fender. The main power filters literally filter the high voltage in your amplifier. The electrolytic chemical inside of these caps can dry out over time, especially when left unused for long periods. It is important for the performance of your amp that these caps be good & functioning properly. Bad power filters can cause serious damage to your amp! Many times they will blister, leak, or rupture (usually on the rubber seal on the +positive+ side of the cap) – damage that is visibly detectable.
The electrolytic cathode bypass caps in the preamp should also be good. Bad electrolytic caps in the preamp often cause muffled, muddy tone, unpleasant distortion and/or “motorboating”.
Electrolytic caps tend to need replacing more frequently than the other type of capacitor, the signal cap.
The other type of capacitor is the called coupling cap or signal cap. They get this name because they “couple”, or join one stage of the amplifier to the next, and the instrument signal passes directly through them. They allow AC voltage (the instrument signal) to pass through them while blocking DC. There is no electrolytic chemical in these caps, and these caps rarely go bad. If they do go bad, they might “leak” allowing DC to trickle through. If this happens, they need to be replaced.
Signal caps all sound different. The vintage Astrons in the Tweed amps and the blue “molded” caps in the blackface & early silverface amps sound fantastic and should NOT be replaced unless absolutely necessary!! On the other hand, silverface Fenders with the brick red “chocolate drop” caps or the navy blue blobs might be greatly improved by a signal cap upgrade. Those blue “molded” caps in the late tweed amps, blond, brown & blackface amps almost never go bad and sound amazing. Unfortunately, the yellow Astron signal caps found in most Tweed amps and some blondes & browns do have a tendency to get “leaky”, allowing some DC voltage to trickle through, requiring replacement.
The “Ceramic Disc” vs. “Silver Mica” debate is another matter of personal preference and worthy of a case by case listening test. Some people prefer the silver micas and some prefer the ceramic disc sound. I have heard silverface Fenders that sound better with silver mica caps and others that sound better with ceramic disc! Often a combo of the two works wonderfully. They both have a slightly different voice, especially noticed when the amp starts to “break up”. I find that the silver micas tend to sound warmer and rounder at lower, cleaner volumes, but tend to get a little grainy or edgy when pushed into overdrive. The ceramic disc caps might be said to sound a touch more “brittle” at lower volumes, but they have a VERY nice, smooth & creamy breakup quality.