Post
by oid » Fri Mar 30, 2018 9:06 pm
The metal film resistors used in the tone stacks and for coupling the gain stages do start leaking dc voltage with time and this throws off the bias of the tubes, so need to be replaced after a few decades. As far as upgrading goes, I think this is largely a myth caused by a some really crap capacitors being used and the hifi/recording crowd talking about the negative effects capacitors can cause. There is a difference in the sound of caps, but you are hard pressed to hear that difference between any good quality audio grade caps. Orange drops are a good example, they are nothing special and far from high end, but almost no one complains.
There is a case against caps when talking reproduction/recording and the design goal is accuracy, but in guitar amps we like color.
Another common cause of blaming the capacitor brand/type is the wide tolerance of especially old caps, +-20% was common and when your cap is defining the frequency response of the amp this can lead to a great difference in sound. Even today tolerances in the sort of caps amps use are quite wide +5%-10% is common and if memory serves is the tolerance of the old faithful orange drop. This difference in value from cap to cap is a very audible effect and can shift the frequency response enough to make one amp sound lifeless and another sound great. I suspect this is a great amout of the change in sound when swapping caps, it is just the values are not the same, much of the difference in sound starts to disappear when you match caps and do a blind tests. Differences are heard, but no one can consistently pick out one cap from another unless their qualities are wildly different.
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