Replacing Vintage Capacitors (Always)

[abelljo76]

I'm not a professional tech, I've just been working on my own equipment and building my own amps for close to 20 yrs. I know enough to just get by let's say, but not as much as most of you here. I recently acquired 2 vintage Gibson amps. A GA40 and a GA55V. Both are sounding amazing right now. The GA40 I had to only fix minor cosmetic issues, but the GA55V was purchased as is and not in working condition from GC in California. The contractor tech, who was more of a pedal guy decided to tackle the amp's vibrato not working and disconnected a bunch of wires and components. I was lucky to find the schematics here. I got the vibrato running and replaced all the caps in that circuit. I also replaced the main cap can, but was not able to find suitable cans for the two 10+10 at 350 or 400 volt ones that would fit and look good. I've read that these are many times good even in very old amps. I did measure the capacitance and they are dead on. Not to say they might not fail sooner or later.

My immediate concern is all the caps under the main amp circuit board. The vibrato circuit board was removed by the tech making it easy for me to replace all the coupling caps etc, but not the main one. I'm sure there are leaky capacitors underneath, but I'm afraid of tackling the board of such an old and fragile amp, especially when I know many of these wires are I'm sure weak from age alone. The amp is sounding great and it is super quiet, no hums crackles or pops. I don't want a leaky cap to cause a major issue in the future though. I'm a jazz guitarists and both the GA40 and GA55 were what I was looking for in a long time to get that old vintage tone. Your thoughts?

[abelljo76]

Sorry, I guess the question is. Should old capacitors be replaced always? Or take the "if it ain't broke don't fix it approach"?

Thanks!

[shooter]

Or take the "if it ain't broke

I take this approach if it's my stuff, A customer, I give the option, once I've fixed the reason it came to me.

[abelljo76]

Thanks, I'll take that route

[pompeiisneaks]

Electrolytic capacitors can, and will fail, they have an electrolytic in them that will dry up over time, and cause them to no longer function.  Most capacitors have a life measured in hours on their datasheet.  Oddly if they're used more, the last more, the lack of use seems to speed up the drying of the electrolytic.  one of the first things most people notice when they replace electrolytics is that the amp just sounds cleaner.  I highly recommend replacing them every time you get an amp that's 20 years old or older, as almost all caps have an hour range that's well exceeded by that point.  This is a heavily debated topic, many people swear by just leaving them in, but the techs that have had to replace multiple parts of the amp when a cap fails know that it could have been prevented by just replacing it when it got old enough.  I have now recapped about 4 amps that were over 15 years old and could audibly hear how much cleaner sounding the amp was, and how much tonally the amp improved with new filter caps. 

That's my 2c.

~Phil

[shooter]

was not able to find suitable cans for the two 10+10 at 350 or 400 volt ones that would fit and look good.

One "trick" that some have done, IF, you can cleanly remove the old can, you can gut it, and use the small modern caps inside the old can.

[92Volts]

Besides the first cap following a rectifier tube, you can freely increase power supply cap values. This can change the amp's tone, but when feeding a preamp that effect would be tiny. Besides, small values aren't always "what the designer intended", they're what they could afford.

So a 20+20 (or whatever you can find that fits) could replace those 10+10 cans. At any voltage of 400 or higher.

You can also install a terminal strip with modern caps under the chassis, since they've gotten smaller, and you could even leave the can installed for appearance but disconnected.

Depends on your priorities of exact restoration vs. good, working amp.

Paper in oil caps are even sketchier than electrolytics while ceramic caps are usually fine. Other types are a crapshoot. I'd examine coupling caps, especially the final one feeding the power tube since any DC leak will screw up your bias and potentially kill tubes, transformers, etc.

I'm more comfortable with "if it ain't broke, don't fix it" with tone, bypass, and even preamp coupling caps.

One "trick" that some have done, IF, you can cleanly remove the old can, you can gut it, and use the small modern caps inside the old can.

I recently tried this on an old Zenith radio cap and mangled every part of it from the can, to the composite base, to the contacts I was planning to use for the new caps  your mileage may vary! My failure was probably 20% due to the way the cap was built and 80% due to bad technique