Amp Restoration

Capacitor Restuffing

This section is about some techniques and ideas for restoring old equipment.

First section is on Cap restuffing.

On vintage amps and radios we often want to retain the amps capacitors since they such a part of the look. Particularly true when they are visible like on Ampeg flip tops.

In the above case of a delightful Park 50 amp several of the caps had dried up and various repair solutions had been tacked on with tape or glue. This conscientious amp enthusiast wanted to restore it properly and asked me for some options.

In this case because the cap cans are quite big 35mm and 25mm it was easy to restuff them. What is restuffing? Well it is what it sounds like. Replace the innards and have new capacitors that the eye cannot see.

It is a time consuming that involves cutting into the aluminium can of the capacitor, heating up its insides so that gunk and glue inside melts and can be removed. Next we need to find suitable replacements such as the excellent miniature range from Rubycon that are between 10 and 12.5mm diameter and similar height so fit into the cans directly.

I like to use two capacitors in series, with equalisation resistors to make the working voltage that each cap must deal well within its range. On This park B+ was over 450 the same rating as the caps but in fact when I checked the spec the caps peak is 450 and working is 450vdc.

Care needs to be taken with adequate insulation material since the series cans will have high voltage on one of the cans. I like to do a PAT insulation test on the the cap to be sure that there is no possible short as well.

Once the caps are in the can then the wires are brought through tiny holes to join on to the old cap terminals. Don’t forget that our original cap can is in two halves now but we can join together using wire diameter heat shrink and optionally some plastic plumbing tube internally is strength is required. This sounds a bit flimsy but don’t forget that the cap is going into a metal holder and that will provide a firm grip on out new but old looking part.

Visually you can only tell that cap has been restuffed if you look very closely. You can see the wires coming through the top but that is all.

And it’s great to know that you are running with brand new caps. In the case of this Park 50 the amp was a little bit hummy beforehand so has benefited from the new caps although it was not so bad to have needed them doing.

Restuffing a capacitor like this typically takes a couple of hours time, great patience but the results are rewarding.

Visually hard to spot