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Step 6Replacing the filter 'can capacitors'
Many guitar amps have the larger electrolytic caps in sealed "cans." Quite a few, this one included, have multi-capacitor cans--more than one capacitor together in a single package. They usually are the power source filter caps.
These can be replaced, but--
--Replacements are 5-10X as expensive as a handful of single caps
--Even if you can find an electrically-identical replacement, finding the same physical dimensions is difficult (height, diameter, etc.)
--Fewer options with cans. If circuit changes are made, and not all the sections are used, it's just a waste.
--You don't always know if you're getting new cans or 20 yr old 'N.O.S' ('New Old Stock'--good for tubes, bad for caps.)
--Even brand-new caps have a failure rate. Much less painful to replace one $7 cap than a $40 "can" type...
So it's quite easy and common to replace with individual polarized capacitors. I removed all the wiring from the can, then rerouted to the new caps with a terminal strip. Terminal strip are cheap, simple to use, and very common in vintage gear.
The terminal strip was attached to the chassis with a steel pop rivet and steel rivet backing-plate. The hole was pre-existing (cool!) but drilling a new one would be easy.
The old 'can' was left in place for looks, and for a reference to whoever 'recaps' this amp in the future.
The filter caps, originally rated at 150V, were replaced with 350V versions. Yes, you caught me--the 20 uF bypass cap is still only 160V, because that was the only one I had on hand (This is part of Mod A--I've since replaced it with a 10uF 350V cap...)
When replacing the filter caps, don't exceed the capacitance values by much (I went from 40 uF spec to 47 uF.) More capacitance sounds good, right? Less hum, you say? Unfortunately, amps with tube rectifiers can't handle high capacitance values--they cause voltage spikes that wear out the rectifiers quickly. 60 - 100 uF is the recommended max for this type of rectifier...stay near the spec values here.
This link has a tutorial on encapsulating (hiding) new caps inside an old cap can:
http://www.nmr.mgh.harvard.edu/~reese/electrolytics/
These can be replaced, but--
--Replacements are 5-10X as expensive as a handful of single caps
--Even if you can find an electrically-identical replacement, finding the same physical dimensions is difficult (height, diameter, etc.)
--Fewer options with cans. If circuit changes are made, and not all the sections are used, it's just a waste.
--You don't always know if you're getting new cans or 20 yr old 'N.O.S' ('New Old Stock'--good for tubes, bad for caps.)
--Even brand-new caps have a failure rate. Much less painful to replace one $7 cap than a $40 "can" type...
So it's quite easy and common to replace with individual polarized capacitors. I removed all the wiring from the can, then rerouted to the new caps with a terminal strip. Terminal strip are cheap, simple to use, and very common in vintage gear.
The terminal strip was attached to the chassis with a steel pop rivet and steel rivet backing-plate. The hole was pre-existing (cool!) but drilling a new one would be easy.
The old 'can' was left in place for looks, and for a reference to whoever 'recaps' this amp in the future.
The filter caps, originally rated at 150V, were replaced with 350V versions. Yes, you caught me--the 20 uF bypass cap is still only 160V, because that was the only one I had on hand (This is part of Mod A--I've since replaced it with a 10uF 350V cap...)
When replacing the filter caps, don't exceed the capacitance values by much (I went from 40 uF spec to 47 uF.) More capacitance sounds good, right? Less hum, you say? Unfortunately, amps with tube rectifiers can't handle high capacitance values--they cause voltage spikes that wear out the rectifiers quickly. 60 - 100 uF is the recommended max for this type of rectifier...stay near the spec values here.
This link has a tutorial on encapsulating (hiding) new caps inside an old cap can:
http://www.nmr.mgh.harvard.edu/~reese/electrolytics/
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