AB763 70uf Power supply caps

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tubejunky
Posts: 3
(3/21/04 5:57 pm)
 AB763 70uf Power supply caps
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 I'm working on a Super Reverb clone, and have a couple of 47uf, 500v Illinois caps that I'd like to use in place of the 70uf power supply caps that the AB763 schematic calls for. So what results can I expect from this substitute? Less low end punch? The AA763's have a lot less filtering here, and I just don't know if I want to shell out the $$ for 80uf Sprague Atoms right now, plus I don't think they'll fit the chassis.. Thanks!
 
GroundhogKen
Forum Moderator
Posts: 2334
(3/21/04 8:48 pm)
 Re: AB763 70uf Power supply caps
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 If you are using Doug Hoffman's AB763 layout, then just use a single 47uF 500v cap. You will actually have more filtering than the 35uF ( two 70uF caps in series ) that the original design had.

Just make sure the 500v cap is on the cold side of the standby switch as in Doug's layout so you don't exceed its voltage rating.


Ken
 
tubejunky
Posts: 4
(3/21/04 10:22 pm)
 Re: AB763 70uf Power supply caps
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 Doh!  

So was it because of heat buildup in the caps (questionable tolerances?), or matter of availability that Fender went to the dual cap first stage? Or was it something else? Any ideas?

Thanks again!
 
GroundhogKen
Forum Moderator
Posts: 2335
(3/21/04 11:13 pm)
 Re: AB763 70uf Power supply caps
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 Yep, since Fender put the first filter stage on the hot side of the standby switch, many amps had greater than 500V on the caps when the amp was in standby.

Big caps with big voltage ratings were expensive ( still are ), so Fender used two 70uF 350v caps in series. It's equivalent to a 35uF 700v cap. The 220K bleeder resistors insure that neither cap ever takes the full B+ voltage.


Ken
 
 
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