Fender filter caps - raising values?

[lincolnic]

So I'm soon to mod my Pro Reverb to the blackface AA165 circuit, and replace the filter caps in the process. Raising the two 70 uF caps to 100 uF seems to be a standard move, but I've also seen some people raise the middle cap from 20 uF to 40 uF (Gerald Weber recommends this). Does anyone have any info, advice, or experience regarding 20 uF vs. 40 uF for that capacitor?

[pryde]

So I'm soon to mod my Pro Reverb to the blackface AA165 circuit, and replace the filter caps in the process. Raising the two 70 uF caps to 100 uF seems to be a standard move, but I've also seen some people raise the middle cap from 20 uF to 40 uF (Gerald Weber recommends this). Does anyone have any info, advice, or experience regarding 20 uF vs. 40 uF for that capacitor?

There is a lot of opinion and debate on higher filter cap values = tighter bass response or different tone. Really though the point of filters is to smooth rectified voltage ripple to keep hum and motor-boating noise minimal/gone. Designers chose original values using scopes to validate the absent ripple so I guess I am on the side of stick to somewhat stock levels (or at least the closest common values above the original values) and they will get the job done.

Of course no harm in experimenting between 20 and 40uf values.

[timbo_93631]

I'm on the fence about this too, but one thing I would recommend is using F&T caps, they have always been good in my amps and are a little more compact than Sprague Atoms.

[gtr2]

I've been using F&T too...

[culturejam]

I really don't understand how making the power filter caps bigger has an effect on overall bass response.  Seems to me it would just change the corner frequency of the LPFs coming off the bridge rectifier.

What am I missing?

[lincolnic]

Thanks, guys - I don't understand how making the filter caps bigger would affect bass response either, which is why I asked! Unless anyone's actually experienced it, I'll probably stick with the stock values.

[Morgan]

I really don't understand how making the power filter caps bigger has an effect on overall bass response.  Seems to me it would just change the corner frequency of the LPFs coming off the bridge rectifier.

What am I missing?

When you hammer a low E or strike a big power chord, it puts a demand on the output section to supply more current. That current demand will basically empty the filter caps and the amp temporarily "falls apart" and the low end gets all mushy while they recharge. Upping the filter cap values allows for more current reserve, and the bass stays tighter in those situations. Really only happens when you are pushing the amp fairly hard. They designed them to be played clean; so running them at breakup introduces situations they didn't design for.

[alanp]

IIRC, valve rectifiers (rather than 1N4001 diodes) introduce a wrinkle to this, in that there is a maximum amount of capacitance a rectifier can handle attached to it. I could be wrong.

[electrosonic]

If you read the rectifier data sheets there are limits on the size of the first filter cap. I am assuming this is to limit the initial surge current when the amp is first turned on (an empty filter cap initially looks like an open circuit to the rectifier). If the rectifier current goes through a choke before the first filter cap, then its not as much of an issue - the choke opposes the surge of current and limits the initial surge current. 

For my amps, I might bump the capacitance a bit, maybe 50% max. I figure on many of those old amps they used the minimum capacitance they could get away with to save money.

Andrew.

[Morgan]

Yeah, you gotta watch the value of that first filter cap when you have a tube rectified amp, else you can blow the rec.

Data sheet says max of 40 for a 5U4GB. But lots of guys question that. I've seen both 60 and 80uf as "comfortable" figures. If it were my amp, I wouldn't worry about putting 2 100uf caps in series there (50uf total). Really, I'd go with whatever value about 100uf or less that is easy to find.

[Morgan]

Depends on if it farts out on you or not though. If it doesn't ever do that, why not go easy on the rec and install 70's or 80's.

[culturejam]

When you hammer a low E or strike a big power chord, it puts a demand on the output section to supply more current. That current demand will basically empty the filter caps and the amp temporarily "falls apart" and the low end gets all mushy while they recharge. Upping the filter cap values allows for more current reserve, and the bass stays tighter in those situations. Really only happens when you are pushing the amp fairly hard. They designed them to be played clean; so running them at breakup introduces situations they didn't design for.

Interesting.

Is this only in push-pull amps? I would think a typical Class A amp would be wide open pulling max current 100% of the time and regardless of what notes are being played (or if any are being played).

[Morgan]

No, not only in PP amps. In class A, yes the tube is conducting the whole time, but that doesn't mean it is also pulling max current the whole time. The current draw is still going to vary based on how much signal the amp tries to amplify.

[culturejam]

Hmm, okay. I guess I don't understand how tubes work. 

My reading of some of the stuff on Aiken Amps' site led me to believe that a tube rectifier in a class A amp won't give you sag because the current draw is constant, whereas there is sag in push-pull because the output tubes start to pull more current when you play. I guess I just don't get it still, as that was my basis for thinking filter cap size wouldn't have an effect on bass response.

[Morgan]

Well...take what I say with a shaker of salt. I'm a proud hack. Aiken is much more smarter than I.  But also, rectifier sag and filter cap sag are separate things. Upping the filter caps in my tweed champ did make a noticeable difference.

The takeaway should be that jiggering the filter cap values don't actually change the frequencies that are being amplified, so increasing the value doesn't actually give you more lows, it just ups the available reserve power and helps the amp stay tighter at higher output.