Fender filter caps - raising values?

[culturejam]

Upping the filter caps in my tweed champ did make a noticeable difference.

And I totally believe that. Unlike most things people claim make a difference in sound, I actually believe this one to be true. I just didn't quite understand how it worked. But now I do, so thanks!

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.

Noted and taken away. 

[lincolnic]

Thanks for chiming in, Morgan - this is super educational.

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.

My Pro actually has a GZ34 rectifier, which has a 60 uF max according to the datasheets I checked. So I don't see any harm in going to the 100 uF caps, though like you said, it'll probably depend on what's easiest to find.

[Morgan]

I was doing some reading on Merlin's site today and came across some sage wisdom about dealing with the first filter cap when using a GZ34. Thought I'd share:

Ripple current and valve rectifiers: When the reservoir capacitor charges up with each input cycle, it draws a large current pulse from the rectifier and power transformer. This is known as the ripple current, and the larger we make the capacitor the larger it will be...this is why you should not use a ridiculously large value capacitor, as the increased ripple current would put a great strain on the rectifier and power transformer...

Silicon rectifiers can handle larger ripple currents well, but valve rectifiers cannot. For this reason, if you are using a valve rectifier you must check the data sheet for the maximum allowable reservoir capacitor. THIS VALUE MUST NOT BE EXCEEDED or too much ripple current will flow and the valve rectifier will almost certainly be destroyed. For the GZ34 the limit is 60uF, and it would be a good idea to always use a value smaller than this, rather than push the valve to its maximum limits, especially since there is rarely any need for a capacitor larger than 60uF.

http://www.valvewizard.co.uk/smoothing.html

So, again, using two 100µf caps in series for a value of 50µf total in the 1st cap position is pretty ideal for your amp.

[lincolnic]

Thanks for sharing your finding! Glad to know that I'm on the right track.