Replacing Filter Caps Value

[vmazz]

I am just trying to get opinions on replacing the filter caps on my late 80s fender champ12 . I have heard different opinions about whether you can increase the value,I know you can increase the voltage rating but these caps are 47 uf and 350v ..what would happen if I went with 80 uf 500v caps. Any ops would be appreciated.(some guy at a shop told me not to increase as it would change some part of the rectification..I didnt understand it as I am a noob still)
Thanks Val

[Chuck H]

Increasing the uf value of the first filter cap in most Champ amps could be a problem because those amps used tube rectifiers. Tube rectifiers have a max capacitance limitation. This is probably what your friend was talking about. Your amp uses a diode rectifier. You can use almost any uf value you want to for the first filter cap.

The Champ 12 has an unusual rectifier circuit (at least for guitar amps). I haven't seen it before, but it appears to be a half wave voltage doubler. In this case I think the 47uf value is very small and hum level of the amp could be improved by increasing the value of that first filter. I might try 80uf or even 100uf. You shouldn't notice any difference in going with higher voltage rating but that wouldn't be a problem for the amp if you want to.

Chuck

[stokes]

Increasing the input capacitance should give you a tighter response,particularly in the bass.This may or may not be noticeable.Like Chuck says,with a SS rectifier you can go as high as you like,but I have found that with tube rectifiers the limitations listed in the manuals are very conservative.I have an amp with a 5R4 rectifier with a 100uf input cap and its been running a couple years with no problems.Had a 220uf in the amp for a while,again no problem.The manual says a 20uf is max for that tube.I did this as an experiment to see if the rectifier could take the extra capacitance and it would seem there are no problems.Will the tube wear out faster than with the lower cap?Probably so,but so far I've gotten my money's worth out of it.

[tubessence]

Also, there is a filter modeling program over at Duncan's Amp Pages, that allows you to build your filter and the check the output ripple, etc. Pretty Cool! jb

[vmazz]

THANKS ! I really appreciate the advice. I suppose this means ther is no chance of damaging the diodes in the ss rectifier circuit. Also Im not sure which cap is the input c25, c102, or c103. Ill try to attach the schematic.
I am always blown away at the wealth of info available on this sight its really allowed me to make changes I never would have made..and its resulted in much better sounding gear.
Thanks

[tubeswell]

C101 does the plate supply - interestingly the cap is installed with a sort of a 'half-bridge' rectifier.

C103 and C102 form part of a CRC filter for the screen node

C25 is your pre-amp filter/decoupling cap

[stokes]

The first cap after the rectifier is the "input cap" C101 in this case.

[Albert Kreuzer]

D101/D102 and C101/C103 form a voltage doubler. So they get 480V DC from one 180V AC winding.
The junction of C101/103 is like a "center tap" which is used for the preamp supply.

If you want to change cap values, always make sure C101 and 103 have the same size. IMO you can go to 100µF/350V without any problems.

Cheers,
Albert

[stokes]

Sorry,a closer look at the schem and I see what Albert pointed out.You must change 101 and 103.Like he says use the same value for both,thay are "totem poled" and your actual uf value will be half of what one measures.I would suggest using 220uf so you will have a total of 110uf value.But even a couple of 100uf will double what you have stock.

[vmazz]

one more question

Thanks everyone..and just one last ? Since Im replacing the power filter caps should I replace c25 for the pre anyways and could the value be different than 47uf also and how would values effect the sound there? Thanks again
Val

[Chuck H]

If the amp is over 10 years old and your replacing caps in the power supply, you may as well replace them all. Those caps do have a lifespan of a few thousand hours or around 10 to 15 years. Thats just common practice and typical circumstances. Sometimes they can work fine for 25 years and sometimes less than 10 if the amp is not played much. Point is, since the caps do go bad, and you already have an iron heated up, you may as well replace them all.

I don't think you'd do well to increase the value of the preamp filter. BF type preamps can sound a bit farty and splaty if you use high value filters. And I don't think going above the standard value will have any noticable effect on ripple or hum, since your raising the first two filter values to improve that, they will handle alot of that work.

Chuck

[vmazz]

the last one more question

Just to make sure Ive covered all questions in my head. Would it be safe(I mean not detrimental) to replace c 102 with a higher value as well. I understand c101 and c103 should be the same value. What is the purpose specifically of c102? Thanks Everyone for the help.
Val

[defaced]

C102 is the filter cap for the power tube's screen. It can be adjusted independently of C101 or C103. If you increase the capacitance of C101 and C103, it will also improve the filtering seen at node C and every node after it. I wouldn't put too much money on increased capacitance of C102 making a huge difference in the way the amp sounds.

[tubeswell]

My take on this is that doubling the filter caps won't add much difference tonally - maybe make it a tiny bit cleaner/less squishy and (based on previous experience with doing this with a 5F2A) you'd get maybe another 8-10V on the B+. Since you are running a diode rectifier, it won't hurt to triple, or even quadruple, the capacitance.

The only place you really need to be concerned about the overall size of the filter caps is where you need to stay within comfortable cooee of a tube rectifier's recommended maximum rated reservoir capacitor value (but even then most rectifier tubes will take a bit more here than what it says is safe on the data sheet).

The effect of the capacitance is in how much charge the cap holds, and therefore what the 'rate of release' of that charge is. Higher capacitance caps stay more charged for a longer period of time. This means therefore - in the case of the 'input' filter cap(s) - getting the B+ up a bit more, and - in the case of the 'output' filter cap - making the residual ripple a bit smoother.

However (to digress a bit) increasing the capacitance of the pre-amp decoupling cap(s) probably won't add even as much difference as that, because the main function of decoupling caps is to hold the B+ steady against swings in voltage resulting from signal amplification (on the plate in the case of a typical gain stage). The frequency of preamp plate signal swings is mostly higher than the 100-120Hz ripple you get from filtering a rectified Mains AC supply (therefore the pre-amp supply doesn't need as much filtering/decoupling capacitance (because of there being less 'release' time needed to the caps to hold the B+ steady) - well that's my take on the situation. Anyone else care to chime in?