Filter cap values?

[smothaudio]

Hello, bit new to this, so apologies for a potentially dumb question...

I'm trying to find a replacement for a Mallory multi cap can (type FP). The original value's were 30uF, 30uF, 30uF, 20uF at 400V, 350V, 350V, 350V respectfully.

My question is how much do the values matter? I know you can't go under value, but what would happen if I went over? say put 4- 40uF's , or an 80, 40, 30, 20? Wouldn't it do a better job? Would it make any difference?

Thanks in advance.

Smoth

[dwhutchens]

It makes a difference if you have a tube rectifier. Too big a capacitance on the first filter can fry a tube. Don't ask me how I know this.
David

[elkym]

Yeah, you can go up and down a bit... for example...

Caps in a fender champ, tweed, are rated at 450v, and 16uF, 8, 8. But you can take that first one as low as 8uF and as high as 30-40 without frying the 5y3gt rectifier tube (the second 2 caps are a bit more flexible than the first-- but I was told never go below 8).

If you have a solid state rectifier circuit, you can go as high as you want, I think... (Somebody correct me if I'm wrong!) But most people don't seem to go over 80uF or 100uF, even with a SS rectifier.

I would say though, don't ever go below the original voltage ratings... if you can, go up instead. at Tedweber.com I'm pretty sure you can get the values you're looking for (or pretty darn close) in 500v and 630v-- but maybe not in a cap in a can... take a look around...

[Enzo]

Keep in mind that when these circuits were first made, typical filter caps had tolerances of - and I am not making this up - -20% to +80%. So your 30uf cap could measure as high as 54uf and still be on spec. Or as low as 24uf.

These max value rules are rules of thumb, not critical numbers. If you want to replace a 30-30-30-20 with a 40-40-40-40, go right ahead.

Those early Champs had a tiny 8uf first filter cap, but they also had a choke in the filter. By 5F1 they had cheaped out on the choke and ran the first cap up to 16uf. By the time the AA764 Champ came around that same 5Y3 faced a 20/20/20uf filter lineup.

In the old days, caps were a relatively more expensive part in an amp. SO 8uf might have made more sense to the budget than a 20. That it worked less well as a filter might have added to the charm of the amp tone, but not everyone feels that way.

Plenty of guys here who will tell you they ran their rectifier tubes into "too much" capacitor and the amp worked fine forever. The limiting factor is the recto tube itself. A solid state rectifier - diodes - can handle a large inrush surge. Without looking, I think the 1N4007 has about a 30 amp surge rating.

[smothaudio]

Thanks guys, good to know!

I never would have thought about damaging a rectifier tube that way, makes sense though. Since I am using solid state, I think I'll just grab the 4- 40uF's.

Cheers,

Smoth