advantage of using a cap can?

[Bluefinger]

Hi,

Is there an advantage in using a cap can? I am building a simple amp top loosely based on a 5F6A. There is enough space inside the chassis to mount the filter caps on the main circuit board. Is there another reason for mounting them outside the chassis besides space? heat, hum, ... ?

thanks!

[trevorus]

Hi,

Is there an advantage in using a cap can? I am building a simple amp top loosely based on a 5F6A. There is enough space inside the chassis to mount the filter caps on the main circuit board. Is there another reason for mounting them outside the chassis besides space? heat, hum, ... ?

thanks!

I could see space and heat being the biggest advantages.

[R.G.]

There are some electrical disadvantages. Putting them away from the things they filter on long wires makes them less effective as filters, and more susceptible to picking up magnetic field hum. From an electrical standpoint, a filter cap needs to be right at the point it's filtering with as close to zero lead+wire length as can be done.

Magnetic field pickup happens in conductive loops. A capacitor plus leads makes a conductive loop, and this extends outside the cap can. Poor wire routing can pick up fields from the PT, OT, and choke. Twisting the incoming and outgoing wires together may help.

The cap can is not an effective shield against magnetic field pickup in the conductor loops. Metal boxes are shields for electrical field/capacitive pickup, and to some extent thermal sources.

This must be balanced against the need to (a) keep them away from heat sources, for longer life and (b) make them easy to get at and fixed when they do fail.

All that being said, Fender's done it for a long time, and it works OK. I suspect part of it was to get the caps out of the way physically and away from heat sources. If they had the small modern caps we have now, it might have turned out differently.

[JoeM]

My 5F6A clone has all the filters on the main board. I have all the filter caps close to their associated stages for grounding purposes. Heat would be the only issue as R.G. mentions, but the amp is dead quiet.

[jpfamps]

Can caps historically had higher ripple current ratings than axial caps.

Looking at the data sheets of modern caps, you can now get much smaller caps with very impressive ripple ratings, so I don't think that this is so much of an advantage now.

[Austin]

I think axial caps work great (and they are cheap) and that cans are for restoration of vintage gear.

[JoeM]

Ok, is the OP referring to a multi-cap can, (like a Princeton or Champ would have), or the filter cap 'doghouse', as on the larger Fender amps where the filter caps are mounted? I assumed he meant the latter.

[R.G.]

I thought he meant the doghouse, too.

[R.G.]

I thought he meant the doghouse, too.

[Bluefinger]

I was refering to a 5F6A type design with four filter caps in one external housing.

I have built a harp amp with a 5F4 type preamp section and a 5F6A type power amp section and the filter caps are all on the main board. It worked great so I wondered why I should use a cap can at all if I can fit them all onto the main board. TAD caps for example are high quality caps that aren't even half the size of the typical blue Atoms. They fit onto the mainboard easily, especially if I rearrange the bias circuit which takes a lot of space in the original 5F6 layout.

[Enzo]

For a given value and voltage rating, caps today are MUCH smaller than they were 50 years ago when those cap vaults were used. Those caps would not have fit on the eyelet boards they used, and they had the room outside, so they used it. If your modern parts fit inside, there is absolutely no reason to put them outside.