Anyone used alternatives to electrolytic filter caps? How were the results?

[greengriff]

Hi Everyone,

My first post, so go easy on me :-). The title says it all really. Before I shell out for some new filter can caps I'd love to hear people's opinions on alternatives to electrolytic caps they've used. I'm enthusiastic about the idea of not ever having to do cap jobs!

How well did they perform, did they have any noticeable effect on tone etc..?

[MWJB]

Certain builders do use non-electrolytic caps, (I believe Carr do in some models at least?) limiting factor is often the physical size of the caps, e-caps can usually be found to fit in any existing amp...try finding 30-40uf's worth of non-electrolytic caps to stuff into a 5F2A right after the rectifier, or fitting them in space left by 2x 350/500v electrolytics as main filters for a BF/SF Fender.

If you gen up on physical cap sizes, then build in adequate space from scratch with a suitable chassis, then you should be fine. In a lot of cases though it still makes sense to go with long life e-caps & just anticipate a reasonable part life.

[redelephant]

I use film caps now and then. I don't bother with 'audiophile' types that cost their weight in gold, but the DC-Link types are usually ok price-wise. Tone-wise I am too poor a player to notice much difference but they certainly aren't any worse than 'lytics. My main reason for using them is long life, and of course the better specs gives a nice warm feeling.

I've used the square type:
http://www2.mouser.com/catalog/643/907.pdf
and the round type that fits into the clamps used on 'lytics:
UNL6W30K-F Cornell Dubilier DC Link Film Capacitors
and Solen film caps (same as Carr uses)

Basically if u dont mind the extra cost, and have room in the layout, I see no reason not to use them.

[greengriff]

Thanks for the responses. Much appreciated!

[R.G.]

Also consider HVAC motor run capacitors. These are polypropylene film types, and are designed for power handling. They are usually cheaper than electronic equivalents because of the large volumes.

[greengriff]

Thanks. I'd not considered those. Price wise - here in the UK at least - they don't seem much different from the kind of poly caps I had found:

VISHAY ROEDERSTEIN|MKP1848 710 454Y5|CAPACITOR, MKP, 500V, 100UF | Farnell United Kingdom

[greengriff]

Unless you know a cheap source of course :-)

[Steve Conner]

Just search for "motor run capacitor" on Ebay UK. I tried there, 6 pages of the things.

[greengriff]

Yep, pretty obvious thing to do really, especially considering I use ebay pretty regularly!

One thing, they all seem to be max 450v, whereas the marshall clone I'm currently building requires 500. I don't want it to go 'pop'! Has anyone successfully used 450v in a 500v application without an explosion :-) ?

[Steve Conner]

Is that 450V an AC or DC rating? If it's AC you can multiply by 1.4 to get the DC rating.

PP film capacitors don't have "a rating" as such, it's a curve of voltage vs. temperature vs. lifetime.

[greengriff]

They're rated in AC, so it's good to know I have some leeway! Any idea what the max likely DC volts they will ever be subjected to is? I can't recall ever measuring more than about 460 in a Marshall type amp, but I have a memory like a goldfish!

By the way, many thanks for your detailed and helpful replies!

[redelephant]

The psu caps should handle the unlikely event that the amp is turned on with tubes removed, in which case the unloaded supply can go way above design. If u aim for near 500volts, I'd use 600V rating. That said, film caps are better at handling overvaltages than 'lytics. But a 450AC rating is (as mentioned) good for 630VDC.

What do u mean PP caps have no rating Steve? U mean no voltage rating? If so that is untrue, each rating is related to the thickness of the insulation between each layer, and for a higher voltage rating, u need thicker insulation.

[Steve Conner]

Redelephant: Insulation rated for 600V doesn't last 100 years at 599V and explode instantly at 601. Think about it, then look at the lifetime curves on the DC link capacitor datasheets.

[redelephant]

Agreed, as I said earlier films handle overvoltage better than electrolytics. But u get film caps rated 40V. So obviously they have ratings.

[Steve Conner]

Yes, but they're pretty negotiable.

However, people choose film caps because electrolytics don't last long enough. So it wouldn't make any sense to overvolt the film caps.