[quote author="learner"]The problem is that do film cap come in such a big capacitance value? I can't seem to find values such as 47uF 400V. You only seem to be able to find electro caps of that size, may be I haven't looked hard enough? Also the cost would not be as cheap?[/quote]
They're out there, but they're quite expensive and
huge.
BTW, do you know how long the electro usually last?? and do they die out ealier if they get used more often??
Life of an electrolytic is typically anywhere from 5 to 20 years, depending on its quality, the temperature, how much it's used, etc.. On the one hand, the worst stress for an electrolytic comes when you turn it on. On the other hand, leaving it on all the time to avoid that stress generates heat, which causes the electrolyte to dry out, so you can't really win.
[quote author="pstamler"]The choice of cap size is usually dictated by the load impedance and the desired low-frequency response. The cap size is defined by:
C = 1 / (2 * pi * R * F)
where C is the size of the coupling capacitor, in farads; R is the load impedance/resistance, in ohms, and F is the desired 3dB rolloff point in Hz. You would need a 100uF cap only if you were driving low impedances (which tubes don't want to do, hence the earlier comment) or needed exceptionally low rolloff frequencies (into 10k, for example, a 100uF cap will generate an 0.16Hz rolloff). [/quote]
[quote author="learner"]Right, I guess large capacitance is necessary for low freq roll off point and to hold enough current inside the cap to accomodate current peak swing to drive the low impedance load. [/quote]
The problem is, you can't drive a low-impedance load with a typical tube circuit anyway. Realistically speaking, small-signal tubes need to operate into a load impedance of at least 5k and preferably 10k, or they'll distort like mad. To drive lower impedances, you need a stepdown transformer. With such a transformer, the tube is seeing (typically) a 10k load, so the cap can be smaller, or the transformer is direct-coupled and you don't need a cap at all.
In other words, there's no situation where a really big cap is needed; the impedances low enough to require them are also low enough that the tube won't work properly.
What I am curious about is that would there be any adverse effect or audible disadvantages to use such cap for normal 10K loads and above???
Yes; because it effectively has to be an electrolytic, the sound will suffer. Or, if you do manage to find a huge film, there will be an adverse effect on your wallet, not to mention the space inside the box.
Eventhougn its seem to be an overkill, would this help to extend the operating life of the cap since its barely working to its normal operation point?
No. A smaller (say, 5-10uF) film cap will outlast an electrolytic.
Peace,
Paul
