Why only electrolytics as cathode bypass?

[greiswig]

Other than cost and availability of higher values, why aren't film caps used for cathode bypass? Is there another advantage to electrolytics there?

Just curious.

[El_Martin]

Hi greiswig!

I have used MKT and none polarized electrolytics with good results.
Low ESR and some other "signal processing factors" -in lack of a better word- DO make a difference to me.
It's not always better, same is with paper-in-oil caps...

It depends...use YOUR ears and your gear...might not work for everybody.
Somehow electrolytics may round out the sound a bit
->fighting harshness

Ciao
Martin

[keithrick]

Size. Maybe that doesn't apply as much today.

[Andy Le Blanc]

film caps are used in bypass applications
usually in hi fi
sometimes parrallel with electrlytics
play with it if you want
the bypass cap controls degenerative feedback
you can use them in the power supply circuit as well

[greiswig]

Great info, as usual. Thanks!

[gregarious]

George, not trying to hijack the thread, just take it sideways

Carr uses Solens for filtering, was wondering whether using a mix of electrolytes and film caps would cause the electrolytes to "stress" due to different response time of the film caps, where the first filter node is electrolyte, followed by film in second and third nodes. with node four electrolyte -
40uF, 22uF, 22uF, 20uf.

Why? I've got two left over 22uF 630v polyprops!

[Firestorm]

gregarious --

Go for it. I'm not sure whether the film caps "fill up" faster than the electrolytics, but the internode resistors limit charging voltage anyway. All power supply caps start out looking like a dead short after all. If there are different discharge rates, too, that might be interesting since it could encourage or discourage sag, depending on where the caps were placed. See how it sounds, you won't blow anything up.

[greiswig]

George, not trying to hijack the thread, just take it sideways

Not at all...that's why I asked the question: to learn!

[6G6]

I think the main reason that you usually see E caps in filters, cathode bypass, etc., is that back in the dark ages, when amps began, that was the only choice for any meaningful values.
It's been that way so long that everyone just assumes there is no other choice.
Of course, if you are Fender or Peavey, and building thousands of amps, the price difference makes them still the only choice.

[David Root]

I use nonpolar bypass caps wherever possible, which is most everywhere with the high value low voltage nonpolars available today.

No it's not the cheapest way, but they don't have to be big either. Take a look at a 22uF /6.3V Black Gate NX type. Tiny.

So then the only electrolytics in the amp are the PS caps. If you have plenty of space and even more plenty of $$$ you can do the PS in nonpolar caps too. I don't. Modern electrolytics are light years ahead of where they were before the mass production of personal computers. Having said that there are still some unreliable cheap s**t electrolytic caps out there.

With regard to gregarious' notes about the order of mixed types in a PS, some Hifi types will tell you when using multiple different types of caps in parallel to a supply node, the nonpolar should be the last one immediately before the node. Something about impedance balancing IIRC.

[klingo]

I tend to prefer ELNA silmic and F&T bipolar electrolytics but haven't extensively A/B'ed dif cathode caps though.
when using a relatively small value (~2/3uf in a preamp) loosy Ecaps may be less ear-pearcing than cheap MKT
I remember an audiophile article telling not to use MKT for cathode cap when bias is under 10v but MKC (or MKP) but the guy never say why

[LarryN]

No it's not the cheapest way, but they don't have to be big either. Take a look at a 22uF /6.3V Black Gate NX type. Tiny.

I always see just a few volts on preamp cathodes at idle. Do they jump up under load, to require a 25 volt rating? I'm also curious if 1/4 watt resistors are OK in preamp cathodes. Sorry for the mini-hijack!

[drz400]

No it's not the cheapest way, but they don't have to be big either. Take a look at a 22uF /6.3V Black Gate NX type. Tiny.

I always see just a few volts on preamp cathodes at idle. Do they jump up under load, to require a 25 volt rating? I'm also curious if 1/4 watt resistors are OK in preamp cathodes. Sorry for the mini-hijack!

yes of course you can !

[LarryN]

Thanks!