Film type capacitors in cathode bypass positions

[ampgeek]

Any thoughts and/or experience in using film type capacitors in pre-amp gain stage cathode resistor bypass situations?

I have only ever used electrolytics there but can't come up with a reason to not use adequately rated and sized film types.

TIA,
Dave O.

[menger66]

You can, sozo use a non- e-cap .68, for by-pass as does mustards, it's kinda hard to find that value in a e-cap, though i've never seen a film 25uf-25v cap, What value do you wanna use, A old timer once told me "i just stick a orange drop in there" when i asked him the same question. mark

[Andy Le Blanc]

sure... used quite a bit in HIFI... I liked a 1.0 oil and foil for a high gain first stage.
the larger the value the more gain towards the low end.
you can tailor the bass response thru a pre with critical values of the by pass no matter what the type is
as long as its rated for the voltage.

look at the back of your RCA tube manuals at the R-C amp tables, notice how small the by-pass cap values are.

[iknowjohnny]

Tried several but i always seem to like electros for some reason. Also have notice a pretty considerable difference in a couple different brands of electros believe it or not. I think most sound alike but i found a certain brand that sounded quite different and they measured .68uf just as they said. Anyways, i used several film caps and for some reason the electros just seem livelier or something. been a while so i don't recall the difference exactly.

[ampgeek]

Thanks mates!

I was scouring around all of the my normal suppliers for a few axial 0.68 electrolytics and came up empty. Came across a handful of Mallory films in my stash and wanted to get a read from ya'll on their use.

Thanks again,
Dave O.

[rp]

When going below 4.7mf I always use films, usually 100V. Above that mf value it's a size problem. I never really A/B'd film and electros but I didn't hear anything I didn't like from the films. BTW I find it hard to hear a diff after 5mf up to 25mf. Not sure I can even hear up to 250mf which is pretty much wide open. Never tried that many different cabs though, maybe testing with a sealed 4x12 I'd hear it. I mention cause if you can't hear it either, then you can just use a 5mf/100v film that will never drift off spec and should never need replacing.

[vibratoking]

...BTW I find it hard to hear a diff after 5mf up to 25mf. ...

^+1

[ampgeek]

Good stuff. Thank you for sharing!

Yep. My experience (all with electrolytics) regarding perceivable differences in tone is very similar. Any reasonable values below 5uf are no different to me. I occasionally hear a difference (like you said...hard to tell) somewhere between 1uf and 10uf depending on the circuit. Same thing between ~10uf and ~25uf. Always some difference between 1uf and 25uf.

Thanks again!
Dave O.

[Structo]

When I was experimenting with by pass cap values and type of cap, some changes were very subtle.

The differences I noticed in the size of the V1a cathode cap was that with the Marshall type (0.68uF) was that the bottom wasn't as loose/ flabby, just a clearer tone.
This is with humbuckers, never have got the strats to sound great on it.
As far as types and brands of caps, I played around with it for a while but I think the cumulative total of the components is what gets it.
You just have to find the right combination of parts that pleases your ears.

Dumble amps, fun at any speed.

Also, I think bedroom players (me) that don't crank up the volume to gigging levels may have a tougher time dialing in what you want.

I've seen guys use the fat Solen Fast Caps for cathode bypass.

You have to plan accordingly since they are a fat diameter.

[briane]

yep -I use the solens fast sometimes.

Their the bomb - but they do sound a tad different - so use at your own discretion.

99% wouldnt notice.

[Firestorm]

Little off topic: I read somewhere that with the crazy large 250uF-320uF caps in Bassmans and Marshalls, there's something else going on besides low-frequency bypass. Obviously much too big to be only that. Anyone see something about this?

[M Fowler]

What are those solid white 25uf/25v caps made of I have some of those in my stock, they're axial type. They don't look like e-caps.

[Firestorm]

What are those solid white 25uf/25v caps made of I have some of those in my stock, they're axial type. They don't look like e-caps.

The ones Fender used, or something else?

[roberto]

I prefer poly caps for the cathode bypass of my designs, as all caps are between 220n and 2u2.

For clean amps I prefer ecaps, as for higain designs à la engl.
But usually I don't go for anything bigger than 4u7 or 10u in both cases.

[rp]

What are those solid white 25uf/25v caps made of I have some of those in my stock, they're axial type. They don't look like e-caps.

The ones Fender used, or something else?

I'm curious too what those early 70s white Fender ones were. I figure they had to be e-caps of some sort, they were too small to be anything else at that time, or this time too, I looked and there are no 25/25 films out there.