Metallised Polyester Capacitors Installation direction

[ziggy007]

G'day all,
Pls excuse my ignorance but does it make any difference on the installation direction of Met Poly caps when used as coupling caps. The ones I have have a line like on the film/foil caps which I think (if memory serves correct) that indicated the outer 'foil' end which was traditionally supposed to go the end with the lowest potential.
I may be way off here but just thought I'd ask.
Thanks
Gavin

[Rhodesplyr]

Gavin,

Metallized film capacitors have an outer foil like film and foil caps. However, it's not lowest potential, by which I assume you mean voltage. The general rule is to try to orient the outer foil towards the lowest impedance to ground, which is often the plate of the driving stage. The grid resistor of the following stage is usually a higher impedance to AC ground than the plate resistor of the previous stage even though the plate is at a higher DC voltage.

David

[ziggy007]

Thanks David,
Yeah I think I was trying to say that.
What would be the consequences of installing the wrong way? I have just recapped an old amp and a few (due to my incompetence/rushing) went in the wrong way. I am not convinced there is a difference sonically although the amp is definitely brighter. I will change them around but my brain always wants to know WHY??????
Cheers
Gavin

[Steve Conner]

Wouldn't worry about it. After all, the outer foil is coupled straight to the inner foil by, believe it or not, a capacitor. To me that says that any hum, etc. picked up by the outer will go straight through to the inner.

That goes for large values such as coupling capacitors. If it was a tiny capacitor like a bright cap, then it's worth putting the outer foil to ground.

[ziggy007]

Thanks guys, appreciate the responses
Gavin

[Rhodesplyr]

Gavin,

It's not so much about sonics as it is about noise reduction. The principle is that orienting the outer foil towards the lower impedance allows it to act as a partial shield for the inner foil. You can see this with an oscilloscope. If you connect a capacitor directly across the scope's leads with the scope on a low-level setting and touch your fingers to the outside of the capacitor, you'll see more disturbance of the trace if the outer foil is connected to the signal input than if the outer foil is connected to the scope's ground. I use this method to determine the outer foil of all film capacitors I use in repairs, mods, or builds.

I don't know that it always helps, but it can't hurt :-)

[ziggy007]

Neat trick with the cro, Ta, will adopt as standard practice from now on.
ta mate

[Enzo]

I just hate this argument. I always hear that the outer foil will shield the inner. From what? Those foils are all connected to the wires running around the amp, Whatever that outer foil might shield from the inner, the wires connected to the inner would pick up anyway. More than that, to signals, the cap is more or less a wire in the first place. The point of the cap in the circuit is to pass signal along. Whatever is on the one end will be on the other. I think that is what Steve said. There is no shielding.

To me this argument is like aguing whether to restring a guitar bottom strings first or high strings first. Or which end of your strap to button first.

[Gaz]

i agree with enzo. after some experimenting of my own, i haven't been able to notice any noise reduction or sonic changes whatsoever. changing my strings or switching the thickness of pick i use has yielded more tonal benefits than some mods that took hours to complete, namely the one mentioned in this thread.

[ziggy007]

Thanks guys, That's what I couldn't get straight in my head, knowing how the cap works and passes/blocks signal, How it could make a difference?? it was just something i remember being taught as an apprentice yonks ago, But as Rhodesplyr said, I guess it cant hurt
got a good debate going thou

[Enzo]

Yes, it absolutely won;t hurt to run them some consistent direction. When I build things or install parts, I like them all facing the same way, just for a neater appearance. All the caps facing the same way, all legends reading the same direction, all resistors with tolerance band at the same end.

Where outside foils can matter is in high frequency circuits. Not treble, I mean RF circuits. Bypassing a grid for example.

[ziggy007]

aha!!!
now that makes sense. I knew it was in there somewhere. My apprenticeship was in Avionics, hence the RF connection. Thanks Enzo . Another satisfied customer.