Choosing the correct coupling capacitor

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abbey road d enfer

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I read somewhere that in tube stages much larger foil caps can sometimes be used to good effect with regards noise . The idea was that by making the cut off frequency of the filter much lower , the output impedence of the preceeding valve could help reduce low frequency noise at the following valve grid.
Anyone any thoughts on this ?
I believe it would be a sign of a defective tube (grid current?). Or wrong implementation (capacitive coupling?).
 

Newmarket

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I never heard this before. So that's pretty much the case for all 500 modules. Interesting.

Well it rarely gets mentioned. And it's difficult to quantify and know if it's a significant factor or more academic.
IIRC I came across it some years ago at Power Supply Design Seminar hosted by one of the big semiconductor companies - likely International Rectifier or Texas Instruments. But I've still put boards into kit where the electrolytics are horizontal (vertical mount PCB).

On a related topic does anyone remember the "Capacitor Plague" problem ?

Capacitor plague - Wikipedia

I think it's worth bearing in mind that an ac coupling capacitor in an audio circuit gets a much easier life than one in a power supply etc.
The two things that really hurt them are heat and ripple current. Although I guess the temperatures inside 'Big' mixing desks can get pretty high.
 

JohnRoberts

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Indeed. Other factors too eg standard 'wet' electrolytics are (arguably) best operated in a vertical position as this helps with the continual 'self-healing' process. But there are loads of cases in which they will be horizontally orientated eg a PSU pcb is mounted to a chassis vertically or they are on a PCB in a vertical rack type system (eg Schroff).
I never heard this in decades of work with electrolytic capacitors. Radial electrolytic caps are clearly designed to be used vertically, but I never saw a failure related to horizontal mounting.

I can imagine a heavy radial cap, mounted horizontally putting stress on the leads that would normally be relieved by resting on the PCB. Allowing my imagination to run wild, this lead stress may affect the bung seal and lead to electrolyte loss. But I repeat I never saw evidence of this.

Note, not that long ago, most electrolytic capacitors were axial and I am not aware of significant internal design differences other than lead attachment.

JR

PS: In hindsight this post is full of stuff I do not know, so caveat lector.
 

Newmarket

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I never heard this in decades of work with electrolytic capacitors. Radial electrolytic caps are clearly designed to be used vertically, but I never saw a failure related to horizontal mounting.

I can imagine a heavy radial cap, mounted horizontally putting stress on the leads that would normally be relieved by resting on the PCB. Allowing my imagination to run wild, this lead stress may affect the bung seal and lead to electrolyte loss. But I repeat I never saw evidence of this.

Note, not that long ago, most electrolytic capacitors were axial and I am not aware of significant internal design differences other than lead attachment.

JR

PS: In hindsight this post is full of stuff I do not know, so caveat lector.

Yeah - I did experience a couple of electrolytic failures in horizontal, and in one case coincidentally did observe the actual failure. It was kind of aesthetically beautiful as the 'cross' in the top of the cap: ruptured and a spiral of electrolyte 'smoke' formed a moving spiral.
That was before I'd come across the idea that orientation was something to be considered.
But I'm not saying that the Horizontal was significant. I had inherited the design.
Actually the cap would have normally been vertical but the instrument was placed on its side for access to the electronics. Changed the capacitor based on ripple current rating and didn't see the issue again in several years. This was a serious power and voltage DC/DC converter non audio product.

But the idea is based on the wet electrolyte and gravity.
I did think of mentioning axial devices but thought I'd keep it short 🙃
 

JohnRoberts

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Yeah - I did experience a couple of electrolytic failures in horizontal, and in one case coincidentally did observe the actual failure. It was kind of aesthetically beautiful as the 'cross' in the top of the cap: ruptured and a spiral of electrolyte 'smoke' formed a moving spiral.
The X scored in the top of the package is an intentional weakness to encourage ruptures from over-pressure to occur there (you don't want that pressure contained). That sounds like a ripple current, overheating stress failure rather that loss of electrolyte.
That was before I'd come across the idea that orientation was something to be considered.
But I'm not saying that the Horizontal was significant. I had inherited the design.
Actually the cap would have normally been vertical but the instrument was placed on its side for access to the electronics. Changed the capacitor based on ripple current rating and didn't see the issue again in several years. This was a serious power and voltage DC/DC converter non audio product.
yup
But the idea is based on the wet electrolyte and gravity.
I did think of mentioning axial devices but thought I'd keep it short 🙃
Bung on bottom is so leaks make less of a mess, but there are no good leaks. IIRC the typical bung also serves as a pressure relief.

JR
 

Newmarket

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The X scored in the top of the package is an intentional weakness to encourage ruptures from over-pressure to occur there (you don't want that pressure contained). That sounds like a ripple current, overheating stress failure rather that loss of electrolyte.

yup

Bung on bottom is so leaks make less of a mess, but there are no good leaks. IIRC the typical bung also serves as a pressure relief.

JR

The X scored in the top of the package is an intentional weakness to encourage ruptures from over-pressure to occur there (you don't want that pressure contained). That sounds like a ripple current, overheating stress failure rather that loss of electrolyte.

yup

Bung on bottom is so leaks make less of a mess, but there are no good leaks. IIRC the typical bung also serves as a pressure relief.

JR

Yeah - understand about the cross being a deliberate pressure relief thing. Agree that it was likely ripple current / internal heating due ESR. It was a new capacitor.

I've had the bung go too but I know that was a high voltage / PCB layout thing due to offset pins. Quite a mess on the PCB 🙄
 

JohnRoberts

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Yeah - understand about the cross being a deliberate pressure relief thing. Agree that it was likely ripple current / internal heating due ESR. It was a new capacitor.

I've had the bung go too but I know that was a high voltage / PCB layout thing due to offset pins. Quite a mess on the PCB 🙄
Back in the 60s as a wet behind the ears junior technician I blew up a couple... One time on a dare from older technicians I plugged a small wet electrolytic cap into an outlet and it sounded like a rifle shot when it blew apart.
===
I blew up a second cap actually doing my job. Back then I was working on a very early DC to DC switching supply. We had a large metalized film cap across one of the transformer windings to absorb the inductive spikes. Luckily I was away from my bench across the room when that one blew up. It sounded like a fire cracker going off, and my bench was covered by a light sprinkling of what looked a little like snow.

Modern wet electrolytic caps have pressure relief engineered in to reduce damage, I never saw another film cap fail that spectacularly.

JR
 

Newmarket

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I blew up a second cap actually doing my job. Back then I was working on a very early DC to DC switching supply. We had a large metalized film cap across one of the transformer windings to absorb the inductive spikes. Luckily I was away from my bench across the room when that one blew up. It sounded like a fire cracker going off, and my bench was covered by a light sprinkling of what looked a little like snow.
Oh yes - "Indoor Snow" I recall that filling a small industrial unit I was working in sometime in the 90s.
The prototype PSU was just sitting there during lunch but off it went !
 

bone

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I remember back in the 60s I was responsible for the amps used by this Band and while on stage the PA amp (valve/tube) blew an elec capacitor in the cathode of one of the output valves. What a noise, and also a little panic fron the band and the audience. He he. Had to temporarily use the instrument amps for the mics as had no spare with us. Almost had to wash out the chassis to get rid of the bits of aluminium foil. Back up and running though for the next gig a week later. Remembered to bring spare amp this time :)
 

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