signal cap "polarity", finding lower impendence point via schematic

[lorkp]

Hello, please tell me if I have this right or maybe provide a better explanation. Looking at this layout: http://www.cnjradio.net/fenderamps/pro_reverb_aa165_layout.gif , the outer foil would be on the south side of the schematic (from overhead point of view), because it connects to a pre amp tube?

[VacuumVoodoo]

To make it simple: outer foil to the signal source i.e. to where the signal comes FROM. That will be plate (anode) of the tube. A cap whose one end is connected to ground will have outer foil grounded.

[BJF]

Hi,

If you're looking into a real amp of this type it may have or may not have caps that have 'inner foil' written on them and if so those would be blue. It will have no significance imho which direction the cap is used in a circuit like this. Also many amps of this type does not have caps that indicate which side of the foil goes to what leg of the cap.

In some circuits you'd like to connect the outer foil to lowest impedance point to reduce microphonics, also from mechanical vibration, but that is in extreme high gain amplifier stages and in those you may also see things like a wire wrapped around a cap and that wire having one end grounded.

The type of amplifier described in the schematic has no where near the gain to warrant such techniques, though you may have trouble in the tremolo coupler- check waveform across lightbulb: if it is clipped or squarewave you'll get a ticking sound. Fender adviced you'd bypass this with cap while if bias to coupling triode is adjusted so there is a sinewave across bulb ticking goes away. Then also feedback onto B-tree is a lot less.

I wouldn't worry one bit about it

Just a few thoughts
BJ


Affiliations:
www.bjfelectronics.com
www.mpamp.com

[Sarasmiles]

Hello, please tell me if I have this right or maybe provide a better explanation. Looking at this layout: http://www.cnjradio.net/fenderamps/pro_reverb_aa165_layout.gif , the outer foil would be on the south side of the schematic (from overhead point of view), because it connects to a pre amp tube?


I am not sure which brand of signal cap your using, But there is a fairly large misconception on the outer foil orientation on signal caps. ESPECIALLY on orange drops or for that matter, Any signal cap made by Sprague.

This article from The manufacture of Sprague signal caps might be interesting to read.
http://www.sbelectronics.com/images/PDFs/Outsidefoillocation.pdf
Cheers!

Sara :)

[Blue Strat]

I am not sure which brand of signal cap your using, But there is a fairly large misconception on the outer foil orientation on signal caps. ESPECIALLY on orange drops or for that matter, Any signal cap made by Sprague.

This article from The manufacture of Sprague signal caps might be interesting to read.
http://www.sbelectronics.com/images/PDFs/Outsidefoillocation.pdf
Cheers!

Sara :)

Good article. The end marking on these (and very likely all other) caps is random and does not mark the outer foil.

[scottl]

I use a scope and mark my caps. Makes me feel warm and fuzzy. Makes me feel like my amps have extra mojo. ;)

[BJF]

Hi,

Feeling warm and fuzzy is good ;)

Measuring components on the scope before mounting them is maybe adding mojo and will quite possibly affect outlook on amplifier as a precisiontool, regardless if it will effect sound, but knowledge affects feel.

I have worked on extreme gain circuits where maybe one or two capacitors benefitted from being poisitioned in a certain way and where the lead lenght of the component had a direct effect, but those were perhaps Spandex amplifiers with so much gain that both hands could constantely be on the fretboard and sound just resonante. Direction and placement could be determined from using the amp as a Theremine.............oh well that's fun too ;)


Have fun
BJ

[VacuumVoodoo]

Well, things like that usually start somewhere as a real solution to a real problem and then the "news" spreads through the grapevine and becomes a rule in applications that have little o nothing in common with original problem.

"Outer foil to low impedance point" was a rule in High Frequency equipment like very sensitive (= very high gain cascaded multistage front end preamp) Short Wave receivers etc etc. Specialized caps for HF applications still have outer foil marked, these are not your run of the mill orange drops, blue tablets or brown pralines:). Multilayer HF caps do not need this marking by reason of their design.

Cap orientation doesn't really matter in most guitar amp circuits. Old SW habits die hard so I still do the outer foil thing even if mostly for the sake of aesthetics.