Which direction should banded (foil case) side of signal caps go in an amp circuit?

[mikepukmel]

I read someplace that the banded end should go towards the lowest potential. So, if a cap is coming off the junction of a plate and a resistor that feeds the plate, the cap band should not be on the junction side, but the other side, i.e. towards the next input stage.

Just read a post someplace where they had them in several directions in an amp, and said that the banded side should go towards the plate, in tube amps.

Is there a right way universally, or is it not so cut and dried?

[Enzo]

It is not so cut and dried because it is not so important at all.

On the internet, you will find people to argue about ANYTHING. And whichever side they take, they will put great energy rationalizing reasons for their choice.

Outside foil makes darn little difference in a guitar amp. I makes a lot more difference in say a short wave radio, which our guitar amp is not.

I like my parts to face the same way on the part board. To me that cosmetic difference is worth more than some outside foil argument.

On the other hand, if you pick one approach, and defend it to your death, you won't be wrong.

[mikepukmel]

It is not so cut and dried because it is not so important at all.

On the internet, you will find people to argue about ANYTHING. And whichever side they take, they will put great energy rationalizing reasons for their choice.

Outside foil makes darn little difference in a guitar amp. I makes a lot more difference in say a short wave radio, which our guitar amp is not.

I like my parts to face the same way on the part board. To me that cosmetic difference is worth more than some outside foil argument.

On the other hand, if you pick one approach, and defend it to your death, you won't be wrong.

Thanks Enzo!! Im thinking about taking a spray can, and painting the resistors all bright red, and leaving a little tiny window just to leave teh capacitance and voltage rating show. All those bazillion old classic amps, most don't have any bands anywhere on those caps.

[catalin gramada]

Thanks Enzo!! Im thinking about taking a spray can, and painting the resistors all bright red, and leaving a little tiny window just to leave teh capacitance and voltage rating show. All those bazillion old classic amps, most don't have any bands anywhere on those caps.

do that and you will modify thermal derive of you components, so take this in account

[mozz]

The foil goes toward the side with the lowest ground potential. But i have to ask, do any modern caps have the outside foil marked?

[Chuck H]

I've read that some caps have the markings arranged so that you know where the outside foil is by which side, say, the value is printed on. I can't remember any specifics on this and the caps I'm referencing are actually made by another manufacturer now, so it's really no holds barred. But my point is that there MAY be info out there for SOME cap models that indicates where the outside foil is.

And your right, sort of. I've read that the outside foil connected lead should be nearest to ground, but also that it should be to the lowest impedance. They can be two different things and exist on either side of the same circuit. Orange drops use to be marked for their outside foil. I experimented a little with orientation. I can't say I eliminated every possible scenario at all but I never found orientation to make any difference in noise or tone.

[Sowhat]

Mr. Carlsons Lab on YouTube has an excellent video on this very subject. Now, he does a lot of radio work so capacitor shielding is pretty important to his work however as Enzo pointed out, musical instrument amplifiers are pretty much immune to this type of mismatch. He shows that even though there is a band on the capacitor it does not necessarily mean that is the case side of the foil. He finds them randomly marked but has come up with a rig using a scope and your own body as an antenna to determine the outer and inner leads of a cap regardless of it's markings. Interesting stuff, check it out.

https://www.youtube.com/watch?v=BnR_DLd1PDI

[DrGonz78]

There is a very long thread from a while back here on MEF about this topic too.

http://music-electronics-forum.com/t39867-2/

[Steve A.]

One quasi-scientific answer...

I read someplace that the banded end should go towards the lowest potential. So, if a cap is coming off the junction of a plate and a resistor that feeds the plate, the cap band should not be on the junction side, but the other side, i.e. towards the next input stage.
• Just read a post someplace where they had them in several directions in an amp, and said that the banded side should go towards the plate, in tube amps.
• Is there a right way universally, or is it not so cut and dried?

With 630vdc 418P and 715/716P orange drop capacitors you can actually see and feel which lead is connected to the outer foil. In a critical circuit having the outer lead connected closer to ground potential can add some shielding, at least in theory. It probably makes absolutely no difference at all most of the time.

When I was experimenting with my Blue Express amp builds inspired by Trainwreck schematics in the early 2000s I discovered that I had wired the .001uF coupling cap after the 2nd stage backwards. I switched it around and found that the amp had lost a definite on-the-edge aliveness that was very responsive to my playing. So I switched it back and that responsiveness returned.

My guess was that there was some sort of parasitic activity going on for lack of a better explanation. To experiment with intentional parasitics you can wrap an insulated wire around an orange drop coupling cap and then connect the other end to the plate of a preceeding tube stage. If you go back 1 stage you would get negative feedback, if you go back 2 stages you would get positive feedback. (To reduce parasitics you can connect the wire wrapped around the cap to ground.)

So why the heck would anybody want to introduce positive feedback to a guitar amp circuit?!? My own theory (entirely unconfirmed by reality) is that Ken Fischer would experiment with the layout of each amp he built on perfboard until the parasitics were just right and the amp had just the right amount of positive feedback to get as close as he dared to the edge of oscillation.

Here is the reasoning behind my theory. We know that Ken built his amps on perfboard which would allow him to orient each component in all 3 dimensions. We also heard the report that he would spend up to a week fine-tuning and tweaking the design. As shown in the layout drawing of a Liverpool or Express clone there are only 27 components on the preamp board, 11 capacitors and 16 resistors so what the heck was he doing in his basement for a full week?!? I think that he was moving the main coupling and tone caps, et al around until the sound was just right.



I thought it was amazing that he could fine tune an amp in his basement and know that it would deliver the goods in a large venue like Madison Square Garden. I am sure that there was something he listened for and when the amp had that special something he knew it was golden.

Enough for my quasi-scientific explanations which will be debunked by anyone with any knowledge about electronics. But I know what I heard in my Blues Express experiments and I have been unable to find any other explanation.

Steve A.

P.S. I had first tested Orange Drop caps for the outer foil lead using a 1/4" cable with two alligator clips plugged into a guitar amp. I would clip the test clips first one way and then the other to see which hummed more while holding the cap in my hand. Sure enough the bulge in the cap indicated the outer foil lead every time.

I have not bought any OD caps for many years so perhaps they have changed. (I usually use Mallory 150's instead of orange drops since they cost less and take up a lot less space.)

.

[mikepukmel]

Wrote a too long post, erased it, but I think the whole thread makes a lot of sense now, thanks everyone. If you have an amp builder guru, like Fisher or Dumble, who would toil for weeks or more to make one amp, they maybe they will sift through a huge pile of cap's, and fiddle with orientation and positioning of all the caps in an amp to "tune" it, and maybe they have some way of figuring out when it makes a difference. But for the 99.99% of us, replacing one or two caps, or even reversing all of the caps in an amp, it probably won't make an audible difference.

On amp #1, The FrankenDeluxe, it had a certain sound. I can't describe the tone, but it had a sound character. I pulled all the components off the old board that was not intended for reverb, soldered all the components back down to a new board, and wired it back up, and ... it sounds like a completely different amp. Same caps, same resistors, MUCH neater wiring, and added 2 new grid stops to V2 and one to one side of the reverb driver (I think 5.6K) Didn't change tubes, speaker, etc. I think the orientation of the "signal" caps is about the same. The second wiring,, for the lack of a better description, is 'darker'. Has more boomy bass (not a huge amount), doesn't sound as bright. The tone controls all work better. Before, the bass control didn't do much. I could hear a difference, but now its more pronounced. The highs are slightly less caustic, a little more clear. Admittedly, my ear isnt so good, like a recording engineer who can hear the tiny fizz and ring here and there, but it is noticeably different.

I really wish I had a recorded raw guitar that I could use as input (so it would be exactly the same), and do an AB comparison, but I didn't record anything before.

Maybe this is also partly due to what you're talking about Steve, little parasitic loops throughout the circuit that are now just tuned differently now.

Based on my ad hoc, unintended experiments, I can see how a mfr could produce 1000 amps, all the same parts, same person wiring, same mfr tubes, and get some that sound noticeably VERY different. I wish I had the smarts and technical background to be able to figure out what I did that caused the difference in tone!

[J M Fahey]

It is not a *potential* issue at all, but an *impedance* one.
The outer foil is a shield by definition, a metallic foil wrapping the rest of the capacitor, and as we already know, shields to do their magic, must be grounded.
Ever saw a guitar cable with center conductor grounded and shield carrying signal?

If not actually grounded, best (or least bad) is "the point with lowest impedance to ground"
Imagine a 100k resistor tied to a plate, and a 1M resistor tied to next grid ... which one has lowest impedance to ground?

That said, you are shielding just a part of a path which may be much longer.
If plate gos to a grid 1 inch away, fine; if signal travels through 4 inches of wire or more, won´t do much.

As of paint influencing thermal behaviour of a capacitor, IF the capacitor dissipates power (what a resistor does) the thin layer of paint will increase thermal resistance and resistor will work (a little) hotter, but capacitors do not dissipate significant power, so .......
Capacitors take whatever ambient temperature surrounding them, whatever it is; a paint layer in theory will make that process infinitesimally slower .... unmeasurable.

Even if the capacitor is wrapped in styrofoam, it will work at ambient temperature, it will just take a little longer to reach it (suppose you just turned your amp on).

[Dave H]

Imagine a 100k resistor tied to a plate, and a 1M resistor tied to next grid ... which one has lowest impedance to ground?

I have an amp with a switchable coupling cap between the 100k plate and 1M vol pot. If I run the amp chassis out of the box the hum increases as the coupling cap is switched from 10n in stages to 500p. I assume this is because the impedance at the vol pot is higher for smaller coupling cap values because the vol pot can't "see" the lower impedance plate through the impedance of the small cap.

[g1]

Based on my ad hoc, unintended experiments, I can see how a mfr could produce 1000 amps, all the same parts, same person wiring, same mfr tubes, and get some that sound noticeably VERY different. I wish I had the smarts and technical background to be able to figure out what I did that caused the difference in tone!

Not sure if it was your intention, but you have identified one of the big reasons manufacturers like printed circuit boards, and one of their greatest benefits: consistent products.

[bob p]

Mr. Carlsons Lab on YouTube has an excellent video on this very subject.

I've seen that video before. I had forgotten that Mr. Carlson was the guy who lives in a house that was actually built out of old test gear.

[mikepukmel]

I've seen that video before. I had forgotten that Mr. Carlson was the guy who lives in a house that was actually built out of old test gear.

[ATTACH=CONFIG]48482[/ATTACH]

That looks like the coolest places in the whole world. I can imagine what my wife would say if she saw that lab!