12AX7 HUM

[big_teee]

The outside foil is marked with a dash line on the case of the capacitor, the outside foil faces the source (the outside is the input).
The outer foil should be on the plate.
I didn't invent it, I just follow the tradition.

Electrically, does that really make any Difference?
InQuiring Minds want to Know!
B_T

[jmaf]

Electrically, does that really make any Difference?
InQuiring Minds want to Know!
B_T

It easily picks up noise when put backwards.

[Chuck H]

It easily picks up noise when put backwards.

The difference in impedance or resistance must be mice nuts because I've put this to the test and heard no difference between orientations. And this is in guitar amps. Where the gain is thousands of times greater than normal amplifier applications.

[dai h.]

I think it's kind of bogus also (as far as practical value--not to say that there isn't a difference with some of the tests come up to ascertain outside foil). Maybe orientation (physical), distance more important (for noise).

[km6xz]

The foil coverage does make a difference, you can measure and hear the difference. For bypass operations, the foil end should be closest to ground. For coupling service, the lowest Z connection point should be at the foil outside. Tubular caps might not have a marking, if not mark it yourself. The most popular simple method is to connect the cap across the your scope probe and put the cap in a field, like holding in your hand and see which orientations has a lower noise amplitude. That is the foil connection end that is connected to the probe ground.
Most old caps are marked but most of them have deteriorated by now so not many caps used in new builds will have bands.
There used to be shielded caps that had a metal enclosure that was was grounded while the cap leads were brought out without regard to orientation. Those were most used in the 30s up to late 40s for higher end or industrial gear. They were mounted something like a narrower cross-section stud mount can electrolytic. I had an old E.H.Scott(that was before H.H. Scott who made consumer products) receiver for short wave from the mid 30s that all the caps were metal tubes sticking out of the top of the chassis like thick porcupine quills. The chassis was chrome plated heavy gauge steel and looked like new when I got it in the 1970s. Never had to change any of those caps. During WWII larger caps were mostly oil filled metal modules bolted to the chassis. Those "bathtub" caps are usually still making spec now.

[jmaf]

The difference in impedance or resistance must be mice nuts because I've put this to the test and heard no difference between orientations. And this is in guitar amps. Where the gain is thousands of times greater than normal amplifier applications.

Hmm. How did you test it? I read this in a Gerald Weber book years ago, so I decided to test some capacitors back then. When the foil is out on the grid sides and you hold the cap tight with thumb and index finger, there is a thunderous hum. When I inverted it it would not hum that way. I would pass the soldering iron by it too, try other noise sources, the advice seemed sane.

[Jazz P Bass]

Nice info to have on hand.
Thanks.
Which brings up the question: If the caps are that "picky", why even bother using them in a guitar amp?

[Chuck H]

Ahhh... I have noticed this too. But at a different time than I did my orientation experiment. I'd say it's worth orienting the outer foil if only for this reason.

My amp layouts are a bit roomy compared to some. Closed in a chassis there is no noise difference in my amps between orientations. So I think dai might be onto something with the theory that proximity may be more important than foil orientation. But the next time I have one open I'll grab a cap with my fingers and see. Since I don't plan to hold the caps while playing I probably won't trouble to swap them around though

[dai h.]

Hmm. How did you test it? I read this in a Gerald Weber book years ago, so I decided to test some capacitors back then. When the foil is out on the grid sides and you hold the cap tight with thumb and index finger, there is a thunderous hum. When I inverted it it would not hum that way. I would pass the soldering iron by it too, try other noise sources, the advice seemed sane.

I get the sense somewhat that some might think correctly orienting the outer foil is some sort of cure for a noisy amp, and that wouldn't seem to be true (for me). Being confimed to be less noisy when noise sources are brought closer is useful to know but I'm thinking (in the normal course of doing the layout) you don't bring noisy things close (or orient them for the least interaction--such as right angle, orthogonal, or whatever) anyway.

As far as someone trying to obtain less hum, noise I'd look into (google, search archives, etc.) noise theory, layout grounding, plus study (good) existing layouts, possibly finding amps where things have to be more stringent (high gain, more noise sensitive) and studying how things have been dealt with.

[soundguruman]

It is more noticeable in hi fi amps, where the frequency response (unlike guitar amp) is wide. And the speakers (unlike guitar amp) is full range.
I am always trying to tweak out noise and distortion that may not be audible, but I see it on the scope, and I want it gone.
It may not always be hear-able, but I want the performance to be clean, technically. And then again, some of it is I think, noticeable.
There are different types of listeners, some who notice less and some who notice more.
I am the type of listener who wants all the noise and hum removed. But many people would probably not notice it at all....

[soundguruman]

It is more noticeable in hi fi amps, where the frequency response (unlike guitar amp) is wide. And the speakers (unlike guitar amp) is full range.
I am always trying to tweak out noise and distortion that may not be audible, but I see it on the scope, and I want it gone.
It may not always be hear-able, but I want the performance to be clean, technically. And then again, some of it is I think, noticeable.
There are different types of listeners, some who notice less and some who notice more.
I am the type of listener who wants all the noise and hum removed. But many people would probably not notice it at all....

Its just that some try to build as much gain as possible, with as little noise as possible, and we are taking cues from the classic amplifiers.
Not everybody wants that much gain in a guitar amp. And not everybody notices the noise, or is bothered by it.

[jmaf]

Ahhh... I have noticed this too. But at a different time than I did my orientation experiment. I'd say it's worth orienting the outer foil if only for this reason.

My amp layouts are a bit roomy compared to some. Closed in a chassis there is no noise difference in my amps between orientations. So I think dai might be onto something with the theory that proximity may be more important than foil orientation. But the next time I have one open I'll grab a cap with my fingers and see. Since I don't plan to hold the caps while playing I probably won't trouble to swap them around though

You're right it's not a big of a deal, at least in my lesser experience.

It does work to quiet some amps down, it has worked especially well for me one particular time, on one of the coupling resistors out of the phase inverter. Weird pesky 120hz hum that wouldn't go away, it had a beating to it, like another frequency of hum with the 120hz, in desperation and unable to find the cause for hours, I held the two nearby phase inverter caps like so, one of them hummed, it was the one at the edge of the board, closer to the power supply sector....I tried inverting it and it worked....it was picking up some vodoo from the PS....

Something related: when you hook up the cap straight to the potentiometer lug and you've got it with the grid side on the outside foil, if you just tug the cap against the potentiometer body it will quiet some amps down. I had to use straps around caps on pots one time to keep the cap tight against the pot body. This also worked for me once.

The photo below is of Komet nr. 133, I think it was built by Ken Fischer. Note the second cap tugged against the pot body. Kinda obvious, but some may overlook this, as I've in the past. (Edit: I'm not saying they did that Komet for that reason(noise), it's just an example of how I meant "tugging" it would be.)


Edit: Reworded first paragraph, it was portuenglish.