Checking cap foil lead polarity w/o oscilloscope

[King Fan]

more difficult with smaller-value caps than it is with larger-value ones.

Good question. This may be true when testing by ear, but I've pictured it the other way around; on a scope, Aiken says, "... start with a 0.022uF cap or thereabouts... the induced signal is smaller at 60Hz with larger value capacitors, and is more difficult to see on the scope." But I'll admit I've been puzzled sometimes; I'd be happy to hear from folks who've done this a lot.

 

[chas.wahl]

Mallories have sometimes puzzled me. OTOH, I hope they're the ones that matter less as to noise in the amp.

I'm not sure which type of cap "Mallories" refers to. The CDE 150 series (metallized polyester film, which presumably does have the outer end of the dielectric/metal connected to one lead). Or the CDE WMF or WMC series, which are polyester film and foil (in that way similar [except in color, shape, lead configuration and various adherents' perceptions of their their tonal qualities] to CDE 225P orange drops) -- constructed the way such caps were before direct metallizing of the dielectric film was a thing? I would imagine that these, too, have a lead connected to the outer corner of the foil. Anybody remember how long metallized polyester caps have been around?

Your puzzlement regarding Mallories may have something to do with the small value these often have in a guitar circuit. From what I think I understand from research online (and in my limited experience with trying to do this myself heretofore), the hum difference is less pronounced with these than with larger caps.

Aiken's article cites as exceptions: "ceramic disks, multi-layer ceramics, or silver micas", and CDE 716P (polypropylene film and foil) in the higher-voltage categories, which 'use a "series-wound" technique that has two separate sections, side by side, with a common "floating" connection layer, usually at the bottom of the layer stack.' CDE's datasheet confirms the differing construction of lower- and higher-voltage types of 716P: "Units rated 100 through 600 VDC are single section designs constructed of plain polypropylene film with extended foil. Units rated 800 VDC and above are series-section designs with extended foil and incorporate a floating common of metallized polypropylene. All units are non-inductively wound."

 

[Short on cash]

I don't want to "start anything" but is this really a thing ?
My first exposure to it was in the 80's with "hi end" stereo.
Do you think Fender or Marshall was concerned with the polarity of this type cap ?
Again, I not trying to start an argument but would like an honest answer.

 

[sds1]

I don't want to "start anything" but is this really a thing ?

The premise is rooted in facts but practical outcomes are indeterminate.

 

[King Fan]

I don't want to "start anything" but is this really a thing ?
My first exposure to it was in the 80's with "hi end" stereo.
Do you think Fender or Marshall was concerned with the polarity of this type cap ?
Again, I not trying to start an argument but would like an honest answer.

It's a thing, honestly, but @sds1 is right. Real? It's simple engineering. Aiken, Rob, and lots of other people a lot smarter than me have documented it. You can usually hear the noise in my simple test -- sometimes one orientation is twice as loud.

But is it a thing *you have to worry about?* Most folks don't. Does *noticeable* noise come out the amp's speaker? I imagine the sum of all that noise is probably quite small. Honestly, I forgot (blush) to do it on my recent (simple) Bassman Micro and the amp is really quiet (with Mallory *150s* ). OTOH, Astron and Ajax and Sozo and Jupiter and Fender boutique caps and others already had / have the outer foil marked -- and they didn't / don't do that for pixie-dust reasons. The way I test is easy, takes about a minute, and I figure reducing potential noise never hurt anyone.

 

[Kevin Wolfe]

I have read the Aiken papers and tested and marked caps. I’ve also changed cap orientation in amps and not noticed any real difference. Maybe it maters more in a higher gain circuit.

 

[Short on cash]

I will say in a funny side note.
At a "hi end" stereo show in the 80's all the displays would have all the interconnects and speaker cables
all arranged "pretty" and zip tied so it looked nice.

There was one display where a well known reviewer came buy and said if they would let her (yes, a woman. Not that that makes a differance. ) rearrange the cables she would guarntee (?) it would sound better. Story goes she rearranged the cables, it looked like a rats nest, but everyone agreed it did sound better.

I was not there to say one way or another but it was in a well known mag.

Just say'n.....

 

[chas.wahl]

And I doubt that Fender paid much attention to it unless it was easy to do so (caps marked by mfgrs), which the yellow Astron and blue Ajax molded caps appear to have been.

 

[joulupukki]

I wonder how much they (Fender) are paying attention to it in their hand-wired modern amps (like the Deluxe Reverb)? I was looking at pictures of the guts of some and noticed the outer foil is marked on the caps.

 

[King Fan]

You guys are killing me. Is it real or is it a conspiracy? Did Fender truly care? Do they care now? Maybe cap makers were (and are) going to all that trouble and expense just for fun?

And I doubt that Fender paid much attention to it unless it was easy to do so (caps marked by mfgrs), which the yellow Astron and blue Ajax molded caps appear to have been.

Fender appears to have paid perfect attention to it for the many years of Astron and Ajax caps, ie, for all those iconic tweed and BF amps. For that matter, maybe the brown and blue blobs were made like their successor ODs, without an outer foil? If so, Fender only stopped paying attention when it didn't matter any more. Now, if you mean did they test every unmarked cap on a scope, then no, they didn't care *that* much. But they built more amps in a day than we all build in a year; I can afford the few minutes it takes me to test.

As for modern Fender, I don't know if they do it 100% per Aiken, but it sure looks like they're paying attention. In all the pics I hunt up, the caps are oriented the same way. @joulupukki , I think you've been looking at '64CDR pics. Are they oriented consistently?

 

[joulupukki]

As for modern Fender, I don't know if they do it 100% per Aiken, but it sure looks like they're paying attention. In all the pics I hunt up, the caps are oriented the same way. @joulupukki , I think you've been looking at '64CDR pics. Are they oriented consistently?

View attachment 1081596

Yup, the ‘64CDR is what I was referring to. There are a few discrepancies between the orientation that they’re using in it vs. what @robrob shows on his DR layouts. But I’m guessing the discrepancies make very little difference, if any.

As a side note, I know the OP wanted to figure things out w/o a scope and that’s how I’ve been trying to do it. But, I recently found a very lightly used digital scope and bought it. Testing caps is quite a bit easier and I double-checked a few that I had to guess on with the traditional way listening for differences – and discovered I was wrong.

Like you, @King Fan I also didn’t pay attention to the orientation when I built my Bassman Micro (in my case because I didn’t really know this was a thing to do). It’s still extremely quiet (self noise) but I wonder if that has more to do with it being such a low-wattage amp?

 

[King Fan]

Helpful, thanks. I noticed Fender didn’t follow Rob in every spot, but it did look like they're consistent — ie, paying attention. Now they may have a different 'rule' in mind; good rule, who knows. Aiken points out some experts get rule details wrong, but I don’t know if Aiken is 100% right, or if there are spots where impedance-wise it isn’t that clear, or if Fender is just doing what they did in 1964.

 

[Lowerleftcoast]

I don't want to "start anything" but is this really a thing ?

Technically it is a thing. In practice, the outer foil *can* act somewhat as a shield. Whether *this* shield is actually keeping the amp quieter is not clear. Imo, it doesn't matter which direction the cap is installed, until it does.

 

[King Fan]

LOL, so I got curious about what Fender *was* doing in the 60s. Here's a '66:

Here's the reissue (from Rob's page) that I posted above:

Here's Rob's drawing:

Here's a reference-quality build that Rob, um, refers to on his AB763 mods page:

My guess is Fender was following a first-order rule, pointing 'paired' caps the same way, while Rob is thinking through the Aiken destinations and downstream signal flow. Notice the third cap from the left is oriented the same way (stripe up) in all four versions. That's the AC shunt cap on the LTP PI -- and stripe up is the 'right' way. This is done 'wrong' by folks who just decide to point all stripes 'down.' So I'm guessing Fender then and now was 'oriented' to outer foil principles, but with some differences from Aiken in interpreting them.

 

[chas.wahl]

Well, as threatened previously here, I went ahead and constructed a device for connecting a cap to the input of an amp, and reversing polarity of the cap almost silently and within a second. It works just as intended. Keeping one hand on the switch side, and using the fingers of the other hand to pinch the capacitor body, I can press down on the whole assembly (activating the momentary SPST "doorbell button" switch on the bottom) while listening, moving the slide switch from one side to the other extreme, or parking the slide switch in the middle (it's DP3T, middle position shorts and holds the input) for removal and marking of the cap, and insertion of the next victim into the alligator (crocodile for you participants whose native fauna lacks alligators) clips.

PDF attached below

The biggest problem is that one of the clips seems to be a bit challenged about gripping a small-gauge cap lead, leading to an unholy buzz noise when the device released from "short" mode. But moving a slightly too-tight plastic sheath back onto the wire fixed that temporarily, until I can replace that clip or sheath.

It's in an old Radio Shack plastic enclosure I had that measures 3" l x 2" w x 1" high. The parts needed are:
• enclosure
• wire (used 20 AWG solid for everything except the external leads, and 18 AWG power cord conductors that are stranded)
• two small grommets for the leads
• two alligator clips
• DP3T slide switch (on-on-on); I used CW G1128S
• SPST momentary switch, push button (NO [off-(on)]) -- I just got a surface-mount doorbell switch
• mono jack to accept amp instrument cable plug

If I had to do it again, I'd probably use a slightly larger enclosure, and get a less protruberant button switch (an NKK Series SB, for instance) for a lower profile. There are such things as DP3T toggle switches, too, though they're more expensive: $12 as opposed to less than $3 for the slide type. I had to Dremel out some fins in the way within the Radio Shack enclosure, mount the jack off-center for clearance at the slide switch, and thin out the box walls where the small grommets are installed.

My experience with it is that it definitely makes it easier, perceptually, for me to distinguish with confidence which polarity produces the least hum. Wiring is such that the clip on the side toward which the switch is thrown when making a quieter hum is the one connected to the cap's outside corner of foil. I tested about 10 or so caps, mixed in type (metallized polyester film [CDE 150], polyester film/foil [CDE WMF and CDE 225P "orange drop"], plus a Vishay metallized polypropylene 510 pF x 1000 V (also orange-drop-ish). Values of the polyester caps ranged from 0.1 uF to 0.0022 uF (2200 nF), and 400 to 630 V.

Observations:
a) The difference in hum magnitude when polarity was reversed, for two caps bought at the same time, with exactly the same specs, varied surprisingly; one being very faintly different, and the other quite obvious. Also, their general levels of hum (say, the average) might vary.
b) I didn't notice that larger-capacitance-value caps typically made a bigger hum than lesser-value ones. These two observations make me think that some individual caps just have a greater hum quotient, and some a greater difference in hum when reversed in polarity, and I don't know why that is.
c) I did physically reverse several of the caps, and do the whole procedure over on them, and detected no difference in the result for a given cap, when installed onto the leads in either direction. That's what I would have expected, but I felt the need to try it.
d) The polypropylene cap exhibited absolutely no hum level difference dependent on polarity. Randall Aiken has said this lack of shield polarity was to be expected with high-voltage PP film caps (citing CDE 716P, but not Vishay), because of internal construction is not roll-up-like, but like interleaved fingers.
e) All that being said, and this final observation calls into question the "necessity" of such a device for testing in this way: a few caps that I tested which had been previously tested and marked then with our best guess as to lesser hum by my me with my son (much more expert amp builder and guitar player than me) a couple months ago using "rudimentary" equipment (a jack with two leads and alligator clips, swapping polarity manually) ended up testing exactly the same way using this new device. Still, doing this unassisted, I was much more confident in my judgment about which polarity hummed less, and made, I think, a lot less extraneous noise while executing the testing. So I'm happy to have it.

 

[__HKGuns__]

If I didn't have an O-Scope, I'd probably build one too. Nice job working that out.

 

[King Fan]

Yeah, that rig is impressive. Having done it the old-fashioned alligator-n-ears way, I admit I feel better knowing the switch just made it easier to confirm your impressions.

 

[58Bassman]

You guys are killing me. Is it real or is it a conspiracy? Did Fender truly care? Do they care now? Maybe cap makers were (and are) going to all that trouble and expense just for fun?


Fender appears to have paid perfect attention to it for the many years of Astron and Ajax caps, ie, for all those iconic tweed and BF amps. For that matter, maybe the brown and blue blobs were made like their successor ODs, without an outer foil? If so, Fender only stopped paying attention when it didn't matter any more. Now, if you mean did they test every unmarked cap on a scope, then no, they didn't care *that* much. But they built more amps in a day than we all build in a year; I can afford the few minutes it takes me to test.

As for modern Fender, I don't know if they do it 100% per Aiken, but it sure looks like they're paying attention. In all the pics I hunt up, the caps are oriented the same way. @joulupukki , I think you've been looking at '64CDR pics. Are they oriented consistently?

View attachment 1081596

Look at the stripes, even on non-polar caps- they're consistently on the end that's more negative- doesn't need to be grounded, just more negative and that makes the - end the cathode. Look at t he resistors, too- the first bands are all on the same end.

I don't remember where I saw it- might have been a page from BillM, but I made a harness with a 1/4" plug on one end and two alligator clips on the other, with the Red clip attached to the tip of the plug. This plug is inserted into an input jack on any guitar amp and the outer foil is found by touching the outside of the cap- if it hums, reverse it. If the hum stops, mark the end that connects to the outer foil and move on to the next cap. It mattered in my Bassman.

 

[Killing Floor]

9V battery tongue method?

 

[Peegoo]

You guys are killing me. Is it real or is it a conspiracy?

It's real, because noise in a circuit is cumulative. If you're building something with one cap in it, probably not much of a concern. But when you add more caps to the mix, each one of them presents an opportunity to allow noise to enter the signal. Every little thing you can do to reduce noise does matter because a bunch of 'littles' adds up to one 'big'.