Outer foil direction - EQ and pre PI coupling cap

[pjd3]

Hey there, thanks for stopping by.

While I realize this is no show stopper for many builders, I figure its only about 15 minutes to "ear test" a handful of caps to establish the outerfoil, and only a few more seconds to mark the foil lead and position that to lowest impedance direction, in attempt to minimize buzz/noise and not feed this interference onto the grid of the next stage.
I don't consider it a waste of time for me even it if wouldn't make much of a difference I can at minimum learn something about how to think of some circuit things.

In this Sluckeys Plexi build I bounced this consideration off of Robs Blackvibe who happened to illustrate the recommended direction of the outer foils of various caps. This helped to generate some theory considerations about how to translate this to the plexi preamp, but I did have reservations primarily on how to think about the eq bass capacitor and what could be considered the path of least impedance.

The mid .022uF cap seemed to be fairly obvious - it goes right to a 25K pot and straight to ground
The pre-PI .022uF coupling cap seemed fairly obvious as one of the ways points straight into the grid of the PI - I had to suspect that directing the foil to the 1M pot to ground may be the way to go with that.
The bass .022uF cap eludes me a bit but, I suspect that directing the foil toward the 33k slope resistor and 100k cathode resistor to ground (of the previous stage cathode follower) seemed like the way as opposed to directing the foil towards the grid of the next stage.

I'd be interested on your thoughts and what you may choose to do if you had to!
Thanks for your feedback
And for a bit of findings on my part, I found over these 2 builds that the Mallory 150's and Sozo's are very easy to determine the foil ends with my POD and headphones, as opposed to the Jupiters. I could barely hear a diffence with the the Jupiters. The Mallories were especially obvious, like 2-3 times as loud one way verses the other!

Thank you very much,
PJD3

[wpaulvogel]

Ground is not what you are pointing the outside foil towards, it’s the lowest impedance you are pointing it at. The plate 100k into the dc supply is lower impedance than through a 1meg pot. The cathode resistor to ground is also lower impedance than the 1meg grid leak. Make sense. You’re going to find that people put the Sozos in wrong, the black stripe doesn’t always point the same direction. It might look nice with them all facing the same way, but it’s wrong.

[pjd3]

Thanks wpaulvogel,

So lets see, by what you describe, the foil for bass cap, mid cap and pre-PI coupling cap would all be directed back toward the input direction (down stream), which amounts to pointing the outer foils toward the cathode/100k resistor/Ground of that previous cathode follower stage. Sound like I'm on the right track?

Damn, and there was a day I used to ace tests on impedance vector addition, and draw out impedance curve charts with Freq vs Impedance. I think its time for my old brain to take a refresher course!

Let me know if I'm in for any HUM Buzz !

Thanks wpaul,

PJD3

[wpaulvogel]

That’s right, you will usually always face the stripe towards the previous stage.

[pjd3]

Great wpaul.,

What I do for all the eq and coupling film caps is set up my POD amp modeler and headphones, and do the finger/thumb squeeze, and when I have determined the cap polarity that makes the loudest buzz, I mark the side of the cap connected to the input tip, just put a little dot on the edge of the cap with a sharpie. I've heard often enough that you can't go by markings on the caps. And I have found that the orientation of the labels does not determine the orientation of the outer foil so I always check and mark them all.

Thanks!

PJD3

[wpaulvogel]

I have a scope and set it really low and watch the waveform, the lowest amplitude hooked to the ground lead is the outer foil. I mark it. I found that Sozo and Synergy Royal Mustards so far are correctly marked. The CDE 715 and 716 must be checked and marked. I haven’t used any Mallory 150’s in a while and I’ll have to mark them also. It’s a good idea to put them in correctly. I wish I had been doing this all along.

[pjd3]

I keep saying I'm going to look at my caps on a scope in work but, keep getting impatient and just do the headphone/POD method. It does seem to work pretty good (except for the Jupiter caps I put in my Blackvibe). I am really interested though to see what the noise looks like on a scope. Ah someday. Maybe on project #3.

My 6L6 Blackvibe is a very quiet amp and I was pretty vigilant about the cap orientation but then again its only a 2 gain stage amp not including the PI. Its a quiet clean machine, just how I wanted it. The 6V6 plexi however I imagine would be more revealing of noise or buzz harboring elements so, things like this I take more seriously with this build

PJD3

[frankdrebin]

i wonder if mr Dumble has ever followed these old rules,
i guess not.

[wpaulvogel]

It’s hard to say about Dumble, but I wouldn’t put it past him. I believe he’s kinda OCD about doing it right. He may know something and he might get really lucky too.

[martin manning]

I keep saying I'm going to look at my caps on a scope in work but, keep getting impatient and just do the headphone/POD method. It does seem to work pretty good (except for the Jupiter caps I put in my Blackvibe). I am really interested though to see what the noise looks like on a scope.

Either way works fine. (Spoiler alert) The wave form looks like the beat-up sine wave you see coming out of your mains power.

i wonder if mr Dumble has ever followed these old rules,
i guess not.

I'm guessing he did, given that vacuum tube radios were still common in the era in which he was learning electronics, and his generally meticulous attention to detail.

[hitchcaster]

the caps are polarized sonically.. so think of the stripe side as your + and the side with no marking as your -...
maybe their are exceptions but thats basically the "rule"... or another way to look at it.
and yes, dumble knew which way to put his caps

[Dingleberry]

I’ve never checked the orientation of the film caps before, but will definately do it with my coming Marshall build. Found and watched a very thorough video about it and now I kind of feel stupid that I haven’t paid attention to this subject before although I’ve known it matters
From now on I’ll be doing this

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