Outer Foil Test

[Funkalicousgroove]

The Dumbles I have seen all seem to have the Marked side of the Cap toward the PT except for the .02 at the entrance of the PI, I wonder..........

Not sure what you mean by PT. But they normally point towards the plates. On the PI, the low impedance side is on the input side. The grid side of the PI has an impedance of several megs due to the bootstraping of the PI.

Gary

EDIT: Actually the grid side is probalby in the order of 20 to 40 meg. A DVM will load the heck out of the grid voltage.

PT= Power Transformer. By marked I mean the side with the writing, none of them seem to have the foil end marked, but some have their actual value written on them.

[yeahyeah]

this is one of those things I have yet to fully grasp when it comes to tube circuits.

i think i *kind of* understand but i'm not sure.
With the inner and outer foil stuff, the outer foil should be towards the lower impedance. So, with a cap in the tone stack the outer foil would be closer to ground correct? In coupling positions, should the outer foil be closer to the grid of the following stage, with the inner foil closer to the plate of the previous stage?

do i have this right or am i backwards?

could someone enlighten me?

[ampdork]

Tone stack caps are coupling caps if you think about it. I always orient towards the plate or the lowest impedance path to ground.
Also I don't use foil or fingers. I simply put my scope leads across the cap and run the cap over some ac line. Sweep the cap across and then again with the leads flipped. A little practice and it's easy and quick. For the record too Gerald Weber wrote it backwards in his book. Aiken got it right.
If you look in classic fenders you can see they took the time to orient the foils (all the ones I have checked so far were properly marked btw) yet they never bothered wiring the filaments to buck hum. Look at the old fenders for a lesson on how they ought to be oriented.

[yeahyeah]

Ah ok,

so i was backwards.

[toneman]

I wonder when some one who has a scope is going to offer this sevice(HINT)

[Structo]

For the record too Gerald Weber wrote it backwards in his book. Aiken got it right.

Hmmm, could you elaborate on this.
I have Gerald's Amplifier Essentials book and that's where I heard about orienting foil caps.
What did he make a mistake on?
Thanks

EDIT:
Ah, I found it at Aikens site.

[chocopower]

I found this...

http://www.geocities.com/wa2ise/radios/ofoil.htm


Could be a good option for non-osciloscope-owners?

[gearhead]

Also I don't use foil or fingers. I simply put my scope leads across the cap and run the cap over some ac line. Sweep the cap across and then again with the leads flipped. A little practice and it's easy and quick.

You mean you wave it over an active AC line (EM Field)? If so, how do you tell which is the foil-connected lead; the greater measurement with the + scope probe?

[Fischerman]

Whether you use the AC line or your fingers...the way that shows the least noise has the outer foil connected to the ground lead. The most noise means the ground lead is connected to the inner foil. It's about a twice as noisy with the ground lead to the inner foil...in many cases I didn't need to clamp it with my finger or use an AC line...the noise was just there and it was enough to see the difference...but I did have my amp chassis (which was plugged in but not on) sitting right there.

[JimiB]

On the PI, the low impedance side is on the input side. The grid side of the PI has an impedance of several megs due to the bootstraping of the PI.

so in this case the old fenders are wrong? They have the lines towards the 1M resistors.

[JimiB]

please I would like to get this right.

[glasman]

please I would like to get this right.

I would personnally place the foil away from the Grid for best noise performance. So I guess I would have done it differently than Fender. That said, I have not swapped the caps around in my 66 Super Reverb

Gary

[Tdale]

Ok..

So caps that are grounded, have the outside foil towards ground?

caps in tone stacks have the outside foil towards the tone pots?

coupling caps have the outside foil towards the next stage?

Is that about right?

[HiGain]

Ok..

So caps that are grounded, have the outside foil towards ground?

caps in tone stacks have the outside foil towards the tone pots?

coupling caps have the outside foil towards the next stage?

Is that about right?

Here's my understanding. The outer foil serves as a shield from ambient noise. When you connect the outer foil to the lower impedance to ground side, you are in effect providing the lower impedance path for that noise to be conducted to ground, keeping your signal more pure.

Remember that your Power Supply is more or less AC coupled to ground.

To orient your cap, essentially you just look for the side of your cap in question to determine which has the least resistance to ground. In the case of a coupling cap preceded by a 100K plate R, feeding a 1M pot, the lower impedance side is facing the 100K plate R. For caps in tone stacks, determine the most common ranges for the bass and mid pots and contrast the resistance to ground through the pots at those settings to the resistance back through the Slope R and Plate R.

Yes, for caps connected directly to ground the outer foil points to ground.

Hope that didn't ADD to the confusion!