From: Steve A. (steve_ahola@yahoo.com) Date: 11/24/2000 7:52 AM Subject: Outer Foil on caps (Continued) Both Randall and Bruce Collins recommended connecting the shielded outer foil terminal to the point with the lower impedance to ground. I believe that I ran across an exception to that rule in my Blues Express amp project: http://www.blueguitar.org/bluzxprs.pdf -or- http://www.blueguitar.org/bluzxprs.gif The cap in question which made worlds of difference in the noise level of the amp (especially with the Volume control set to 0) was the 0.001uF coupling cap I used after the second stage. In this amp I used a 100k plate resistor for the second stage and a 56k grid load resistor for the third stage. In lieu of actual impedance measurements I used the dc resistance scale on my DMM. In any case, with the outer foil lead connected to the plate of the second stage, I noted a definite reduction of noise in this amp. However the dc resistance at this point is higher than that at the other end of the cap (with the 56k grid load resistor). So I believe that the orientation depends on the actual circuit in question. Let us pretend for a moment that I decided to replace the 56k grid load resistor with one that was 150k (as is listed in many of the Express schematics). In this case, the cap lead going to the 2nd stage plate would have a lower resistance to ground so it should be connected to the outer foil lead (which was how I wired it up to lessen the noise in the amp). Now if we were to lower the grid load resistor to 100k or less, does that mean that we should flip the .001uF cap around to minimize the noise level (by following the lower impedance rule)? I would have to say "no" because the circuit still works basically the same whether the grid load resistor is 56k, 100k or 150k. So I believe that logic tells us that there are exceptions to this lower impedance rule. As for the orientation of the bass and mid caps in the tone stack, I thought that the noise level improved with the outer foil lead connected to the 100k slope resistor, but it was not as drastic of a change as with the .001uF coupling cap so I might have been mistaken... In any case the pot side of the mid cap is much closer to ground than the slope resistor side, yet it sure seemed like the noise level was reduced with the outer foil lead connected to the slope resistor. ;) Respectively submitted by- Steve Ahola From: Randall Aiken (reaiken@aikenamps.com) Date: 11/24/2000 1:46 PM Subject: Re: Outer Foil on caps (Continued) In lieu of actual impedance measurements I used the dc resistance scale on my DMM. In any case, with the outer foil lead connected to the plate of the second stage, I noted a definite reduction of noise in this amp. However the dc resistance at this point is higher than that at the other end of the cap (with the 56k grid load resistor). You can't go by DC resistance measurements. The actual source impedance on your second stage is 100K in parallel with the internal plate resistance of the tube (around 62.5K typically), which gives a source impedance of 38.5K. This means the plate is indeed the lower impedance source, and is where the coupling cap should be connected. You'd have to load the next stage with a resistor less than 38.5K to make it a lower impedance, which is a mighty hefty load to drive. Even so, since the plate source impedance is so low to begin with, it is better to have the plate connected to the outside foil rather than the grid regardless of the value of the following load resistance, because at frequencies where the reactance of the cap is high, you end up with a great big ol' capacitor plate hanging in the air picking up stuff, and the low impedance on the plate will not attenuate it much at those frequencies. For instance, the capacitive reactance of your .001uF coupling cap at 60Hz is a whopping 2.65 Megohms. If you connect the outside foil to this point, you'll get little or no benefit from the shielding, because the cap "looks" like a 2.65 Meg resistor going back to the 38.5K plate. All the noise on that plate gets coupled into the grid, because of the high level of isolation provided by the 2.65 Meg "resistance". Stick to the rule on coupling caps for lowest noise pickup. Randall Aiken From: Steve A. (steve_ahola@yahoo.com) Date: 11/25/2000 1:18 AM Subject: Re: Outer Foil on caps (Continued) Randall: Are we agreeing to disagree, or disagreeing that we do agree? ;) ... it is better to have the plate connected to the outside foil rather than the grid regardless of the value of the following load resistance... I believe that I had suggested exactly the same... You'd have to load the next stage with a resistor less than 38.5K to make it a lower impedance, which is a mighty hefty load to drive. Well, several people had wired up this basic Express preamp with a 33k grid load resistor (or even lower!) No point other than just pointing that out... ;) For instance, the capacitive reactance of your .001uF coupling cap at 60Hz is a whopping 2.65 Megohms. Actually the background noise in question was more of a "sh-h-h-h-h-h" white noise than a 60Hz or 120Hz hum. Just for the heck of it what would the capacitive reactance of the .001uF cap be at 1000Hz? Stick to the rule on coupling caps for lowest noise pickup. Which rule? The one about going with the lower impedance point, or the one about connecting the outer foil of a coupling cap to the plate? Tone Stacks The bass and mid caps of a BF style tone stack got me scratching my head. The mid cap with the mid control set to 0 is actually at ground so it seems like the outer foil should be connected to the mid pot. But with the bass cap, the lower impedance point may depend on the value and setting of the bass pot. Any suggestions on this? --Thanks! Steve Ahola From: Randall Aiken (reaiken@aikenamps.com) Date: 11/25/2000 10:08 AM Subject: Re: Outer Foil on caps (Continued) Are we agreeing to disagree, or disagreeing that we do agree? Heh...I *think* I was attempting to tell you that when you thought you were connecting it "wrong", you were really connecting it "right", which is outside foil to plate. When you thought you had found an exception to the theory, you were actually confirming it! :) ... it is better to have the plate connected to the outside foil rather than the grid regardless of the value of the following load resistance... I believe that I had suggested exactly the same... Yes, I was confirming that, and elaborating on the reasons why (the change in reactance with frequency stuff). Whether your noise is hiss or hum, it is still better to connect the outside foil to the previous stage's plate. The formula for capacitive reactance is: Xc = 1/(2*Pi*f*C), so at 1kHz, your cap "looks" like a 159K resistor (well, sort of!). Now, here's the deal on why it makes so much difference: For the sake of an example, assume the offending noise source "looks" like a 10V source with an impedance of 500 Megohms. If this noise is applied to the outside foil plate of the cap, when it is connected to the grid/56K point, it will generate a voltage of 10V*43.6K/(43.6K+500Meg) = 0.87mV. The 43.6K is figured as the parallel combination of the 56K and the effective 159K in series with the 38.5K output impedance of the previous stage. The 10V noise source, when applied to the voltage divider created by this parallel combination and the noise source impedance, results in a 0.87mV signal at the grid. Assuming a voltage gain of 60, you end up with a noise of 52mV at the plate, which is then further amplified down the line. If the cap is now reversed, and the outside foil is connected to the plate of the previous stage, and the same 10V/500Meg noise source is again applied to the outside foil, it generates a voltage of 10V*32.7K/(32.7K+500Meg) = 0.65mV. This induced noise is then further attenuated by the voltage divider created by the effective resistance of the cap and the 56K, so the noise signal that appears at the grid is now 0.65mV*56K/(56K + 159K) = 0.17mV. Again, assuming a voltage gain of 60, you end up with a noise signal of only 10mV at the plate, compared to the 52mV when the cap was reversed. This is a reduction in noise of 5.2 times, which is quite significant. If you run the numbers at 60Hz, by changing all the 159K's to 2.65Meg, you end up with 1.1mV noise at the grid in the "wrong" direction and 16uV in the "right" direction, a reduction of 69 times, which means a huge reduction in potential hum pickup. As you can see, the smaller the cap value or the lower the offending frequency, the more attenuation you get (at that frequency) from the "correct" outside foil orientation. Which rule? The one about going with the lower impedance point, or the one about connecting the outer foil of a coupling cap to the plate? The "rule" I am speaking of is the one that connects it to the lower impedance point, which is almost always the driving source, even if you have a lower resistance at the grid (which is seldom done because of the extreme load line this presents - draw it on a set of 12AX7 characteristic curves and you'll see what I mean). The reason you connect it to the plate of the previous stage is because that impedance is not only usually lower, it stays relatively constant and relatively low over the entire frequency range of interest, and you gain the benefit of any attenuation between the plate and the following grid, which reduces the noise level further, giving an effective increase in the signal-to-noise ratio compared to the reversed situation. The tone caps are tricky. To correctly figure that one out, you have to take into consideration the position of the controls, the value of the slope resistor, and the output impedance of the previous stage. In general, though, it is probably best again to align the outside foil towards the previous stage output on all caps, because of the potential problem of having the plate effectively connected to the grid of the next stage as shown in the above example. Now for the fine print: Your mileage may vary. No warranty, express or implied, is given for this off-the-cuff explanation of why outside foil polarity matters, especially considering I am writing it at 5AM after driving to Tennessee and back today. :) Randall Aiken From: Steve A. (steve_ahola@yahoo.com) Date: 11/25/2000 4:54 PM Subject: Re: Outer Foil on caps (Continued) Randall: Thanks! BTW as for the mid cap in a BF tone stack it is *almost* wired up as a capacitor bleeding off the high frequencies to ground so I have a hunch it might be quieter if the outer foil end was connected to the mid pot, since its resistance to ground would be between 0 and 10k... There was a post here awhile back that explained the function of the three tone caps in a BF or Marshall tone stack: 1.) The treble cap mainly affects the mid frequencies 2.) The bass cap mainly affects the treble frequencies 3.) The mid cap mainly affects the high frequencies Did I remember that right? The only one I am sure about is the first... Steve Ahola P.S. If I recall correctly the formula is: Xc= 1/IIRC :D From: Randall Aiken (reaiken@aikenamps.com) Date: 11/25/2000 7:45 PM Subject: Re: Outer Foil on caps (Continued) You may be right about the mid cap, although I'm not quite sure what happens when the bass pot is turned to the point where it is shorted out, and the treble is turned all the way down - then the cap outside foil is connected to the grid of the next stage (if the volume is all the way up). I think you'll still get some attenuation of intereference if the outside foil is on the side near the slope resistor. I'd say in either case, it is best to try it both ways and see if there is any discernable difference at any extremes of the pot rotations. If you want to get a "feel" for the way the tone stack caps affect the various frequencies, I'd highly recommend downloading Duncan Munro's most excellent tone stack program at http://www.duncanamps.com/ . P.S. If I recall correctly the formula is: Xc= 1/IIRC The formula for the complex impedance of an ideal capacitor is Zc = -1/(j*w*C). There is no real component of resistance, just the imaginary component. To get the reactance, you just get rid of the 1/j, and end up with Xc = -1/(2*w*C). The "w" (which is supposed to be a lower-case "omega", but I don't know how to make one of those characters!) is equal to 2*pi*f, so the capacitive reactance is equal to Xc = -1/(2*pi*f*C). When speaking of the magnitude of the capacitive reactance, we drop the negative sign (capacitive reactance is negative, inductive reactance is positive), because the magnitude is the square root of the sum of the squares of the real and imaginary parts. There is no real part, and the square root of the square of a negative number is always positive. Randall Aiken From: Steve A. (steve_ahola@yahoo.com) Date: 11/26/2000 5:41 AM Subject: Re: Outer Foil on caps (Continued) Randall: P.S. If I recall correctly the formula is: Xc= 1/IIRC That was a joke (IIRC is the internet abbreviation for "if I recall correctly"— Speed had mentioned here that he always thought that the II was "pi"... ;) ) I tried to insert an Omega sign in this post but I guess that HTML doesn't recognize the True Type Symbol font... :( --Thanks! Steve Ahola From: Randall Aiken (reaiken@aikenamps.com) Date: 11/26/2000 8:08 AM Subject: Re: Outer Foil on caps (Continued) That was a joke (IIRC is the internet abbreviation for "if I recall correctly"— That's funny. Actually, it's not funny. I'm getting too old to see a joke when it is staring me in the face. That happens when you turn 40, I guess. :) RA From: Steve A. (steve_ahola@yahoo.com) Date: 11/27/2000 8:20 AM Subject: Re: Outer Foil on caps (Continued) RA: Your reply was so much over my head that I couldn't tell if you were in on the joke or not (the square root of the square of a negative number, and all that... ;) ). If you think that 40 is bad wait until you see what 49 has in store for you! I just read about a scientific study that indicates that the human mind loses its short time memory in a linear fashion after the age of 20... That makes a lot of sense— I recall knowing everything about everything back when I was 20! :D --Thanks! Steve Ahola From: Bruce (MissionAmp@aol.com) Date: 11/25/2000 6:24 PM Subject: Re: Outer Foil on caps (Continued) Hey, I could of said all that too... well, if I knew how to. :D Sheesh!! Bruce From: Richie{~}==::: (richhall@zoomnet.net) Date: 11/26/2000 4:03 AM Subject: Re: Outer Foil on caps (Continued) Heres a little thing i found,and a few good tech tips on the page also.. but this is what they posted about the caps they sell. And has a picture of the caps, so you'd know what side they are talking about.. Here are there words.. The inner foil of coupling capacitors is the non-shiny side of the capacitor encasing. The outer foil is the shiny black epoxy side of the casing. When installing these caps, point the inner foil towards the output of the circuit (towards the the grid of the output tube). The inner foil is also marked with the red insulation over the lead-out wire. Richie{~}==::: From: Steve A. (steve_ahola@yahoo.com) Date: 11/26/2000 5:30 AM Subject: Re: Outer Foil on caps (Continued) Richie: Heres a little thing i found,and a few good tech tips on the page also.. but this is what they posted about the caps they sell. And has a picture of the caps, so you'd know what side they are talking about.. Here are there words... What company/page are you referring to? Bruce's tests on the caps he had around indicated that the orientation was fairly random, although I believe that the 418P's I get from Torres are fairly consistent (looking at the side with the writing with the leads down the outer foil seems to be consistently on the right). So what type of caps are you referring to, and do they sound good? --Thanks! Steve Ahola From: Richie{~}==::: (richhall@zoomnet.net) Date: 11/27/2000 6:16 AM Subject: Re: Outer Foil on caps (Continued) Steve: To be honest..i been searching so many pages i think i lost it.. but i was on Vishay,Aerovox,and RS, not radio shack..lol and here is a link to some good reading..I think the Vishay page has the tech stuff,and pictures of how they are made.And also some experts talking of how the OT is such a major part of the sound and tone.. http://www.spectrum.ieee.org/select/0898/tube.html http://www.vishay.com/docs/mkt1813.pdf http://www.wima.com/ http://rswww.com/ Richie{~}==::: From: Nate S. Date: 11/29/2000 5:41 PM Subject: Re: Outer Foil on caps (Continued) I was always under the impression that if so labelled, the end of a cap with the "band" was the outer wrap. I realize this doesn't apply to orange drops but I usually use CDE WMF series caps (polyester film/foil) which have the "band" marking. And sound nice too.. YMMV Nate From: MBSetzer (QualityAna@aol.com) Date: 11/30/2000 12:45 PM Subject: Re: Outer Foil on caps (Continued) I would have to say that the marked end of orange drops is there for a reason. I previously thought it was for the same reason as other caps, to indicate the outer foil. Now it appears it is only there as a legacy for when they were made by Sprague and they really cared. Typical '90's mentality . . .