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Thread: Does cap voltage affect tone?

  1. #71
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    So this chemical is a good fire retardant on clothing. But does it have adverse effects on people? Let's grab 100 university students, give them $25 each to keep this patch on their arm for a day. After a day goes by the students get checked, nothing visible under the patch, the chemical gets the green light. What is wrong with this picture? The patch is not a real world application, this is not how the cloth will be worn. The clothing will be worn on the whole body in environments less temperature controlled than a classroom. The same cloth that passed in a patch test can cause burning of the skin when some people get hot and perspire. Real case.

    Now the question of a pair of electrolytic caps and them behaving differently in a circuit when one is rated at 6.3V and the other for 350V. And since we have used 22 uF caps as bypass caps for our signal stage cathodes do you think trying a typical circuit with a 12AX7, 1.5k and 100k resistors would be in order? Well I thought so also when about six years ago the question came up in a forum at tdpri.com. I didn't know the answer but a knowledgeable member said the high voltage cap would not work as the low voltage one. I had some 22 uF 350V caps and I looked through my box of parts I was too lazy to sort and found a 6.3V 22 uF cap. It was tiny.

    So I bread boarded a stage of gain using the 12AX7 and the normal Fender complement of parts, set up my signal generator, scope, and ran through the normal guitar spectrum swapping the low voltage part for the high voltage one (before I forget, checked the capacitance with my meter and selected from my 350V ones to get the same reading as the 6.3V one). I took pictures of the scope traces, and so everyone remembers the bias on the cathode is roughly 1V, (I know from seeing familiar names most know this, more directed to the new guy as I do not know his frame of reference). With the traces of both side by side you could see the gain was exactly the same, the sine wave looked exactly the same. And if Photobucket would not have gone to the dark side I could pull up the pictures for you. You may have to take me on my word for that.

    A network analyzer would have been something that might have been useful when I did some work for a aerospace manufacturer that had me come up with a filter network to keep RF out of rocket wiring so they do not light off prematurely. They gave me nothing for a budget and had no idea what it took to get the job done. Oh yea, make it small, great rejection, and try to keep the cost low. When I did get a bunch of parts together that I thought would do the job I decided to test it in a TEM Cell, (an expanded coax cable of sorts. Made one with the center conductor using some copper sheet, the cell was made of cardboard that I glued tin foil on (ok everyone, let's get our hats on). I got laid off before I could test it, a week after I showed them the prototype. Came back a year later for my original department (What the heck did I know about 20 GHz anyway? The top frequency it was suppose to reject.) I asked how it worked when I met the engineer that took over (he was suppose to have had the job but the satellite project was running late). He said it seemed to work and that he used my arts and craft Tem cell up to half a Gig.

    Moral of the story? Don't underestimate people, as an Instrument Technologist my world ends about 20 Hz, 50 Hz if you want to push it. And if there was one person on the forum that I take seriously it would be Mr. Fahey. But then again I have read many of his posts and have found they were backed up by experience. Just saying.

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  2. #72
    Bent Member Chuck H's Avatar
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    Hey Juan. What about the sub harmonic frequencies that occur with stiffer power supplies (like Marshall used) that result in "beating".?. Sum/difference harmonics that occur IN the audio spectrum BECAUSE there are frequencies below the audio spectrum. This is actually a cool effect sometimes and probably wouldn't happen if the amp weren't trying to reproduce frequencies below "guitar amp" spectrum. It's just another mojo part of the coveted tonal qualities of our vintage favorites. I'm sure there are similar phenomenon happening in the HF as well. May be more or less relevant here. Just keeping it real

    EDIT: Then there's Enzo's consideration of context. At this level it's more about listening tests than it is about loop and pole theory (I think). Actually understanding "beating" probably won't even help a designer to reproduce it short of the original designs that do it already because there are, indeed, other complex influences in play. Trying to isolate them with even the most advanced bench gear (or believing you have?) hasn't proved out much in the real world. Nothing should be ignored, but anything claimed should be proven. I think that's where we are in this thread.

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    Last edited by Chuck H; 07-08-2018 at 02:35 PM. Reason: typo
    "...less ear-friendly but handy for jazz." Leo_Gnardo

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  3. #73
    Senior Member Malcolm Irving's Avatar
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    Quote Originally Posted by Chuck H View Post
    … What about the sub harmonic frequencies that occur with stiffer power supplies (like Marshall used) that result in "beating".?. Sum/difference harmonics that occur IN the audio spectrum BECAUSE there are frequencies below the audio spectrum. This is actually a cool effect sometimes and probably wouldn't happen if the amp weren't trying to reproduce frequencies below "guitar amp" spectrum. It's just another mojo part of the covented tonal qualities of our vintage favorites. ....
    I connected my guitar through the sub-woofer channel on a big PA once, just to see what happened. I was amazed how much stuff comes out at frequencies below the 82Hz of the low E. It was how you strike the strings (palm muting and such like) that caused the low thumps and bumps. OK, a guitar speaker would not reproduce this stuff very well, but if we allow it through stage 1 of the amp it could have a significant effect on blocking distortion etc. in later stages.

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  4. #74
    Bent Member Chuck H's Avatar
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    Quote Originally Posted by printer2 View Post
    ...if there was one person on the forum that I take seriously it would be Mr. Fahey. But then again I have read many of his posts and have found they were backed up by experience.
    Which is exactly as you have done here. And very well IMO. Here's my second

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    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

  5. #75
    Old Timer J M Fahey's Avatar
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    Oh, the sub-audio frequencies (I tend not to call them sub-harmonics simply because they are not exact multiples and submultiples of the note being played, although they are created by playing) cant be heard by themselves, as a pure sinewave tone, but we can be definitely hear their effect on very audible sounds.

    Just think *tremolo*: a 2 to 10Hz band oscillator modulates audio frequencies and is so strong and easy to hear that we use it as a special effect

    By the same token, string to string beating can create all sorts of low frequencies, even as low as 1 Hz of even less if they are *almost* in tune, what matters is difference and you can get as close as you wish.

    And no need for tubes or even *electronic* stuff in the path, Piano tuners for ages have counted beatings per second between double and triple strings hit by a single hammer key for a richer sound and consistency.

    So in an overdriven amp, which is the King of Intermodulation by definition, and where chords are being played, and you feed it the proper material to intermodulate, yes, lots of things will appear, as experienced by Malcolm Irving.

    As another easy to experiment fact: just look at, say, Twin Reverb speakers with the grill cloth off.
    I mention that particular amp because its easy to pull it out of the way and because the amp has "too much Bass", and its easy to see (specially if you look at it from one side) speaker cones pumping in and out a lot, at very low frequencies, a few Hertz and definitely inaudible by themselves.

    And they are created by intermodulation, not fed from the input or part of input program.

    Long ago when I worried about that I tried to stop that wasteful movement (does not contribute to acoustic output and wastes resources) so added an active 60 Hz highpass, 12dB/Oct between preamp and power amp .

    End result?: lowered wasteful movement but definitely didnt stop it , far from that, so it was clearly being created inside the Power Amp itself when driven balls to the wall.

    On the other end, in a single non overdriven stage, when components behave politely well within their ratings, "nothing weird happens" as shown by printer2 :
    So I bread boarded a stage of gain using the 12AX7 and the normal Fender complement of parts, set up my signal generator, scope, and ran through the normal guitar spectrum swapping the low voltage part for the high voltage one (before I forget, checked the capacitance with my meter and selected from my 350V ones to get the same reading as the 6.3V one). I took pictures of the scope traces, and so everyone remembers the bias on the cathode is roughly 1V, ...... With the traces of both side by side you could see the gain was exactly the same, the sine wave looked exactly the same. And if Photobucket would not have gone to the dark side I could pull up the pictures for you. You may have to take me on my word for that.
    Oh, we do, you are confirming by experiment what is the "normal" experience.

    Now if you were going *contrary* to it, then we would ask for evidence, not out of distrust by the way, but out of curiosity.

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    Juan Manuel Fahey

  6. #76
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    Just to demonstrate that you don't need a 10,000$ HP analyzer for comfortable Z measurements, here a few curves taken with my cheap Velleman PCSU200 USB scope+synchronized signal generator:
    Click image for larger version. 

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    The generator source impedance was 100 Ohms. The horizontal blue line is the "Z-response" of a 1 Ohm resistor for reference/calibration.
    The red curve shows one section of a 2x32/350V cap I pulled from my 1967 Marshall. This type has the highest inductance.

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  7. #77
    Bent Member Chuck H's Avatar
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    Excellent! But... Could I trouble you about measured UF values for the caps tested? I think it could be beneficial to the discussion.

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    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

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    Quote Originally Posted by Chuck H View Post
    Excellent! But... Could I trouble you about measured UF values for the caps tested? I think it could be beneficial to the discussion.
    Oh boy, they are all back in their boxes, but I made sure that all were within +/- 10% of rated value. All are at least 20 years old. The C value can be estimated from the lay of the -6dB/octave slopes.

    But what specific discussion?

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  9. #79
    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by Helmholtz View Post
    Just to demonstrate that you don't need a 10,000$ HP analyzer for comfortable Z measurements, here a few curves taken with my cheap Velleman PCSU200 USB scope+synchronized signal generator:
    Click image for larger version. 

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    Thanks for this very useful post, which clear more points than it caims to
    Such as:

    * confirms previous statements and tests that much maligned Electrolytic capacitors are much better than their "internet fame" would say.

    * no excessive inductance *inside the audio range*

    * no weird (or even mild) resonant peaks *inside or outside the audio range*

    * ESR low enough that it does not interfere with expected/calculated hum/noise/ripple filtering action.
    Curves stop sloping down at 6dB/Oct (expected capacitor behaviour) and become horizontal (resistive behaviour) at 30kHz to over 100kHz ... and then we are talking between 1 ohm and fractions of an ohm magnitude, degradation is minimal.
    Consider in RC smoothing stages in a preamp, R values go from 10k to at most 1 K ... compare that to 1 ohm or less in series with the cap.
    "Butterfly in China" effect indeed

    * some people worry and overthink about parts leads or even parts body self inductance ... but a **1 ohm** resistor is perfectly flat to 1 MHz , so a higher value one will be even less affected than a 1 ohm one.

    Just found the Velleman thingie in Amazon ... loved it and will probably order one, thanks for showing it here

    Excellent! But... Could I trouble you about measured UF values for the caps tested? I think it could be beneficial to the discussion.
    I think the curves clearly show that the green cap has somewhat less capacitance than the black one.
    How much less? ... dunno but probably 10% to 20% less, within what Helmholtz selected previous to experiment.
    Notice curves are practically parallel in the "mostly capacitive" area and separate in the resistive area, I guess the green cap has about 3 times the black cap ESR. (because its about 10dB higher in this Log Voltage graph)

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    Bent Member Chuck H's Avatar
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    Rethinking it I don't suppose it matters beyond what the graph shows. My first thought was indeed the green vs black difference. For some reason I momentarily thought that an actual measured value would help disprove a significant capacitance change at different voltages. Only after posting did it occur to me that ALL the test equipment is operating at voltages significantly lower than some of those cap ratings. So...

    Managed to prove out pretty much everything in question (including the need for a bench tool we'd need to make payments on) except whether or not electrolytic capacitor plates are squished together at rated voltage enough to alter the value significantly.

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    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Chuck H View Post
    Rethinking it I don't suppose it matters beyond what the graph shows. My first thought was indeed the green vs black difference. For some reason I momentarily thought that an actual measured value would help disprove a significant capacitance change at different voltages. Only after posting did it occur to me that ALL the test equipment is operating at voltages significantly lower than some of those cap ratings. So...

    Managed to prove out pretty much everything in question (including the need for a bench tool we'd need to make payments on) except whether or not electrolytic capacitor plates are squished together at rated voltage enough to alter the value significantly.
    You could always take a pair if hemostats and clamp one on a cap to prove or disprove that idea.

    nosaj

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    Bent Member Chuck H's Avatar
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    Ha ha! I was thinking more along the lines of measuring a known parameter under controlled conditions that is relative to capacitance AND voltage. Something like reverse analyzing the capacitance by the charge time with other factors, such as ESR already known. This goes beyond my skill set.

    But I like your idea too. Wear gloves

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    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Chuck H View Post
    Ha ha! I was thinking more along the lines of measuring a known parameter under controlled conditions that is relative to capacitance AND voltage. Something like reverse analyzing the capacitance by the charge time with other factors, such as ESR already known. This goes beyond my skill set.

    But I like your idea too. Wear gloves
    Well going by that one guys suggestion that the plates pull together in a cap, why wouldn't squeezing one simulate his idea.

    Heck just a cap out of the junk box put it on a capacitance meter and squeeze it . I doubt it will change but when I get back from the store. I'll put one one the der5000 and see if it changes. I''ll try a film and a electrolytic, The electrolytic will be damaged so it's going to take one for the team.

    Report back this evening.

    nosaj

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    Lifetime Member Enzo's Avatar
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    Prolly not on a 22uf e-cap, but on some film cap, squeezing with your fingers might upset the readings some with your body capacitance.

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    Education is what you're left with after you have forgotten what you have learned.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by nosaj View Post
    Well going by that one guys suggestion that the plates pull together in a cap, why wouldn't squeezing one simulate his idea.

    Heck just a cap out of the junk box put it on a capacitance meter and squeeze it . I doubt it will change but when I get back from the store. I'll put one one the der5000 and see if it changes. I''ll try a film and a electrolytic, The electrolytic will be damaged so it's going to take one for the team.

    Report back this evening.

    nosaj
    Where can I get a der5000!?! I don't want to be under equipped for building guitar amps. I can put it right next to the HP3577 I just ordered based on some readings in this thread. Wow! I'm gonna have the most badass bench ever.

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    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Chuck H View Post
    Where can I get a der5000!?! I don't want to be under equipped for building guitar amps. I can put it right next to the HP3577 I just ordered based on some readings in this thread. Wow! I'm gonna have the most badass bench ever.
    Will you be satisfied squinting at meters? Or do we build/fix these things to satisfy our ears. How many gazillions of amps have been built without the "benefit" of (over)analysis. I gots to wonder... just WHO are the clients that have benefited by having their amps put thru the super duper NASA/NSA-approved network analysis process. By name, not just "a long list of very impertinent people."

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    Lifetime Member Enzo's Avatar
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    I have a couple HP distortion analyzers, I think a 331 and 332. Maybe you could use those?

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Chuck H View Post
    Where can I get a der5000!?! I don't want to be under equipped for building guitar amps. I can put it right next to the HP3577 I just ordered based on some readings in this thread. Wow! I'm gonna have the most badass bench ever.
    I say it's bunk. I put a 220uf cap in the meter(Chuck it's just a $70 LCR meter nothing super badass but beats spinning dials on a Genrad 1650a) Squeezed it slightly no change, harder denting shell moved up 1 uf, crunch it really badly goes up to 330uf but the cap is now damaged. Put a .1uf film cap in squeezed it no change .
    Seem UF only changes if you really apply some force to it and when you see change the cap is damaged.

    nosaj

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    Binkie McFartnuggets‏:If we really wanted to know the meaning of life we would have fed Stephen Hawking shrooms a long time ago.

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    Quote Originally Posted by J M Fahey View Post
    The ubiquitous 25uF value (and notice that almost never it is , say, 10uF or 47uF ... why?) , if you think about it, is weird because it acts way below audible frequencies
    http://www.sengpielaudio.com/calculator-RCpad.htm
    tells us that typical 1k5 + 25uF has a crossover frequency (somebody would love to call it a Pole instead, but wont speculate on the preference) of .... drumroll!!! .... 4.24Hz !!!!!
    Hardly an Audio Frequency, huh?
    Dr. Juan ... I believe you may have confused a Crossover frequency and a POLE... not the same thing... The POLE is the -3dB point you may be referring to...
    Were you referring to Phase Crossover or Gain Crossover ??
    The 4.24 Hz is off by significant factor...
    The Web Calculator does not apply to this topology....
    It's not that simple...since you need to include the impedance "looking into" the Cathode...
    With a 12AX7 and a 100K plate resistor in this Fender circuit... Your looking at roughly 1.54K in parallel with you 1.5K cathode resistor...when you make this calculation...
    I still get your point.. that this was all below the guitar range...

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    Quote Originally Posted by Chuck H View Post
    Excellent! But... Could I trouble you about measured UF values for the caps tested? I think it could be beneficial to the discussion.
    O.K., here you are (I think I picked the same caps as before).

    Measured values (using a PeakTech 2170 LCR/ESR meter with 4-wire Kelvin clamps):
    (the numbers mean C@1kHz/ESR@1kHz/ESR@10kHz/ESR@100kHz)

    black: 23.6/0.46/0.37/0.33 (22/450V, Siemens Long Life, 1992)
    red: 31.6/0.93/0.0.77/0.0.71 (32/350V, 1967)
    mag: 33.0/0.62/0.54/0.41 (32 tantalum)
    green:20.7/1.46/1.24/1.06 (22/35V)

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Helmholtz View Post
    O.K., here you are (I think I picked the same caps as before):

    Click image for larger version. Name: ecapimp.png Views: 17 Size: 10.5 KB ID: 49647

    Measured values (using a PeakTech 2170 LCR/ESR meter with 4-wire Kelvin clamps):
    (the numbers mean C@1kHz/ESR@1kHz/ESR@10kHz/ESR@100kHz)

    black: 23.6/0.46/0.37/0.33 (22/450V, Siemens Long Life, 1992)
    red: 31.6/0.93/0.0.77/0.0.71 (32/350V, 1967)
    mag: 33.0/0.62/0.54/0.41 (32 tantalum)
    green:20.7/1.46/1.24/1.06 (22/35V)
    Here's another

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    "...less ear-friendly but handy for jazz." Leo_Gnardo

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    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

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    I believe you may have confused a Crossover frequency and a POLE... not the same thing... The POLE is the -3dB point you may be referring to.
    The definition of a filter pole seems to have changed over the years. In newer literature the +/-3dB frequency of a first order filter is actually called a (complex) pole. Nevertheless it's just a corner frequency followed by a 6dB/octave slope.

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    Last edited by Helmholtz; 07-09-2018 at 03:27 PM.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Helmholtz View Post
    The definition of a filter pole seems to have changed over the years. In newer literature the -3dB frequency of a first order filter is actually called a (complex) pole. Nevertheless it's just a corner frequency followed by a -6dB/octave slope.
    Indeed! It seems antagonistic to tell someone they're "confused" based on semantics. But then, that isn't the only antagonism in that post either. So if that was the goal, good job. Insinuating that Juan is using a "Web Calculator"?

    Tiny concession at the end... How magnanimous. Where do I get in line to kiss the ring?

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    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

  24. #94
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    By the same token, string to string beating can create all sorts of low frequencies, even as low as 1 Hz of even less if they are *almost* in tune, what matters is difference and you can get as close as you wish.

    And no need for tubes or even *electronic* stuff in the path, Piano tuners for ages have counted beatings per second between double and triple strings hit by a single hammer key for a richer sound and consistency.
    Beats are not power at low frequencies. Two close frequencies create an amplitude pattern that the ear-brain can detect. Creating power at the low frequencies requires a non-linearity. Without the non-linearity, a Fourier analysis would show no power at the low frequency. Beats can do various things in an amplifier. More complex beats than produced by a pair of sine waves might be sufficiently asymmetrical to produce "interesting" effects in an overdriven output stage, where the potential for output transformer saturation exists with even a small amount of power in the Hz range.

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    A few thoughts regarding the claim that HV charge produces a higher C value by foil attraction in an E-cap.
    While I could imagine such (minute) effect in a film/foil cap, I have problems with electrolytics. The reason is that squeezing the foils in an E-cap will NOT increase its capacitance (at least in theory). This is due to the construction/functional principle of an electrolytic: It consists of an anode foil that is fully covered by an insulating layer of aluminum oxide (Al2O3), a second plain aluminum foil and a paper layer soaked with a conductive soup (i.e. the electrolyte) between. Now, the plain foil is NOT the cathode of the capacitor. The real cathode is the electrolyte and the plain foil just serves to contact it. The dielectric is the extremely thin, extremely hard and incompressible (think emery or sapphire ) Al-oxide layer. In other words, the real capacitor is formed by the anode foil, the oxide layer and the electrolyte. As a consequence the C value will not change when the distance between the two foils changes as long as the oxide layer stays the same. Furthermore any attracting force caused by the charge does not act between the two foils but between the anode foil and the molecules (more exactly: ions or anions) of the electrolytic liquid. And I don't think these tiny molecules can effectively squeeze the hard oxide layer to a smaller thickness and thus change capacity.

    I consulted some literature on electrolytics (actually data/application handbooks from the paper age) and found no indication of a voltage dependancy of capacitance. But what I found was that there are two standardized methods for C measurement. The first one evaluates the voltage to current ratio at 100/120Hz. This yields the AC capacitance and is the method used by most C-meters. I did not find any DC bias requirement for this measurement.
    The second method uses the charge/discharge time and gives the DC capacitance. The latter is said to be typically 10% to 50% higher than the AC value. I could not find any details of the procedure.

    The specified value is generally the AC capacitance.

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  26. #96
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    Quote Originally Posted by Helmholtz View Post
    A few thoughts regarding the claim that HV charge produces a higher C value by foil attraction in an E-cap.
    While I could imagine such (minute) effect in a film/foil cap, I have problems with electrolytics. The reason is that squeezing the foils in an E-cap will NOT increase its capacitance (at least in theory). This is due to the construction/functional principle of an electrolytic: It consists of an anode foil that is fully covered by an insulating layer of aluminum oxide (Al2O3), a second plain aluminum foil and a paper layer soaked with a conductive soup (i.e. the electrolyte) between. Now, the plain foil is NOT the cathode of the capacitor. The real cathode is the electrolyte and the plain foil just serves to contact it. The dielectric is the extremely thin, extremely hard and incompressible (think emery or sapphire ) Al-oxide layer. In other words, the real capacitor is formed by the anode foil, the oxide layer and the electrolyte. As a consequence the C value will not change when the distance between the two foils changes as long as the oxide layer stays the same. Furthermore any attracting force caused by the charge does not act between the two foils but between the anode foil and the molecules (more exactly: ions or anions) of the electrolytic liquid. And I don't think these tiny molecules can effectively squeeze the hard oxide layer to a smaller thickness and thus change capacity.

    I consulted some literature on electrolytics (actually data/application handbooks from the paper age) and found no indication of a voltage dependancy of capacitance. But what I found was that there are two standardized methods for C measurement. The first one evaluates the voltage to current ratio at 100/120Hz. This yields the AC capacitance and is the method used by most C-meters. I did not find any DC bias requirement for this measurement.
    The second method uses the charge/discharge time and gives the DC capacitance. The latter is said to be typically 10% to 50% higher than the AC value. I could not find any details of the procedure.

    The specified value is generally the AC capacitance.

    For fun I did some guesstimating of the pressure exerted on the electrolyte. I could not find any good numbers for the dielectric constant or electrode spacing so it's a (barely) educated guess for those. Now keep in mind that the pressure to compress a liquid ( i.e electrolyte) is something like 1100 bar for a 5% volume change. I think you'll agree that the pressure inside is so tiny at 1.1 x 10e-9 as to make any accusations of incorrect guesses moot.

    Here are the numbers for a 22uF cap at 250V
    Capacitance (uF) Volts Charge spacing (m) Force (n) Dielectric Cont Area (m^2) Pressure (n/m^2) Pressure (Bar)
    22 250 0.0055 1.00E-007 2.7225E-006 10 0.0248868778 0.000109395 1.09395E-009


    As far as I am aware, ideas such as ESR zero derive more usually for SMPS design where the effect of ESR on loop stability and residual ripple are of interest. Not of any relevance to a tube guitar amp.

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  27. #97
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    As far as I am aware, ideas such as ESR zero derive more usually for SMPS design where the effect of ESR on loop stability and residual ripple are of interest. Not of any relevance to a tube guitar amp.
    True.

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  28. #98
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    For fun I did some guesstimating of the pressure exerted on the electrolyte. I could not find any good numbers for the dielectric constant or electrode spacing so it's a (barely) educated guess for those. Now keep in mind that the pressure to compress a liquid ( i.e electrolyte) is something like 1100 bar for a 5% volume change. I think you'll agree that the pressure inside is so tiny at 1.1 x 10e-9 as to make any accusations of incorrect guesses moot.

    Here are the numbers for a 22uF cap at 250V
    Capacitance (uF) Volts Charge spacing (m) Force (n) Dielectric Cont Area (m^2) Pressure (n/m^2) Pressure (Bar)
    22 250 0.0055 1.00E-007 2.7225E-006 10 0.0248868778 0.000109395 1.09395E-009
    There is no attraction/pressure between the foils. The electrolyte is conductive and thus carries no field and a dielectric constant cannot be defined. Compressing the conductive electrolyte would not change anything. The dielectric is the Al-oxide and its thickness is around 1m (=10E-6 m) having a dielectric constant of 9.5. So you would need the compressibility of the solid Al2O3.

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    Here is the probably most comforting argument regarding the OP's question:

    If HV e-caps would only show their spec'd C at rated voltage there would be the severe problem of measurement/verification. I have never heard of a C-meter that allows measurement with a bias of several hundred volts. (I have worked with some having an optional bias of a few volts though.)

    Cap manufacturers need to make sure that their products measure within spec/datasheet on the meters used by the incoming components inspection guys of the high volume customers to avoid claims/complaints. If quality verification requires special test conditions they need to specify in the datasheets.

    For a while I was the manager of a QC group of EEs at one of our production sites. We did things like second source component qualification/release, write component specs and define test conditions and gear for incoming inspection. Our component specs were part of the purchase/supply contract. HV e-caps we bought in millions and we even did real lifetime testing in our SMPS products. We did not use/specify a HV charge for C/ESR verification.

    Now if actually some HV e-caps show a 20% increase in C (this is the maximum I would expect from my experience for a new cap, the reason probably being of electro-chemical nature), who cares?
    And my measurements above show that the ESR of the HV types is fine even at zero DC.

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    Last edited by Helmholtz; 07-10-2018 at 06:07 PM.

  30. #100
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    Quote Originally Posted by cerrem View Post
    Sounds like you are clueless..... I could explain for 100 pages and you still wont get it... You don't even know what a Network Analyzer is... Look up a HP 3577 and read... It's an ANALOG signal sweep and it sweeps as low as 5 Hz ... not in the RF range... its not DIGITAL it measures impedance ..Been designing and building audio amps guitar amps for 45 years.. Also design capacitors for a major manufacturer....Stay out of my post and stay out of yours...
    Don't be a cunt.

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    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

  31. #101
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    Quote Originally Posted by Helmholtz View Post
    Here is the probably most comforting argument regarding the OP's question:

    If HV e-caps would only show their spec'd C at rated voltage there would be the severe problem of measurement/verification. I have never heard of a C-meter that allows measurement with a bias of several hundred volts. (I have worked with some having an optional bias of a few volts though.)

    Cap manufacturers need to make sure that their products measure within spec/datasheet on the meters used by the incoming components inspection guys of the high volume customers to avoid claims/complaints. If quality verification requires special test conditions they need to specify in the datasheets.

    For a while I was the manager of a QC group of EEs at one of our production sites. We did things like second source component qualification/release, write component specs and define test conditions and gear for incoming inspection. Our component specs were part of the purchase/supply contract. HV e-caps we bought in millions and we even did real lifetime testing in our SMPS products. We did not use/specify a HV charge for C/ESR verification.

    Now if actually some HV e-caps show a 20% increase in C (this is the maximum I would expect from my experience for a new cap, the reason probably being of electro-chemical nature), who cares?
    And my measurements above show that the ESR of the HV types is fine even at zero DC.
    There were a few meters that allowed HV bias on caps during measurement... A slight modification to the unit would allow this... For example, old Sprague cap testers... One went to 1500V and a later unit TO-6A from the 1960's went to 600V ... This DC voltage was intended for leakage testing...while the Capacitance test applied 35V ac.... With a simple mod to protect the front end, this would allow to use the DC voltage simultaneous with the C test... The main LCR meter I use most often is the HP 4263B ...since the 3577 is a pain in the A$$ to break out for day to day bench measurements....bought it at surplus for a little over $200 and it looks like it was hardly used... The 4263B has some front end protection to allow you to measure a charged cap.... Would need to look at the data sheet to check the limiting test voltage depending on capacitance...they provide a quick formula...
    As for capacitor variance....I did mention that depending on the di-electric material and the cap construction and charge density, the electric field can cause a force for the plate attraction to slightly deform the di-electric material.... it is a small effect but has occurred in some caps at higher voltages..paper in wax, paper in oil..some films have slight compression, modulus of elasticity ....Glass caps have practically no effect... These issues were noted at Maxwell R&D Labs during cap design and manufacturing.....
    Probably the most noted effects of cap variance would be Temperature and Applied voltage.... The di-electric "constant" is not really constant ...since this is not perfect or linear...
    The manufacturers of the dielectric material, such as DuPont, will have data sheets showing the di-electric constant vs temperature.... for example TEFLON will show a negative slope , ie as temperature increases dielectric constant decreases ...for TEFLON the slope is not that steep....figure 2.1 to 1.9 over 20C to 100C .... Other materials can have a steeper slope... There is also a change in di-electric constant withe respect to applied voltage.... The electric field produced polarizes the material depending on charge density and film thickness...construction, package size.. Ceramic caps can such as X5R would be one of the worst offender for voltage gradient vs di-electric constant .....also poor temperature vs di-electric constant...

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  32. #102
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    Manage to find it. I'll just do a copy and past.

    I wondered how much I could see if I scope two different capacitors with the same microfarad ratting but different voltage ratings. I wanted to use standard Fender values but the lowest value I had was 25V in 22uF, I also have 22uF at 350V. Now being that 25V is much larger than the voltage generated across a 1.5k resistor with a 12AX7 and a 100k plate resistor I went searching through some of my unused audio equipment and sacrificed one and came up with a 22uF at 6.3V. Now this is not really a scientific test, I had a circuit that I was breadboarding and I just swapped out the capacitors I had and used the 6.3V and the 350V. I switched between the two using a SPDT switch and took some picture of the scope. The pictures are of an 80 Hz signal, the first is the generator at the input jack, the next is the 6.3V and the third is the 350V. I am really surprised at the results as I would have thought the 350V cap would look terrible. Test setup/capacitors

    Capacitor Bypass TestSignal Generator



    6.3V capacitor



    350V capacitor



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  33. #103
    Lifetime Member Enzo's Avatar
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    And that brings us home.

    We can bat back and forth esoteric electronic phenomena, and tiny effects of this and that. And we can speak of specialized testing gear or modifying existing gear to hunt down vanishing changes. But context is everything. The OP wondered if it would change things in his guitar amp if he used a high voltage 20uf cap instead of a 25v 20uf cap for a cathode bypass. He wasn't looking for a parts per million effect. The scope tends to agree with me that he'd never hear the difference.

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  34. #104
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    There were a few meters that allowed HV bias on caps during measurement... A slight modification to the unit would allow this... For example, old Sprague cap testers... One went to 1500V and a later unit TO-6A from the 1960's went to 600V ... This DC voltage was intended for leakage testing...while the Capacitance test applied 35V ac.... With a simple mod to protect the front end, this would allow to use the DC voltage simultaneous with the C test... The main LCR meter I use most often is the HP 4263B ...since the 3577 is a pain in the A$$ to break out for day to day bench measurements....bought it at surplus for a little over $200 and it looks like it was hardly used... The 4263B has some front end protection to allow you to measure a charged cap.... Would need to look at the data sheet to check the limiting test voltage depending on capacitance...they provide a quick formula...
    Thanks for your feedback.

    I am interested in measurement principles and gear stuff and would love to see the input wiring of a C-meter allowing for HV bias, but I would be even more interested in actual measured values for low/no and HV bias. So if you have some or can produce them, please show.

    I thought about wiring a high F-high voltage cap in series with the meter's input. Something like 2000/350V or higher together with some charging resistor and protection diodes for the input. The C-value of the series cap needs to be around 100 times higher than the cap under test to avoid too low measuring results. Otherwise the reading would have to be corrected by some factor taking care of the series C circuit.

    The operation manual of the HP 4263B states that the max. bias voltage allowed is 2.5V. To my understanding the higher protection voltage does not mean that it allows measurement with charged caps. In fact it is explicitely stated that charged caps must not be connected to the input terminals.

    And please let's not mix e-caps with other types.

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  35. #105
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    Quote Originally Posted by cerrem View Post
    There were a few meters that allowed HV bias on caps during measurement...
    Exact Brand and Model please.
    And copypaste the specific lines of the Manual stating so, your word is not enough. You lost credibility.
    A slight modification to the unit would allow this...
    If you need to modify something it means it does not do so on its own, and to boot most probably is not even intended (or possible) todo so.
    For example, old Sprague cap testers... One went to 1500V and a later unit TO-6A from the 1960's went to 600V ... This DC voltage was intended for leakage testing...
    Which is completely different to what you state.
    while the Capacitance test applied 35V ac....
    With NO HV bias.
    Thanks for recognizing your intentional error.
    With a simple mod to protect the front end, this would allow to use the DC voltage simultaneous with the C test...
    To avoid endless retyping: >>If you need to modify something it means it does not do so on its own, and to boot most probably is not even intended (or possible) todo so.<<
    The main LCR meter I use most often is the HP 4263B ...since the 3577 is a pain in the A$$ to break out for day to day bench measurements....bought it at surplus for a little over $200 and it looks like it was hardly used... The 4263B has some front end protection to allow you to measure a charged cap....
    So now you are switching away from the test instrument you bragged about and you dropped the HV label too, replacing it with an undefined "charged" label.

    Would need to look at the data sheet to check the limiting test voltage depending on capacitance...they provide a quick formula...
    Yes, indeed, you would have to look at the datasheet, but more important paste here the lines stating they apply HV to the capacitor under test. Remembering you lost credibility so we need to see some substance backing your statements.
    As for capacitor variance....I did mention that depending on the di-electric material and the cap construction and charge density, the electric field can cause a force for the plate attraction to slightly deform the di-electric material....
    Who cares? We are not talking about that but E-cap capacitance variation with applied voltage.
    it is a small effect but has occurred in some caps at higher voltages..paper in wax, paper in oil..some films have slight compression, modulus of elasticity ....Glass caps have practically no effect... These issues were noted at Maxwell R&D Labs during cap design and manufacturing.....
    Who cares? Not the subject here.
    Open another thread for them if you wish.
    Probably the most noted effects of cap variance would be Temperature and Applied voltage....
    Now you digress into Temperature.
    The di-electric "constant" is not really constant ...since this is not perfect or linear...
    You are switching from an audible phenomenon, what worried the OP, to a vague "philosophical" realm.
    Butterfly in China indeed.
    The manufacturers of the dielectric material, such as DuPont, will have data sheets showing the di-electric constant vs temperature.... for example TEFLON will show a negative slope , ie as temperature increases dielectric constant decreases ...for TEFLON the slope is not that steep....figure 2.1 to 1.9 over 20C to 100C .... Other materials can have a steeper slope...
    Absolute waste of bandwidth and server space, we are not talking about that in this thread, you are just trying to deviate course for your own purposes.
    There is also a change in di-electric constant withe respect to applied voltage.... The electric field produced polarizes the material depending on charge density and film thickness...construction, package size..
    Please focus on E-Caps and provide specific data.
    Ceramic caps can such as X5R would be one of the worst offender for voltage gradient vs di-electric constant .....also poor temperature vs di-electric constant...
    Not talking Ceramics here.

    I suggest you add these instead.

    Very unrelated of course but at least what you probably dream about:



    Hey!!! I bet you can connect a Million of these in parallel and use them as a cathode bypass!!!

    A couple days ago a Forum member wanted to connect different cathode caps and worried about wiring, this way he could vary them remotely and without clicks or pops, think about the possibilities !!!

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