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Thread: 5F6A Bassman total custom build - super low output

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    Quote Originally Posted by The Dude View Post
    There are inevitably bugs to work out on a build, so don't get discouraged. Your experience is not singular. Actually, given the unconventional layout of that amp I'm quite surprised there aren't more bugs.
    I'm not in the least discouraged! This is fun for me. I love wiring stuff up.

    Twisted pair noise rejection only works if the current in the two wires flows in opposite directions. It's strange to me how tube amp designers of the Fender/Marshall era only worry about the positive side of the signals. I thinks it's because they were trying to make the amps as cheap as possible to manufacture. In industrial instrument wiring you NEVER use the chassis to carry a signal ground. Only Shields go to chassis.

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by tomgilmartin View Post
    Thanks Chuck. It has a funny sort of sizzle at high treble. Still have the bias super cool. I'm not really blown away with the "chime" I was hoping for.
    No chime to be expected from Hi Fi speakers with incredibly thick and heavy cones, incredibly heavy voice coils, and 10dB less sensitivity than whatīs expected in a Guitar speaker.

    The original Bassman was designed, developed and tuned-by-ear using 4 Jensen P10R , as different from your as can possibly be.



    Parameters ..... Jensen ..... Dipole-10
    Resonance ..... 99 Hz ....... 26 Hz
    Moving mass .. 13.7g ....... 54.6g
    X max ........... 0.8mm ..... 12mm
    Sensitivity ..... 95.1 dB .... 86.5 dB

    Pity Dipole10 frequency resīponse is not available, but being an expensive Hi Fi woofer I bet itīs ruler flat within the low band, has little mids and absolutely no treble, while Guitar oriented Jensen P-10 is "pre equalized" with full bass and treble boost built in.
    Just look at its frequency response curve:
    Click image for larger version. 

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    Juan Manuel Fahey

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    Quote Originally Posted by J M Fahey View Post
    No chime to be expected from Hi Fi speakers with incredibly thick and heavy cones, incredibly heavy voice coils, and 10dB less sensitivity than whatīs expected in a Guitar speaker.

    The original Bassman was designed, developed and tuned-by-ear using 4 Jensen P10R , as different from your as can possibly be.

    Parameters ..... Jensen ..... Dipole-10
    Resonance ..... 99 Hz ....... 26 Hz
    Moving mass .. 13.7g ....... 54.6g
    X max ........... 0.8mm ..... 12mm
    Sensitivity ..... 95.1 dB .... 86.5 dB

    Pity Dipole10 frequency resīponse is not available, but being an expensive Hi Fi woofer I bet itīs ruler flat within the low band, has little mids and absolutely no treble, while Guitar oriented Jensen P-10 is "pre equalized" with full bass and treble boost built in.
    Just look at its frequency response curve:
    Click image for larger version. 

Name:	P10R.jpg 
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    Great stuff, I agree the speakers are totally different. At this point, it sounds pretty good!

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  4. #109
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    Can someone explain to me why guitars cords are made with coaxial cable? It would seem any noise picked up on the shield would appear on the hot leg anyway through the coil of the guitar. How does this give any semblance of noise attenuation? <NO SARCASM, I DON'T UNDERSTAND THIS.>

    I suppose because it's the only alternative for the unbalanced output of a guitar. Only other choice when you have only two connection points at each end is twisted pair. One could use shielded twisted pair, but for the shielding to work properly the shield would have to be connected at one end only, and that end would have to be plugged into the amp end. If the shield ground end were plugged into the guitar end, then shield noise would couple through the negative line and thus into the hot leg of the signal.

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  5. #110
    Supporting Member eschertron's Avatar
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    There are certainly better ways to shield the signal in a guitar cable, but decades of tradition... sigh.

    There's a relatively high impedance through the pickups vs. to the chassis via the grounded side of the unbalanced signal. Think of the cable shield as an extension of the chassis itself, there will always be noise 'cast at the amp, but path of least resistance sends that noise signal to ground (well, most of it, most of the time!).

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    If it still won't get loud enough, it's probably broken. - Steve Conner
    If the thing works, stop fixing it. - Enzo
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
    MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey


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    Attached file is an mp3 of a frequency sweep of the speakers. Driven from a phone with a function generator app through my studio amp. Recorded on another phone. So there may be some phone artifacts. File is mono. If you want to get the frequency response, the sweep is 20sec for 20hz-20khz.

    To analyze for frequency then:
    0.1sec = 100Hz
    1sec = 1kHz
    5sec = 5kHz
    ETC.

    Checked this with my old software CoolEditPro to see the waveform and do Frequency Analysis.
    Seems like rolloff doesn't start until about 4kHz, then drops off, then peaks again at 8.2kHz.
    Not an anechoic chamber test, nor high-tech equipment, but it gives some idea of the response.

    Click image for larger version. 

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    Question: How do I get the site not to log me out all the time?
    Attached Files Attached Files

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  7. #112
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    New pics with all wire shielded twisted pair. It took me about 12 hours actual time to wire all this. Pretty much one hour per shielded wire.
    Everything is shielded except these signals which are all unshielded twisted pair:
    • Inverter output to power tubes
    • DC power from power section to preamp section
    • Tube heater filaments
    • Choke wires to DC bus
    • Power Amp to OT
    • Speaker output from OT to phono connector.


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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by tomgilmartin View Post
    One could use shielded twisted pair, but for the shielding to work properly the shield would have to be connected at one end only, and that end would have to be plugged into the amp end. If the shield ground end were plugged into the guitar end, then shield noise would couple through the negative line and thus into the hot leg of the signal.
    I've tried building guitar cable like that, the "proper" way with a separate interior line for signal ground, shield NOT served at the guitar end of the cable. Tried several cable types too, Belden 8412 mic cable, Belden "Brilliance" quad conductor cable, Canare quad, Mogami & some other brands too. It only seems to make a marginal audible improvement in the most noisy environments, even then it's barely detectable over ordinary competent guitar coax, Conquest USA-1 for instance. There's an analogous method for mic cables, called "Westrex wiring." For a balanced cable you need 3 interior conductors plus shield. Here again, not much difference can be heard unless the stage/studio is in a noise polluted zone such as southern Manhattan. By "noise" I mean electrical fields present from AC wiring 50-60 Hz, radio transmissions at all imaginable frequencies, then up to and beyond microwave. I've done work in some NYC studio locations where entire rooms are lined with grounded copper screen to turn them into Faraday cages. In environments like that, copper screening the whole room is the only thing that seems to help.

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    Smoke test passed, but no change in sound. If anything it's worse. More hum than before. Maybe I have a mistake or a shorted shield somewhere. Will check tomorrow.

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    Bent Member Chuck H's Avatar
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    One big leap in a new direction. Two steps forward. One step back. There will be sideways steps too. As long as the forward steps continue to outnumber backward or sideways. Realize that you're brand new to this, doing something "out of the box" (my theme for this thread ) and killing it

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

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

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Don't forget the joker g1's Avatar
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    Quote Originally Posted by tomgilmartin View Post
    Question: How do I get the site not to log me out all the time?
    There should be a box to check off that says 'remember me' near the login button.

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    I'm wondering about this sizzle noise I hear. I have made a lot of mistakes building this amp.

    When I received the speakers I decided to do some slap bass with them just sitting on the railing. No, I didn't drop one, but I did blow one up. Huge loud pop! Then I could hear it rubbing. Sanded down the voice coil a bit to fix the rub, but I can still hear it. I wonder if this is causing a feedback, because it effects the movement of the speaker, which will be present on the leads for sure, then coupled back through the neg feedback. I can see the second and third harmonic in the measured output of the speakers, check out the file I sent.

    Also, while testing DC during early startup, I managed to cause an arc. Twice. I may have damaged a power tube.

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    Frequency response to pink noise. Through the studio amp, close mic. Recorded on phone.

    Click image for larger version. 

Name:	Pink Noise Response.JPG 
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    Still puttering with it. Found a crack in a signal ground that had touched the shield. Fixed that, now it sounds like before. Disconnected my shield ground point and shields show infinite resistance to ground, so no touch points between shield and signal ground except the shield ground point.

    Next up, the output from the PI to the power tubes. I was poking at the wire with the chopstick method (a Sharpie marker) and seemed to help. Looking again at it, the input and output of the power tubes are running parallel for quite a while. Perfect place to pick up a feedback. I might go all the way and shield the output too. I'll try the input first, but I used up the entire sleeve of solder I bought! Gotta get some more.

    Even if nobody answers, writing it out here is helpful to force me to think clearly about it. Thanks again to all who have helped.

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    THAT WAS THE ONE!!! Sizzle GONE! Clean tones abound! Crunch tomorrow when people are not sleeping here. Audio attached.
    Shielded PI Output.mp3

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    Supporting Member Jazz P Bass's Avatar
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    "Even if nobody answers, writing it out here is helpful to force me to think clearly about it."

    I like that statement.

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    Supporting Member The Dude's Avatar
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    Quote Originally Posted by tomgilmartin View Post
    Great stuff, I agree the speakers are totally different. At this point, it sounds pretty good!
    Not to beat it to death, but I think you'd be surprised what different speakers would do. If you have any guitar player friends or if you have another amp, maybe you can temporarily borrow a guitar speaker for testing purposes. Or if you have access to a combo, just use the speaker inside to test. You wouldn't have to go to the trouble to remount anything just yet. Just run some leads to the test speaker(s).

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    Quote Originally Posted by The Dude View Post
    Not to beat it to death, but I think you'd be surprised what different speakers would do. If you have any guitar player friends or if you have another amp, maybe you can temporarily borrow a guitar speaker for testing purposes. Or if you have access to a combo, just use the speaker inside to test. You wouldn't have to go to the trouble to remount anything just yet. Just run some leads to the test speaker(s).
    I've been busy playing and working on guitars lately. This amp is the balls! It's amazing how sensitive it is, like my fingers are pushing the speaker cone directly. It sounds so much like all my favorite recordings. So a small issue - sometimes, when the amp gets hot, even if the guitar isn't plugged in, it will go HUMM-SKRITCH! as if I had plugged in a cable with the volume up. Then is goes away a few seconds later. Seems like it happens maybe once an hour. Am I overheating it? I put the covers on now but haven't drilled any holes yet. It does get hot, but I can still touch the aluminum skin no problem. The tubes are not inside, the front is open so they are getting some air.

    Totally different topic. I have a junky Les Paul clone that I got for literally $5 about 20 years ago. I decided to play with it, so I broke out the belt sander and a 40 grit belt and ground off the heel of the neck. Leveled and rounded off the frets and sanded it all down. Finish going on soon. Going to put some nice DiMarzios in it and Pagey Project push-pull pots. Then I will have a great sounding guitar that I don't have to worry about scratching. Ideal for this amp, no? Neck will be DiMarzio Air Classic and bridge will be DiMarzio 36th Anniv PAF. With coil switching will have 21 different options of in/out of phase combos, single coil tapping, etc. Should be interesting.

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  19. #124
    Bent Member Chuck H's Avatar
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    You should check out the filament voltage issue. At 6V that's low. Not scary low, but it could be where some heat is coming from. You have ample current available from the windings, so what's drawing current such that the filaments are being dragged down? Anyway, Maybe check the PT for excess heat. If it feels comfy to the touch you probably don't need to worry about it. If it's really hot then maybe consider examining the fan circuit since it's the only peripheral thing attached to that winding.

    And just to mention it one more time... Any instability issues you notice from here are almost always going to be relative to the input lead, it's length and it's proximity to other sh-- ah, stuff from later stages, especially the power amp (negative feedback lead, OT leads, etc) or shared grounds on long leads. The input lead to the first stage grid should be alone, shielded and as short as possible (maybe you could drill a port just to get the input jack to the first grid in a short run?)... If it comes up.

    The only other thing that might cause occasional squawking might be an iffy solder joint. I don't envy re-flowing solder on that build, But hey, greatness comes at a price

    So glad you're enjoying it for what it is. It's a great circuit. Typically very fast (even with a tube rectifier) and very strident on playing technique. Some amp designs make every player sound the same (a lot of modern, high gainer fall into this category). Other's show off the differences between players. I think the latter applies to the Bassman circuit.

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

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by tomgilmartin View Post
    Frequency response to pink noise. Through the studio amp, close mic. Recorded on phone.

    Click image for larger version. 

Name:	Pink Noise Response.JPG 
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    This is much better than the earlier one, please repeat it in a way closer to regular speaker sweeps to be of use.

    1) sweep from, say, 50 or 60Hz to about 8kHz.

    2) use a Logarithmic frequency scale, linear is useless.
    The vertical dB scale is fine.
    In any case, just from what is already shown, response falls like a brick above some 1500Hz or so.

    We expect something similar to what I posted on #107

    Thanks and glad you found the main problem

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    Juan Manuel Fahey

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    Quote Originally Posted by J M Fahey View Post
    This is much better than the earlier one, please repeat it in a way closer to regular speaker sweeps to be of use.

    1) sweep from, say, 50 or 60Hz to about 8kHz.

    2) use a Logarithmic frequency scale, linear is useless.
    The vertical dB scale is fine.
    In any case, just from what is already shown, response falls like a brick above some 1500Hz or so.

    We expect something similar to what I posted on #107

    Thanks and glad you found the main problem
    This is not a representative curve, I think. It's an instantaneous response to pink noise, not a sweep. I have to understand better how the freq response curve is meant to be produced. Pink noise is equal energy in all octaves, so it emphasizes the low frequencies because the octave bands there are much narrower. White noise might be more representative. When one sweeps across the frequencies, how does one keep 1 volt on the output as the frequency changes? This is fun, but unfortunately I'm not a testing lab, LOL! There's room noise and mic response that are totally unaccounted for.

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  22. #127
    Supporting Member eschertron's Avatar
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    Quote Originally Posted by tomgilmartin View Post
    Pink noise is equal energy in all octaves
    Which is why Juan suggested a log scale graph. Easier on the eyes, and less cognitive strain on the musical types.

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    If it still won't get loud enough, it's probably broken. - Steve Conner
    If the thing works, stop fixing it. - Enzo
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
    MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey


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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by tomgilmartin View Post
    This is not a representative curve, I think. It's an instantaneous response to pink noise, not a sweep. I have to understand better how the freq response curve is meant to be produced. Pink noise is equal energy in all octaves, so it emphasizes the low frequencies because the octave bands there are much narrower. White noise might be more representative.
    Oh yes it is, just harder to read.
    Pink noise does NOT emphasize the low frequencies but show every octave with same energy.
    White noise is a poor tool because it does NOT have even energy per octave.
    When one sweeps across the frequencies, how does one keep 1 volt on the output as the frequency changes?
    You do not, and thatīs the point.
    You *feed* constant 1V or whatever you chose, output will vary and this variation is tracwed and showx circuit or speaker frequency response.
    This is fun, but unfortunately I'm not a testing lab, LOL!
    Lab or not, you are testing
    There's room noise and mic response that are totally unaccounted for.
    Definitely, but itīs useful anyway.

    As of the "sweep" bit, you are sweeping, just there is two ways to do it (and probably more)
    Classic one is to sweep a sinewave, amplify microphone output with a wideband preamp and plot "whatever you find".
    You will reach a "mountains and valleys" curve.
    Notice the technique: sweep narrow (single frequency) feed > wideband read.

    What you did is the opposite (ends up being the same ) : you
    feed wideband signal (pink noise) > read with a sweeping narrowband filter.

    The curve to be considered is the solid line joining the peaks in your graph:
    crudely drawn by hand and mouse :
    Click image for larger version. 

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    Juan Manuel Fahey

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    Hi guys I'm still here!! Sorry for not responding, but I was busy PLAYING THE GUITAR!!! Many, many observations after a month of playing DRY guitar without effects. I will write some down in case anyone is interested:

    1. With the speakers I have being low-efficiency, the clean headroom of the amp is greatly diminished. Because the amp has to drive hard to make the speakers move, it reaches breakup sooner (i.e. at lower volume).

    2. I bought a Line 6 Spider Valve mk2 112 for $250 for comparison. JUST THE TRANSFORMERS in my amp build cost more than that! The Line 6 has a Celestion Vintage 30 12" in it, a proper guitar speaker. So, of course, I plugged it into my amp. It sounded really cool, much more "chime" as discussed before and WAAAY more clean headroom. In fact, I got it to start being crunchy but it was so loud I couldn't take it, even with the 8ohm 50W pad resistor on the output. Like ear-splitting loud.

    3. I LIKE the sound of the woofers. As I said before, the PERCUSSIVE sound is the BALLS with the woofers connected. SHUKKA-SHUKKA-SHUKKA with the strings muted sounds amazing. I think this is because of the high power output required to move them, and the long excursion of the cones. They compress a LOT of air because they move so far, and you can hear it. I will post some examples, but it's hard to keep the mic from clipping.

    4. JAZZ tones are awesome. My cheapo Made-in-Korea Les Paul copy guitar turned out awesome. Not like a factory build, but it feels great in my hands. Fret 22 is totally accessible now with the (WHY DO THEY PUT THAT THERE?) heel ground off. I have small hands for a tall guy. The Dimarzio Air Classic Neck is the star. With that pickup, tone all the way down it's impossible NOT to sound like Wes Montgomery. It just DRIPS.

    5. There is NO WAY to stop a guitar and amp from making hum. I have been doing extensive research on magnetic interference, etc. I shielded my guitar cavities and had a small improvement. I actually wired my amp inputs for TRS conections and separated the signal negative from the shields at the guitar and amp end. With a 2 conductor shielded-twisted-pair cable there was little to no improvement. Here's the rub - a guitar pickup's PURPOSE is to detect disturbances in a magnetic field. You CANNOT shield it from magnetics as this will defeat it's intended function and render the guitar incapable of producing voltage. So shielding the cables more has a marginal effect because it's the PICKUP that is introducing all the hum by PICKING UP (get it?) 60hz from the ether and INJECTING it directly into the amp gain stages. As stated previously, to stop noise from getting into the pickup you have to shield the ROOM. Maybe I should play the guitar inside a giant aluminum foil bubble. CRAP now I have to buy 50 rolls of foil and line my music room. Cursed OCD. I have learned where to stand in the room for least noise but unfortunately the comfortable spot to sit on my stairs is SUPER noisy.

    6. I always thought the thing about dimmers and fluorescent lights making hum was BS, but I was WRONG. Move the guitar near the dimmer switches and HUUMMMM!!! Why doesn't this effect solid state amps so much, do they have filtered inputs?

    7. Using 2 amps at the same time is AWESOME!! I have been goofing around with the Line6 and my amp running at the same time. With slightly different tones, and the Line 6 with some reverb and a little delay, and crunch on my amp. This sounds sooo thick with lots of thump from the woofers and big power on my amp and nice airy chime from the Line 6. BTW the line 6 power section is 2 12AX7s (Follower and PI) with 2 6L6s designed by Bogner. Nice amp even by itself but combined with my amp it's just stellar sounding.

    OK more later, I want to go play guitar!

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    Quote Originally Posted by J M Fahey View Post
    Oh yes it is, just harder to read.
    Pink noise does NOT emphasize the low frequencies but show every octave with same energy.
    White noise is a poor tool because it does NOT have even energy per octave.
    You do not, and thatīs the point.
    You *feed* constant 1V or whatever you chose, output will vary and this variation is tracwed and showx circuit or speaker frequency response.
    Lab or not, you are testing
    Definitely, but itīs useful anyway.

    As of the "sweep" bit, you are sweeping, just there is two ways to do it (and probably more)
    Classic one is to sweep a sinewave, amplify microphone output with a wideband preamp and plot "whatever you find".
    You will reach a "mountains and valleys" curve.
    Notice the technique: sweep narrow (single frequency) feed > wideband read.

    What you did is the opposite (ends up being the same ) : you
    feed wideband signal (pink noise) > read with a sweeping narrowband filter.

    The curve to be considered is the solid line joining the peaks in your graph:
    crudely drawn by hand and mouse :
    Click image for larger version. 

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    Click image for larger version. 

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    I did it both ways that you suggest. The curves are drastically different, because the sweep one goes UP from 2 sec to 4 sec, corresponding to 2-4kHZ. The pink noise one goes DOWN at 1.5kHz

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  26. #131
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    Quote Originally Posted by Chuck H View Post
    The reason to locate the ground nearer the input has never been obvious. Not to me anyway. It was explained to me here some time ago that ground, WRT the safety, would be earth. That's your chassis earth at the AC mains plug. But ground WRT to signal would be a 0V reference point. Since even small resistance in ground leads can allow for signal interaction it works out that tracing toward the higher current circuits forces the sensitive preamp to encounter the greatest resistance getting to 0V prior to interaction with those higher currents and is therefor more prone to interact. Flip that and it is the higher current, later amplifier stages that experience the greatest resistance while the input and preamp are relatively unimpeded to their 0V reference and less likely to interact. But not only am I not an engineer, I'm not an author or a teacher. So I hope my explanation makes some sense. I had a hell of a time getting my head wrapped around this on the premise that the AC ground connection was as close to 0V as could be "seen" by the circuit. But this isn't so because the circuit prefers that it's ground favor the input end. So even if that input end isn't as exactly 0V as the AC mains ground it won't matter significantly for circuit function as long as the all paths lead that direction. I break this ideal all the time actually because I'm still not comfortable connecting my high current grounds to the input of an amplifier. So I use two (or sometimes three) ground points to chassis. One for the high current power amp and power supply end of the circuit at that end of the chassis and another for low current signal circuits at the input end of the chassis. Which is earth, yes. This is my own misconception because as I said, our goal for the circuit isn't "ground" per se', but a 0V operational reference point.

    The whop whop is a low frequency oscillation. Like a tremolo oscillator circuit. Like any oscillator circuit it is caused by positive feedback. It just happens that in a guitar amp circuit environment this is most likely to happen in the ground scheme or HV rail interactions due to impedance/frequency relationships (short of writing a book here on the matter).

    The squeal, a higher frequency oscillation, is more easily manifested in the higher impedance environment of the signal chain and is typically, but not always, caused by radiant field interaction between signal leads. Although sometimes component location and/or ground scheme and HV rail interactions can also cause them. Since we don't have a scope I think we have to hedge our efforts toward the most common likelihood of lead length and proximity.

    Ok... Going for the aspirin now.
    Chuck, sorry I have been out of the loop for so long. I may have some insight for you on the input grounding issues. I decided to really try to keep the signal ground and shield grounds separate.
    • Re-wired my guitar so that the shielding was separate from the negative signal wire with a TRS jack. Shield not touching minus lead here.
    • Re-wired the amp bright inputs with TRS jacks, + to tip, - to ring, sleeve to front panel as a separate ground. Shield not touching minus at jack only at star point.
    • Used a twisted-pair stereo cable to connect them. Basically separate wires plus, minus, shield.


    The result was that the input went WHUP WHUP WHUP like it had been doing a long time before. Maddening.
    I decided that the jack sleeve connection had too much resistance to ground, so I soldered the sleeve connection point to the shield of the internal wire. BINGO!!! WHUPing GONE!

    So the answer is, make sure that the input jack sleeve is properly connected to ground. The nut that holds it on is not always a low-resistance connection. Even if you do get a good connection through the sleeve touching the nut touching the metal face of the chassis, corrosion can compromise this connection over time. Most diagrams don't show a soldered connection to the input jack sleeve.

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    Last edited by tomgilmartin; 03-27-2018 at 05:41 PM.

  27. #132
    Bent Member Chuck H's Avatar
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    Or... By connecting a lead to another ground point you grounded a ground loop. I've been in this soup as a designer on more than one occasion. The ideal scenario would be to actually isolate the input jack sleeve so you can CHOOSE the ground reference for the input AND thereby keep it off any part of the chassis where currents may fluctuate. We have some good minds here that have helped me through most of my trials. Unfortunately one of those guys was most helpful to me was Steve Conner who no longer participates. We miss him sorely but hope with all sincerity that he's finding happiness in his ventures. I can say "we" because I know I speak for everyone here in that sentiment. Anyway... If you can work cliff jacks into your aesthetic that would accomplish isolation. Or, if you're getting acceptable performance now you can forget about it.

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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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    Quote Originally Posted by Chuck H View Post
    Or... By connecting a lead to another ground point you grounded a ground loop. I've been in this soup as a designer on more than one occasion. The ideal scenario would be to actually isolate the input jack sleeve so you can CHOOSE the ground reference for the input AND thereby keep it off any part of the chassis where currents may fluctuate. We have some good minds here that have helped me through most of my trials. Unfortunately one of those guys was most helpful to me was Steve Conner who no longer participates. We miss him sorely but hope with all sincerity that he's finding happiness in his ventures. I can say "we" because I know I speak for everyone here in that sentiment. Anyway... If you can work cliff jacks into your aesthetic that would accomplish isolation. Or, if you're getting acceptable performance now you can forget about it.
    Originally I did have isolated jacks. They were plastic.

    Also, I definitely didn't have a ground loop when I first set up the TRS connections. The minus lead from the guitar went back to the star point, and the sleeve went to the chassis through the jack nut. They weren't connected at the jack.

    NOW, since I have connected the jack sleeve to the internal cable shield directly, there IS a ground loop through the shield, through the star point, and back through the chassis to the jack nut. But this connection is quiet and sounds fine, I believe because the jack nut connection to the faceplate is poor (high resistance), almost an open circuit. I am planning to get some isolating bushings to force this connection to zero impedance. Well, ZERO is impossible because it will always capacitively couple somewhat...

    So what I am thinking the lesson is: it's more important to be sure the guitar lead's shield is solidly grounded at the amp. This means shield impedance to ground must be lower than the capacitive impedance from the shield into the leads inside the shield. Otherwise all noise on the shield will couple into the leads = NOISE and issues.

    Discuss.

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  29. #134
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    Quote Originally Posted by Chuck H View Post
    The reason to locate the ground nearer the input has never been obvious. Not to me anyway.
    I think the idea is to have one (and only one Chuck ) connection between circuit common and chassis so that there can be no ground loops through the (steel) chassis to pick up stray magnetic fields and cause hum. For safety reasons the chassis is connected to earth through the third wire of the power cord. When the amp is connected to a similarly earthed unit via a signal cable a big external ground loop is formed to couple to stray magnetic fields and there could also be a potential difference between the chassis of the two units causing a current (AC) to flow in the screen of the signal cable between them. I think one of the reasons for connecting the circuit common to chassis at the input is to have this current flow into the chassis and back to source through the power cords and not into the circuit's 0V input wire. A second reason would be to have any RF picked up on the input flow into the lower impedance of the chassis rather than the input 0V wire. I'm not convinced by the first reason as AC has already flowed along the whole length of the input cable screen so the damage is done. Of course the way we normally use a guitar amp there is no external ground loop. The only connection to earth is the third wire of the power cord. The guitar and speaker only connect to the the amp.

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    Last edited by Dave H; 03-27-2018 at 10:00 PM. Reason: spelling

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    Bent Member Chuck H's Avatar
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    Right. I get the concept. But this is where it starts to get hinky. The safety ground is closer to 0V than anywhere else in the amp. And the chassis has far less resistance than any wire. So why is the input the best place for the star point? It's because the 0V isn't actually 0V. It's a reference point that happens to be CLOSE to 0V that the amps signal chain references. Close to the input is best for this because any chassis OR wire resistance (as well as circuit block resistive representations it seems) create currents on the conductor itself that can be amplified by a sensitive input. As most guitar amps have. So... Since the input is the most sensitive place in the amp it's ideal to keep all signal path "grounds" at the input potential and attempt to keep current fluctuations on higher power grounds from interfering. So you have the higher current power amp and supply rail grounds that want one thing and the input (and therefor) preamp signal circuitry that wants the same thing, ideally, but doesn't play well with the other circuits I use a minimum of two grounds in an amp for the sole purpose of keeping any current fluctuations across a conductor off the more sensitive input and preamp ground circuits. It hasn't failed me yet and it's easy to conceptualize when laying out a circuit.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

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

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    I don't think the idea is to have the star point at the input (too messy). The input is just the single point where the circuit common is connected to chassis. The star point (insulated from chassis) with the high current wires is located closer to the power supply where the current originates. The amp circuit doesn't have to be connected to chassis to function if the chassis is only a shield and not part of the circuit. The reason for not having the chassis as part of the circuit with two (or more) chassis connection to circuit common is because the transformer's stray field generates eddy currents in the chassis which cause a potential difference between the chassis ground points which is then amplified resulting in hum. Having said that I've never connected the amp common to chassis at the input. I've always made a star point at the first filter cap and made that the only connection between circuit common and chassis. Like most of us (I suspect) I've used a hybrid bus/star ground system with the bus for the preamp and star for the power amp, the only connection between circuit common and chassis being at the star.

    Anyway, I'm no expert. I feel a headache coming on just thinking about it. If your grounding method works there's no reason to change it. If it ain't broke don't fix it.

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