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  • The basics of tube amp circuit layout to avoid components interfering with one another.

    I've watched pretty much as many YouTube videos as is humanly possible on building, working on, and talking about tube amps. I have picked up loads of tricks on how to avoid adding hum and/or noise to a circuit. I've seen twisted wires, shielded cables for carrying audio signals, not allowing wires to coil around a tube base, and using metal shielding sleeves around tubes. But, I have yet to find an all-in-one resource that ties it all together. So... what I'm left with is a metric ton of mental cobligook where I know you are supposed to twist some wires... but... I can't tell exactly which ones, because some wires in a tube amp appear to not need twistin. I also see tube sleeves on some amps and not others, and even on some tubes but not others within the same amp. I've seen some transformers be mounted inside (under) a chassis, while it's fine to mount others on top of the chassis, Hi-Fi style. Same with tubes. Some amp companies seem to mount tubs right beside big transformers. I've even heard of never having two transformers mounted with their coils "in line" with one another, but to mount on "turned sideways," i.e. perpendicular to the other... but which one?

    Also... just as another example... I'm building a tube based boost / tremolo unit, and I'd planned to build the chassis out of 16 ga rolled steel sheet, bent into a long "C Shape" but leave the ends open, and close them off with some nice wood to add a nice visual aesthetic. However, I don't know if that is a major violation of "shielding" since there would be no metal on the ends? Is that so? Would aluminum foil work to prevent it?

    Rather than address all these ad-hoc type questions, I was wondering if anyone has a resource that is the absolute Bible on guitar circuit "guide to eliminating interference, electrical and RF".

  • #2
    You have to relate tube amp building to cooking.....A cookbook will get you a decent meal made if you follow the directions but it will not give you the meal made by a 4star chef who has a talent.
    When you add multiple things to an equation you can usually come up withe solutions in different ways. So there is no one book you have to take multiple books and experiences to come up with a masterpeice... some people have it and some dont.

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

    Comment


    • #3
      Originally posted by nosaj View Post
      You have to relate tube amp building to cooking.....A cookbook will get you a decent meal made if you follow the directions but it will not give you the meal made by a 4star chef who has a talent.
      When you add multiple things to an equation you can usually come up withe solutions in different ways. So there is no one book you have to take multiple books and experiences to come up with a masterpeice... some people have it and some dont.

      nosaj
      Yeah... I hear you man. Well... about most of what you said. I end up equating almost everything to cooking. When I was 19 I moved to Europe to become a chef. This was before the Food Network even existed, and the only way to learn was to either spend $150K on culinary school, or go let some French and Austrian guys yell at you (a lot!!!) until you just get better. Ha ha ha! And... trust me... in that environment... you get better quickly. So I definitely understand what you mean about a cook book being a great start, but you're very unlikely to get from a recipe, what an experienced chef has learned over time. I also whole-heartedly agree with you when you say there is no "one book" or one resource to teach you everything. I'm 43 now, and over the ~25 years I've been cooking, I can't tell you how many 500 page books I own and have read where I only was able to really learn something new on 3-4 pages. When culinary folks sit down to write cook books, each one of them has their own experiences on their shoulders, and each one of them is going to assume the reader already knows this, or that... so they may not even think to write down certain steps, because they just never could imagine that someone would not just know to do <this> or <that>.

      This is kind of how I've been learning electronics. I watch Uncle Doug videos on YouTube, and at first... it was like listening to someone speaking French. I'd know certain words, and I'd kinda think I caught this or that from what he said... but it was def. not just a "sit back and listen" experience. I ended up having to start taking notes. Now I have this massive Uncle Doug Google Doc with all these terms that I'd hear and go look up later. After ~2 years of watching, taking notes, researching, then re-watching... I've finally gotten to where I can say I know a descent amount about amp electronics. But, without having a physical person who can yell at me (as was the case in France), or without someone to sit and ask "Hey... why'd you do that-right-there?" after a while, you do kind of run out of gas.

      I thought I'd turn to buying some books. So, I even reached out to Uncle Doug to ask him for recommendations on books. On his advice, I've bought the same 5-6 Vacuum Tube amp books that I'm sure you all have on your shelves. I've even bought a few more obscure ones. Unfortunately... and I don't expect anyone to really just "understand this"... I just cannot seem to learn from a book. This is probably the same aspect of who I am that got me into cooking in the first pace... I just look at the words, and can definitely read them. I'm not dyslexic or anything like that. I just... read 1-2 sentences, and either my mind just drifts off, or I start fixating on the fact that I don't understand what I'm reading, or I run across some term or phrase that the author did not intend to be the crux of the issue, but it'll send me off looking up some term I'd never heard, and before you know it... I've gotten frustrated by the rabbit hole, and set it down to come back to later. This is usually when people start to tell me all about having discipline, or... kinda like you said nosaj, that "some people have it and some don't."

      It's that last sentence that just really gives me diarrhea. No offense to you; I'm sure you could be right. I just feel like I'm not at all a stupid person. I am just not the type of person who can learn from rows of words written on pages. Unfortunately, there's just not much money to be made in immersive interactive multi-media video learning experiences for designing tube amps. Ha ha ha!!! But boy... if there were...!!! :-D.

      Thanks man! I appreciate the response!
      Michael

      Comment


      • #4
        Henry Ott's "Noise Reduction Techniques", in hardcover. It too does not tell you "just do this, it's best". And that's because, as nosaj says, the real underlying issues are complex, and what's perfect in one situation is so-so or marginal in other situations. If that seems extreme, You also need the concept of getting away with doing things that are not perfect. As children, we've all had the situation where we should have been hurt but avoided it by accident. Building guitar amps is much like that. The "bible" tells you to do things this way, but that would take years and thousands of your local currency. So you do it imperfectly and sometimes, it's nearly perfect and that's good enough.

        I can give you a few tidbits that might help.

        First up, Faraday cages. A completely enclosing box of highly conductive metal can be shown mathematically to not allow any external electrical, magnetic, or electromagnetic fields to enter it. Good shielding involves making up a conductive metal box with all the surfaces as completely covered in metal as possible, and all the metal electrically conducting along all the seams. The only penetrations should be small holes where the power and signal go through the metal enclosure. The electromagnetic fields math shows that the lowest frequency radiation that can get through a hole is the frequency where the hole's biggest dimension is 1/4 of the wavelength of the field. Incn/cm sized holes push this up into the nearly-GHz range. Yes, you need the end caps to be metal, and a metal bottom if you're going to do a complete job, and all the seams should be conductive all along the seams. Or as close as you can get. You may find you get away without one or another surface being perfectly closed. Real Faraday Cages are imperfect. We can only make them "good enough" for the situation.

        Second, shielded wire. Shielded wire is the Faraday Cage concept extended to wires. If you put a complete metallic shield around a wire, it confers some immunity to radiated fields getting into the wires. That's why shielded wire exists. Note: there are better and worse ways to connect the shield, and it depends on the nature of the signal inside the wire whether it's good enough.

        Third: Source impedance and load impedance. Every signal is generated by some signal source and accepted by some load. But sources are not perfect. Every signal source has some internal resistance, inductance, and capacitance that is not possible to get rid of. This includes not only the desired signal you want going down the wire, but also the source of the interference that's trying to get into the wire. In general, you want the signal source to be the lowest possible source impedance. But with tubes, you immediately run into tubes not being perfect. The plate impedance of a 12AX7 is about 67K. So any impinging interference has to either be a much higher impedance to avoid polluting the plate signal, or somehow attenuated to be "very small" compared to the plate signal. How much "higher impedance" or "very small" is a huge variable. Again each situation will use part placement shielding and distance to get the interference down to "good enough".

        Fourth: Magnetic fields and current loops. Current moving through a wire generates a magnetic field around the wire. A loop of wire with current in it generates a field that's a "donut"/torus around the loop. An external magnetic field cutting across a wire and especially a loop of wire generates a voltage along the wire. Loops are good for making electromagnetic things like transformers, and very bad for trying to NOT pick up a magnetic field. If you want to not pick up magnetic fields in your wires, you can shield the wire, in which case the magnetic field generates a voltage along the shield, or you can twist the sending/returning wires in a pair. A twisted pair is a trick where the wires are set up to have the smallest possible area inside the send/receive wire pair by holding them tightly together, reducing the size of the receiving loop, and also forcing each bit of the wire pair to experiencing equal and opposite magnetic field issues. This helps (but isn't perfect) the magnetic interference to cancel. For high current send/receive wire pairs, like especially heater wires, using twisted pair wire also means that the generated magnetic field is kept inside the space between the wires, and that the loop area is as small as possible so it can't radiate the field as well.

        This kind of thinking just goes on and on. There are EEs who have spent their entire professional career on learning how to reduce EMI. It's useful to remember that there are people who have spent much of their lives studying the bible of their choice, too. There is no quick, easy cookbook for solving hum and noise removal; otherwise, it would simply have been done long ago by the professionals and would not even be a topic for study any more. There is no substitute for going and learning all you can. If it was simple, it would already be universal.
        Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

        Oh, wait! That sounds familiar, somehow.

        Comment


        • #5
          Wow, you got a response from RG--that's like hearing from God's engineer.

          On a more practical matter, the first challenge you'll meet on a custom build is the placement of the PT and OT relative to each other (one of the points you mentioned). This is critical for eliminating the hum you'll hear on power up, even with NO TUBES INSTALLED. It's the sound of the PT radiating 60Hz EM into the OT (and if you like playing quietly at home, you'll kick yourself if you leave this hum in your amp). You solve this by arranging the two transformers with their windings not aligned with each other. I do this by listening to their placement, by using the "headphone trick".

          I no longer twist my heater wires. After 11 years of building, I have found ZERO instances of the heater wires causing radiated hum in my amps. It's almost always the grounding scheme--a whole BOOK of a topic. I do keep the heater wires away from signal wires, but that's just good practice. Regarding grounding, there are plenty of documented ways to get good results. I've settled on a hybrid of stars, busses, and chassis ground points that suits me and gives good results.

          I've never had an issue with power tubes being too close to a transformer or speaker. But it's important to visualize this stuff in 3D before you build, so you don't have trouble when you go to put your completed chassis into your cabinet. (ask me how I know...)

          Chassis shielding has rarely been an issue for me, because I don't build high-gain (i.e., Soldano) amps, only mid-gain (like a recent JCM 800), so having an open bottom or sides never bothers me. I try to only use shielded cable for the first signal run if needed (each run of co-ax cable dumps a little of your high frequencies to ground).

          The rest of the lead dress is a dance between shortest practical path, what looks tidy and pleasing, and what actually is BAD because of parasitic coupling or noise. Things like, it's cool to group the three wires of each triode together, but bad to run your NFB wire from your ouput jacks close to your input signal. You're right, there's nowhere that collects this info, but you learn it over time from a variety of sources, and primarily your own mistakes.

          Oh, one more thing (ask me how I know...) you have to watch for is wires or other components rattling against the chassis or each other while playing. These mystery rattles will drive you bonkers (ask me how I know...)
          --
          I build and repair guitar amps
          http://amps.monkeymatic.com

          Comment


          • #6
            A lot of articles and a lot of books have been written on the topic of grounding, shielding and RFI.
            Personally, I doubt you will be interested in dry theory.
            The attached links of DIY amplifiers will be more useful to you.
            Each schematics is accompanied with component layout, wiring and grounding points.

            http://www.mojotone.com/kits

            https://www.tedweber.com/amps/kits

            1)

            links are educational (not advertising)
            Attached Files
            Last edited by vintagekiki; 01-24-2021, 05:38 PM. Reason: 1)
            Who does not know and knows that he does not know - teach him Confucius)
            Who knows and does not know that he knows - wake him Confucius)

            Comment


            • #7
              My perpesctive on things....

              learned early on that college was not for me. I am more of a hands on type person.
              What i have found is that if I am shown sometime then my mind can make the relational jump to understanding the book stuff. But if you reverse it and have me do book stuff first I can never get to the good stuff(working with my hands)
              Now I love to read so thats not the issue. But reading without seeing the lightbulb never gets turned on for me.
              In high school I could never get into physics because of all the math, but later on watching physics I could understand some of the math. An excellent representation of this is an old ATT film on the characteristics of waves.
              https://www.youtube.com/watch?v=DovunOxlY1k
              Binkie McFartnuggets‏:If we really wanted to know the meaning of life we would have fed Stephen Hawking shrooms a long time ago.

              Comment


              • #8
                Originally posted by Dolmetscher007 View Post
                I've watched pretty much as many YouTube videos as is humanly possible on building, working on, and talking about tube amps. I have picked up loads of tricks on how to avoid adding hum and/or noise to a circuit. I've seen twisted wires, shielded cables for carrying audio signals, not allowing wires to coil around a tube base, and using metal shielding sleeves around tubes. But, I have yet to find an all-in-one resource that ties it all together. So... what I'm left with is a metric ton of mental cobligook where I know you are supposed to twist some wires... but... I can't tell exactly which ones, because some wires in a tube amp appear to not need twistin. I also see tube sleeves on some amps and not others, and even on some tubes but not others within the same amp. I've seen some transformers be mounted inside (under) a chassis, while it's fine to mount others on top of the chassis, Hi-Fi style. Same with tubes. Some amp companies seem to mount tubs right beside big transformers. I've even heard of never having two transformers mounted with their coils "in line" with one another, but to mount on "turned sideways," i.e. perpendicular to the other... but which one?

                Also... just as another example... I'm building a tube based boost / tremolo unit, and I'd planned to build the chassis out of 16 ga rolled steel sheet, bent into a long "C Shape" but leave the ends open, and close them off with some nice wood to add a nice visual aesthetic. However, I don't know if that is a major violation of "shielding" since there would be no metal on the ends? Is that so? Would aluminum foil work to prevent it?

                Rather than address all these ad-hoc type questions, I was wondering if anyone has a resource that is the absolute Bible on guitar circuit "guide to eliminating interference, electrical and RF".
                Some excellent responses, lengthy ones too, and much more so when you read the books.

                Basic amp circuit layout, keep inputs away from outputs. By a couple inches at least, the further the better. Minimum six inches would be good, 15 cm for those metrically inclined, between input jacks plus first stage pre, and output transformer leads and speaker jacks.

                Your proposed chassis - no big problem for simple low gain circuits. Plenty of amps built on c-bent chassis that have open "ends." Foil to complete the "box" well Fender and a few others do that, and I often find the foil ripped out from previous efforts to remove the chassis. Nonetheless hum doesn't seem to be much of a problem for them. Perhaps a more complete Faraday cage "box" is necessary for amps that have high gain, cascaded preamp stages for rip roaring overdrive tone, but there's so many other pitfalls steering clear of hum & buzz with those. Racket from cell phones, that's another problem. Amps seem to pick it up no matter how well shielded so just keep your plastic pocket pal away from your amp, boost box, cables & guitar.

                Transformers, relative to each other. Since I joined MEF also The Amp Garage our very good neighbors, I've seen some mention of the "headphone" technique. Before committing to placement of your output transformer, run its speaker leads to a jack and plug in headphones. If I'm not mistaken, you don't really need all the rest of the circuitry built & running for this check. Run power to the power transformer, and position the OT for minimum hum on the headphones, mark the chassis for drilling and park your OT there. I'm sure there's refinements on this technique, but that's the basics. Some commercially made amps have OT or PT mounted at an odd angle. Crate Vintage Club amps come to mind. If that's what it takes to beat hum to a minimum, so be it.

                My 2 cents worth, hope it helps.

                Enjoy. Every. Sandwich.

                Comment


                • #9
                  Many diy efforts try to clone existing layouts (commercial products and kits and forum member builds), which is practically the safest approach for the less experienced. Many forum threads then seem to arise due to some problem with a constructed amp, such as some hum or noise on the output signal, or just downright unstable output squeal. Then the assessments of the cause or causes start to get teased out in each thread - that is a good form of practical learning curve as it often shows up a particular weakness in a build and how to get around it. Some causes don't show up on a schematic, as they relate to layout of parts where signal from one wire or part couples to another wire or part. Some causes need a schematic to appreciate the problem, like when looking at 2 wires that run near to each other and then realising what signal those 2 wires have on them or connect to. And that is perhaps where you need to make your own list of 'what to look out for' - whether that list is to help with initial construction layout, or fault-finding after you test and find not all is well with your new creation.

                  As indicated, layout infers a level of physical distance/separation between regions of circuitry that are known to be either high level signal sources (the higher the level, whether voltage or current, then the more likely it is to be the origin of a signal or noise), or low level signal pickup points (the more sensitive a section of circuitry is, or the higher its impedance, makes it easier for a hum or noise to get in to and cause a problem). That can be expanded out in to a myriad examples, some of which you have noted, such as a power transformer near an output transformer, and other common examples are wiring taken to front panel pots that relate to high-impedance circuitry and pass nearby to wiring or parts that have high signal levels on them. Maybe there are some nice schematic graphics showing circuit areas that are sensitive, and circuit areas that have large level signals on them - some construction notes for diy amps provide comments on what to be careful of. From that understanding you then assess layout and wiring paths that could be potential problems, and then work out ways to supress interaction.

                  Comment


                  • #10
                    That's you're talking about is called technology.
                    Schematic for itself represents 20%. The quality of the installed components goes up to 40%. Don't save on built-in components. The rest is called labor, experience, knowledge of technical principles ... ...

                    Usually after 2-3 failed constructions, the builder learns where he is wrong.
                    In order to work on development amplifier, it is necessary work experience min 5 years on servicing and repairs factory amplifiers. On servicing, in addition to technically experience, technological experience is gained: component layout, wiring, grounding ...

                    In order for a builder to gain some instant experience, for begin is best copy/ paste (clone) some simple amplifier.
                    Who does not know and knows that he does not know - teach him Confucius)
                    Who knows and does not know that he knows - wake him Confucius)

                    Comment


                    • #11
                      The way I see it, a schematic is just a theoretical ideal, once we attempt to turn that schematic into a real world amp, additional layers of complexity are added to that schematic as the parasitic characteristics each component and conductor are brought into play. We can't escape capacitance etc inevitably connecting everything to everything else, all we can do is to try and mitigate its negative effects by following good practice in regard of layout, 'grounding', lead dress etc. That's our best chance of keeping the gremlins of hum and instability, that can plague things otherwise, at bay.
                      And then just hope it all works out ok
                      My band:- http://www.youtube.com/user/RedwingBand

                      Comment


                      • #12
                        Originally posted by pdf64 View Post
                        The way I see it, a schematic is just a theoretical ideal, once we attempt to turn that schematic into a real world amp, additional layers of complexity are added to that schematic as the parasitic characteristics each component and conductor are brought into play.
                        I'm not disagreeing, but one thing I've not seen mentioned is whether the OP reads schematics. I'll assume so but it is a very critical question.
                        If not, it's the first thing you need to learn. Those who don't never get beyond the 'kit builder' level.


                        "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

                        Comment


                        • #13
                          Find old tube books, earlier the better, 30's-40's-50's. Even some ham magazine articles will throw in hints and tips while describing a project. Grounding,shielding, filtering and parts placement to me is second nature and is always on my mind from the beginning of a project, often overkill but i don't have oscillation or hum problems with my off the wall builds.

                          As was said, years of experience(building and repairing) and being able to read and understand schematics makes a difference.

                          Comment


                          • #14
                            My own real world example....

                            Preface...
                            I'm very much an amateur/hobbyist at electronics. No formal training.
                            That said, I've been mucking around with this stuff for many years. (mucking around being the key words..... )
                            Lots of reading, building book and magazine projects, "inventing" my own circuits (solid state) for unique projects (not amps).

                            ... and all that said... I've never built a tube amp before.
                            Not even a kit.
                            Have repaired a few...

                            I decided to completely gut my Blues Junior PCB amp and build it "old school", with eyelet circuit card and chassis mounted pots, jacks and tube sockets.

                            Goals;
                            Keep all front panel controls functional as original.
                            Add tube driven reverb. (original was SS)
                            Add foot switch for reverb. (original was not foot switchable)
                            Keep "Fat" foot switch.

                            The schematic I came up with...

                            Click image for larger version

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                            Everything to the right of Master Vol is stock Blues Junior.
                            Everything to the left of Master Vol is modified Fender AB 763 circuit.

                            I made my own layout and circuit card... never done that before.

                            Here it is partially completed.


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                            Completed amp.

                            Click image for larger version

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                            The point of all this in relation to this thread....

                            This amp is dead quiet.
                            No hum, no buzz, no nothing..... oh, and it sounds awesome!

                            I was very careful with the grounding scheme.
                            I did have some good reference books to help with the grounding scheme, mostly took tips from Keven O'Connors TUT books.

                            For a first time tube amp build, did I just get lucky the amp is so quiet?

                            Here's what the stock Blues Junior guts look like...

                            Click image for larger version

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                            Attached Files
                            If it ain't broke I'll fix it until it is...
                            I have just enough knowledge to be dangerous...

                            Comment


                            • #15
                              Wow !
                              Nice build.
                              A Boutique Blues Junior.
                              That my friend is how Fender should have built the amp.
                              Although it probably would have them cost too much.

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