I have a problem with formulas & schematics (warning, long post)

martinlb

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When I was a kid I was considered to be "gifted" at math. I actually remember beginning to feel an actual interest in algebra and geometry. Unfortunately there were forces of the dark side that extinguished those. I had some really bad and mean-spirited teachers. Fast forward to near the end of high school and I built a Heathkit combo guitar amp. I followed up on that with a Heathkit head - more power is always better, right?

During the decades that followed I lost interest in building projects until five or so years ago when I decided my garage workshop needed better audio than what I was getting from my computer. This is where my journey into electronics began again.

After buying and returning a couple cheap audio amplifiers I found a 100w amplifier kit on A..zon and figured "wow, that much power for only $40? Why not???" I ordered the kit and was shocked when it arrived. All there was were some puzzling-looking parts and a circuit board with hieroglyphics etched onto it.

Thus began my first steps. I sat and started looking carefully at each of the components, looking things up one at a time, and learned to identify resistors, capacitors and so on. I learned what most of the etchings on the circuit board represented and since I only had $40 at risk, began soldering pieces onto the board. That brought back memories of my Heathkit builds, at least as far as the soldering went.

Not having an instruction manual was a challenge but I finished building one of the two boards I'd received before realizing this kit required an odd sort of power supply. I contacted the seller who offered to sell me a power supply for some number of hundreds of dollars and that was as far as that build went. I was hooked though. And I'd learned a little bit. A $40 educational investment.

The idea of going from a pile of parts and components to something that worked felt really cool. I ordered a $100? $200? stereo tube amp kit from Tube Depot. This kit was built on a pine board. It came with instructions I could understand and after building it and powering up, it worked!!! Now I absolutely was hooked. Big time.

Since then I've painted by the numbers and actually have also learned a bit. That first $40 amp kit I ordered would make sense immediately as far as what goes where is concerned at this point now. I've also picked up a very basic understanding of how tube amps work but only very, very basic. I understand, for example, that tubes need D/C current and that a rectifier converts A/C current to D/C. Whooo hoo, right?

I would absolutely love to really learn electronics and be able to diagnose what's wrong when a kit I've built doesn't work. Even better be able to design a tube amp from scratch. This is where my problem arises.

I go blank when I see a formula, even a basic one like Ohm's law. I just cannot make a connection between what I'm seeing and any real world application. Schematics are a little bit better for me but when I see a schematic I don't make a connection from what I'm looking at and what it represents. What I can do is look at a layout and make that connection.

Maybe I'm just venting or whining. I won't be surprised to see, if I get any responses to this, recommendations that I put on my big boy pants or such. The thing is, though, my inability to make sense of things isn't due to laziness or lack of motivation. Being able to understand these things is very near to the top of my wish list.

I know about different ways people learn and how we can be wired differently (no pun intended!!!) from each other. At some point I want to take a hands-on Amp troubleshooting class. I think it might help me a lot to be able to see the theories in actual use.

If my problems are familiar to anyone here, have you had a chance to take a class or attend a seminar? Did it help? Have you found anything else to break through that wall?

Thanks for taking the time to read this.
 

vampwizzard

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When I was a kid I was considered to be "gifted" at math. I actually remember beginning to feel an actual interest in algebra and geometry. Unfortunately there were forces of the dark side that extinguished those. I had some really bad and mean-spirited teachers. Fast forward to near the end of high school and I built a Heathkit combo guitar amp. I followed up on that with a Heathkit head - more power is always better, right?

During the decades that followed I lost interest in building projects until five or so years ago when I decided my garage workshop needed better audio than what I was getting from my computer. This is where my journey into electronics began again.

After buying and returning a couple cheap audio amplifiers I found a 100w amplifier kit on A..zon and figured "wow, that much power for only $40? Why not???" I ordered the kit and was shocked when it arrived. All there was were some puzzling-looking parts and a circuit board with hieroglyphics etched onto it.

Thus began my first steps. I sat and started looking carefully at each of the components, looking things up one at a time, and learned to identify resistors, capacitors and so on. I learned what most of the etchings on the circuit board represented and since I only had $40 at risk, began soldering pieces onto the board. That brought back memories of my Heathkit builds, at least as far as the soldering went.

Not having an instruction manual was a challenge but I finished building one of the two boards I'd received before realizing this kit required an odd sort of power supply. I contacted the seller who offered to sell me a power supply for some number of hundreds of dollars and that was as far as that build went. I was hooked though. And I'd learned a little bit. A $40 educational investment.

The idea of going from a pile of parts and components to something that worked felt really cool. I ordered a $100? $200? stereo tube amp kit from Tube Depot. This kit was built on a pine board. It came with instructions I could understand and after building it and powering up, it worked!!! Now I absolutely was hooked. Big time.

Since then I've painted by the numbers and actually have also learned a bit. That first $40 amp kit I ordered would make sense immediately as far as what goes where is concerned at this point now. I've also picked up a very basic understanding of how tube amps work but only very, very basic. I understand, for example, that tubes need D/C current and that a rectifier converts A/C current to D/C. Whooo hoo, right?

I would absolutely love to really learn electronics and be able to diagnose what's wrong when a kit I've built doesn't work. Even better be able to design a tube amp from scratch. This is where my problem arises.

I go blank when I see a formula, even a basic one like Ohm's law. I just cannot make a connection between what I'm seeing and any real world application. Schematics are a little bit better for me but when I see a schematic I don't make a connection from what I'm looking at and what it represents. What I can do is look at a layout and make that connection.

Maybe I'm just venting or whining. I won't be surprised to see, if I get any responses to this, recommendations that I put on my big boy pants or such. The thing is, though, my inability to make sense of things isn't due to laziness or lack of motivation. Being able to understand these things is very near to the top of my wish list.

I know about different ways people learn and how we can be wired differently (no pun intended!!!) from each other. At some point I want to take a hands-on Amp troubleshooting class. I think it might help me a lot to be able to see the theories in actual use.

If my problems are familiar to anyone here, have you had a chance to take a class or attend a seminar? Did it help? Have you found anything else to break through that wall?

Thanks for taking the time to read this.
Go watch Uncle Doug on youtube to learn how these things work. He is into cars as well and I think that approach is a good analog. You have to know what the parts are and what they do. Thats step one. A lot of folks figure out how a starter engine works or how to replace headlights etc. guitar amps are similar. As far as formulas and whatnot go, just figure it out once and put it in excel. Then just put measurements in and youre set. Follow the safety rules.. discharge the caps before digging around in a tube amp. Good luck and chip away at it.
 

Mongo Park

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The two above posts are the best advice you can get. Stick with the simple amps champ straight up doesn’t have all that many parts which makes it easier to understand. I think the amps you have dealt with are more complex than a basic champ.
 

Timbresmith1

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I hear you about math trauma. I got hammered for not showing my work, even tho I had the answer. My BRAIN did the calculation for me.
 

24 track

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the basic principles have never changed whether solid state or tube amp, all have a power supply , preamp stages, power stages and output , it how the signal gets to the end stages that differs. becoming familiar with those differences and being able to follow it down is the key .

tube amp High power, or high voltage caps can and will kill you if you lose focus , would watch a ton of you tube on the subject , gerald weber did some good basic vids , although not entirely acurate, but i learned alot from him , read some books first ,Ashton Pittman wrote a great book but again not everyone agrees with him, Uncle Doug as mentioned , and DR Z ( his amps are art to work on, his soldering routine is above and beyond) Rob Robinette or @robrob on this site is spectacular, @Wally is a great resource and a walking encyclopedia of amps and amp history , you also have some of the best resource members on this site. the devil is in the details and how much attention you pay to that.
be careful have fun, and be safe
 
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printer2

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This is a pretty good document. A thing to relating electricity to something some people can understand. Water. The tap has (just for example) 30 psi with the valve closed. All measurements are related to atmosphere right now. Open up the valve fully and there is no resistance to the flow of water. Now put a hose on the valve and kink the hose so there is a resistance to the flow. If you have the valve opened all the way the only resistance to the flow is the 'kink', which offers a resistance to the flow. In electrical terms we would call this a resistor (sure you know this well enough). With the valve all the way open the whole pressure drop is across the 'resistor' and a minimal one across the valve.

But what happens when you cut back the flow at the valve? You will have two resistances in series. What happens when you have the same amount of resistance to the flow in the valve as you do the 'kink', oops, I mean resistor? If they both have the same resistance you will get 15 psi drop across the valve and 15 psi across the resistor. If the valve is closed and has twice as much resistance of the kinky resistor then you have a ratio of 2:1. Together you have a resistance of 3. Across the valve you would have 2/3 of the pressure drop. Across the kinky resistor you have 1/3 of the pressure drop. Since we started with 30 psi we get 10 psi across the valve and 10 psi across the the pinky resistor.

And you can think of the electrical circuit the same. You have a power supply of 30V and a valve (tube) in series with a resistor. Say you have the tube turned up so that it has double the resistance of the resistor. Same thing as above but now you have 20V across the valve and 10V across the resistor. Open up the valve enough to have the same resistance as the resistor, you have 15V and 15V.

Each series string across a voltage source has to have all the voltage drops across each of the resistance add up to the voltage source. So if you had three equal resistances you end up with 5V across each of the resistances. We always designate one point as ground potential. The resistor with one end at the ground potential has the other end of it at +5V compared to ground. The next resistor has 5V across it when using a volt meter measuring across it. But measuring from ground, we have +5 at one end and +10V at the other end of the resistor. The third resistor has 5V across it but referenced from ground it has +10V on the lower end, +15V at the top, which happens to be connected at the power supply.

Now if we have a plate resistor, a triode and a cathode resistor in series we have the same as the above, the tube just is a funny resistor (with a 'control gate'). If the top and bottom resistor are 1k each and the triode is turned on enough to have a 1k ohm resistance across it you have a voltage divider circuit, 15V at the top of the resistor, +10V at the other end of the plate resistor and the valve plate. The other end of the triode has the cathode resistor attached to it and the voltage is +5V there. And that means the cathode resistor has 5V across it.

Now these voltages are kind of low for tubes, reasonable for transistors. Now let us use a 300V supply. Say we have a 12A*7 tube with a 100k resistor on the plate and a 1k resistor. A normal (but what is normal really?) biased triode will have the plate at 2/3 the supply voltage. So let us say the plate is at +200V (referenced to ground). That means that the plate resistor has 100V across it. Now is the tricky part. The cathode resistor is 1k, the plate resistor is 100k, this means the cathode resistor voltage is 1/100 of the plate resistor. The plate resistor is 100V, so that means the cathode resistor has 1V across it. That does add up to 301V, I am going to cheat and steal the 1V from the triode (I like easy) so you have (referenced from ground) +1V on the cathode (also across the 1k resistor), the plate of the triode is +200V (190V across the plate to cathode) and the remainder 100V across the plate resistor. Everything adds up to 300V.

See how easy it is?

Now that you have three devices in series you probably can figure out if you have four in series. But we won't bother with that now, you can take the homework to do later if you like. Now we will drop a resistor, or the valve for now. Ohm's Law. This is a handy one. (the last one is also, you have to look at a circuit and think in terms of where the voltages drop and in a series circuit the numbers all have to add up) E = RI Or sometimes E is represented as V (well quite often, E is short for Electromotive force, it is measured in Volts). I is the current in Amps and R is the resistance in Ohms.

The equation can be turned around instead of V = R x I we can either go V / I = R, or V / R = I

Fun huh? So lets try some numbers. 100V = 10R x 10A. (10R is one way of saying 10 ohms without spelling out ohms. When we are in the thousands we just say 10k, the in the millions 10M, even getting fancy, 2.7k or 2k7)

Now if we have the voltage and the resistance and want to find out the current, 100V / 100k = 100 / 100,000 = 0.001A. When dealing with small current we normally use milliamps (or mA for short) and rather than 0.001A we have 1 mA through the 100k resistor.

Since that was easy enough let us blow our minds. Since our previous example had a 100k resistor and 100V across it, and our using Ohms law we found there was 1mA going through the resistor. And since the triode and the cathode resistor are in series that means there is only one place for that 1 mA to go, through both the triode and the cathode resistor.

So having some fun, we have 1 mA going through the cathode resistor, the cathode resistor is 1k (or 1,000 ohms). We have R (1k or 1,000R), we have I (1 mA, or 0.001A) (remember A is the current or I), heck we have R and I so we can find V (IR=V).

1,000R x 0.001A = 1V

Hey, that is the same as the voltage drop when we were figuring it out with resistance ratios across a voltage source. We must be doing something right. And this is how I troubleshoot circuits. Using the voltage divider rule (anybody remember the name of the rule?) and Ohm's law. I see what pieces of information I have and where I can drop them in the formula. While the numbers are a little different that with a 12AX7 the above is how I work out what is happening (still need more information but that is what the link is for)

The other thing is parallel resistances. If you have a 1R resistor in parallel with a 9R resistor and you have 1V across them you have 1/10 of the current going through the 9R resistor and 9/10 the current going through the 1R resistor. (an inverted relationship) Then you can put two resistors in parallel and have them in series with other resistors, which are in parallel with other resistors. You can have a whole mess of resistors and you can work it out with the ratios and Ohms law to come up with a combined resistance across any two points you chose. A little much to take in maybe but first year Electronics you have problems like this. Thankfully you do not have that many paths in an amp but seeing the components and identifying the series and parallel paths lets you calculate the voltages. Good to know in troubleshooting (did I say that before? Probably said it again as it is important.)

The only other concept to get is capacitance. Think of your water hose filling up a tank. The capacitor plates are just a tank to hold electrons (I mean water). Have a water source (15 psi) and a hose with a resistance (kink ir valve) and the higher the resistance the slower it takes to fill the tank. On the other side, having a tank with a hole in it and the bigger the hole the faster the water drains. And back to the link above, they go through some of that also. Well, related to guitar amps (and that is the only thing we care about, right?). Anyway, I hope this helps, I got to go.
 

Snfoilhat

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I go blank when I see a formula, even a basic one like Ohm's law. I just cannot make a connection between what I'm seeing and any real world application. Schematics are a little bit better for me but when I see a schematic I don't make a connection from what I'm looking at and what it represents. What I can do is look at a layout and make that connection.

Maybe I'm just venting or whining. I won't be surprised to see, if I get any responses to this, recommendations that I put on my big boy pants or such. The thing is, though, my inability to make sense of things isn't due to laziness or lack of motivation. Being able to understand these things is very near to the top of my wish list.

I work as a teacher, and I hope none of my students has ever thought a mean-spirited one. The difference in learning from concrete examples versus learning from abstractions and generalities (a rule, a formula) is a difference a lot of learners have. A lot of instructional material fails to support that style of learning.

As soon as you want to know how much current is running through a resistor (for example to bias a tube), or how much voltage is dropping across a resistor (for example to know why the first dropping resistor in a Champ is 10k Ohms and the second dropping resistor is a 22k), then you can use Ohm's law. It can be question/curiosity-driven, and it can be attached to one particular moment of curiosity, and never mind if you end up asking the same kind of question a bunch of times. Memorizing the general formulas or reading through an entire chapter of a tube amp textbook and forming an abstract understanding of the topic can be a really heavy lift for anybody.

Since you're self-motivated, there's no time limit, there's no need to learn efficiently, there's no quiz or people to impress. A question as simple as 'What would changing this coupling cap here do?' opens the door to some really useful stuff, and a person can pick how far to take it.
 

bebopbrain

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Did James Clerk Maxwell complain about teachers and textbooks? Did he have multimeters and oscilloscopes? Or YouTube? Or Fender amps? Or community colleges? No. He had a cat, an amber rod, and maybe a DIY electroscope. He didn't even have formulas initially, until he wrote them to torture the rest of us.
 

dsutton24

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The easy way to come to grips with formulas and such is to learn the underlying theory. Every time I say this I can hear the channel changing... You don't need a Master's degree from MIT to repair amps, but a half decent basic electricity course would be a great place to start. If you understand theory then formulas become a natural thing.

The substitute for knowledge is experience, and that experience usually comes at a much higher cost and time commitment than just starting from scratch and doing the work.
 

SRHmusic

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Have you tried getting a decent text or working through an open course?

In a decent course you'll start small and work up to more complex circuits, and work through lots of example problems to get familiar with how to apply the concepts.

Also you can do experiments yourself if you have a battery or power supply, some leaded components, and wire, perhaps a breadboard, and a digital meter. Later you can get a low cost USB oscilloscope, and probably a signal generator, as well.

For example:
 

2L man

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If you are going to work with Valves/Tubes while understanding what you do you must learn "loadline principle"!

Begin A-class and you understand how triode pre amp resistive stages are set. There Ohm Law is used to set Operating Point (bias point) to loadline selecting Operative Voltage, Anode Current. OP and cathode resistor.

Then advalce to power pentode which use a Single Ended Output Transformer.

When you increase SE bias current setting negative Control Grid-1 voltage which lift OP up and OT becomes "pre loaded with energy". Same time the end of loadline on right moves to the right and potential for Anode voltage sweep doubles what actual Anode voltage is. To me this is most interesting feature on OT!

Also how the Anode Current potential on left end rise when Loadline rise and there comes possibility to produce power to the OT Primary Impedance making OP move. Then study g1 curves which are needed to make OP move and this is called Drive Signal. Sometimes a Water Valve is used to explain how vacuum tube operates.

Then study OT "winding ratio" and you understand how high voltage/low current tube power sweep is turned to high current/low voltage loudspesker signal.

Then advance to Push Pull AB-operation and then distortion/clipping and you have very good understanding how tube amps operate.

When studying use this loadline calculator. After you select a tube it often pre loads default values and you can change only one parameter and immediately see the effect it has to loadline. At first use 12AX7 which is most common instrument amp tube.

Then select 6V6GT which is used in many guitar power amps. Its SE OP gets too much to the right so first change V1=anode voltage and Bias Current so that OP comes center of Loadline.

If you don't hate e-books you could buy Giuseppe Amato book of loadline designing. It was only $5 when I bought two years ago. Everything is possible to learn in internet pages but it has everything needed. Perhaps it is in paper too but I think will cost more?

 
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andrewRneumann

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Horowitz and Hill, in one of the most beloved and respected electronics textbooks of all time, say this:

We believe strongly that reliance on formulas and equations early in your electronic circuit education is a fine way to prevent you from understanding what's really going on. -The Art of Electronics, Horowitz and Hill
 

2L man

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This attachment is typical 12AX7 loadline. First I set voltage to 300V. Current to 1mA. G1 voltage comes -1,63V and this is typical when both triodes share 820 ohm cathode resistor when current comes 2mA (1,63V/0,002A=815 ohms) and 820 ohm is standard value.

I set headroom to 15V and green bar appears. This headroom value is Vp (peak voltage which here is the left side half of the output. Right side usually comes more of less different and then there comes distirtion to tube output. Anode voltage sweep both side of 200V, just like in actual amplifier. Now from left it go to 185V and on right it seems to go to about 214V so it is "squished" and cause distortion.

I chose headroom of 15V to get drive signal of 0,5Vpp which is realistic what guitar pickup can generate. This 12AX7 gridlines are 0,5V apart.

When output is about 30Vpp and input is 0,5Vpp dividing output with input make the stage gain=60

Using booster, overdrive etc pedal the first gain stage is almost impossible to drive so much that it produce much distortion but when its output is this high the next stage get overdriven so much that it clips the output. To test this takes few seconds just to change Headroom value.

Naturaly this is possible to do draving loadline to the copy of tube datasheet but to use loadline calculator is faster :)
 

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martinlb

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Go watch Uncle Doug on youtube to learn how these things work. He is into cars as well and I think that approach is a good analog. You have to know what the parts are and what they do. Thats step one. A lot of folks figure out how a starter engine works or how to replace headlights etc. guitar amps are similar. As far as formulas and whatnot go, just figure it out once and put it in excel. Then just put measurements in and youre set. Follow the safety rules.. discharge the caps before digging around in a tube amp. Good luck and chip away at it.
Thanks to you and generic202 for the Uncle Doug recommendations. I've watched some of his videos in the past and have enjoyed learning from them. I'll watch the ones he's done about tube amp functioning.
 

martinlb

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The two above posts are the best advice you can get. Stick with the simple amps champ straight up doesn’t have all that many parts which makes it easier to understand. I think the amps you have dealt with are more complex than a basic champ.
One of my problem builds is a Weber 5e3. It "works" in that I can play through it. I actually was able to resolve a powerful, loud, hum that made playing it impossible. Thinking I was being extra careful, I ran ground leads from each of the sleeve connections to ground on the input and speaker jacks. The layout showed they all needed to be grounded, right?. I knew they already were actually grounded via being installed on the metal chassis but the layout showed ground connections specifically being made to those points. I was being thorough! 🥴

I eventually theorized I'd set up a ground loop and disconnected all of those ground connections. That did it for that problem. No more hum.

What I'm working on now is rather than the two channels playing clean the sound is like that of overdriven tubes - it's distorted even at the lowest volume levels. It also breaks up with static-ish sounding noise when I hit the strings hard.

At this point I'm verifying all of the connections I made. I'm partway through that now, and if everything checks out, revealing no problems there, I'll desolder one side of each resistor and cap and test them.

I know that's barely at a level beyond painting by the numbers but that's what I'm capable of at this point and I've got the time to do it so that's what I plan to do.
 

martinlb

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the basic principles have never changed whether solid state or tube amp, all have a power supply , preamp stages, power stages and output , it how the signal gets to the end stages that differs. becoming familiar with those differences and being able to follow it down is the key .

tube amp High power, or high voltage caps can and will kill you if you lose focus , would watch a ton of you tube on the subject , gerald weber did some good basic vids , although not entirely acurate, but i learned alot from him , read some books first ,Ashton Pittman wrote a great book but again not everyone agrees with him, Uncle Doug as mentioned , and DR Z ( his amps are art to work on, his soldering routine is above and beyond) Rob Robinette or @robrob on this site is spectacular, @Wally is a great resource and a walking encyclopedia of amps and amp history , you also have some of the best resource members on this site. the devil is in the details and how much attention you pay to that.
be careful have fun, and be safe
Thanks. Fortunately one of the things I picked up early was a deep respect for the high voltage used with tube amps. I always test with my multimeter before doing anything else.

The help I'm getting here is phenomenal and I'm greatful for it. I'm optimistic about learning enough to have at least enough understanding of these amps to get somewhere with it.
 

martinlb

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Thanks printer2. I've looked at the document you referenced and will do that again (and probably again again 😁) I also really appreciate the time you took to explain as you did. I realize that took some time! I'm planning to read through a few times.

Thanks again, much appreciated!
 




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