Fantastic! What do you recommend for drilling the holes? I'll be working with a hand drill (no ETA yet on when I'll be able to procure a drill press), and the smallest bit I have is 1/16". I've read that smaller than that might tend to break if not used in a press. I've also seen suggestions of using small dremel bit. Given your experience, should I worry about finding something smaller, or is 1/16" (which, if I've done the math correctly, works out to 62.5 mils) plenty big?

Still slow going on the design process, but here is my latest update to my power amp board. I increased most the track sizes and I figured out how to increase pad sizes. I also moved the 100nF cap closer to the IC.


In other news, we just moved into a house from our old apartment. It had some pretty fun electrical properties when we moved in. For instance, one of the light switch cover plates was loose, so I pushed on the screw thinking, "I should tighten this," when my finger started buzzing. Pulled out my trusty voltmeter and sure enough, there was 120V hooked up directly to the screw. The guy who swapped everything out said that the live and neutral wires were backwards on an outlet in the baby's room. Fun times.


Anyway, eventually I will have my workbench cleared off and the fun begins.

Thank you for your reply. I am brand new to PCB design, so I really appreciate your help. I understand what you're saying about "T-ing" capacitors. Now that I notice it, it intuitively makes sense; I just didn't know to look for it. (And to be sure, I think what you're talking about is how I connected the positive node on C3 to VCC. I should have just gone straight through it.)

As far as trace widths, I did thicken them up from how they originally routed. Not on any scientific basis, more just because they looked like they should be thicker. Looking at it now, I see that I could easily double--or maybe even tripple--the width of all the traces, with a little bit of component movement, with the exception of that spot where the VCC trace passes under the IC, which much be smaller to pass between the pads. How big of a problem will it be to make it thinner at this spot? In other words, would it make sense/be worth the effort to add in a jumper wire or something of that nature?

Also, a tutorial I followed said I should fill in the blank space with a pad connected to GND to help avoid interference (of some sort--RF maybe?). I believe the ground pin on the IC is connected to the heat sink, so it makes sense it would help with that too. Is there any sort of theory behind this that I should be thinking about when adding ground planes (or power planes)?

Don't worry that I will rush into production. I knew this was my first draft, so I'm mostly just pleased that the first comments made about it seem so minor (as opposed to, "Why on earth would anyone do that?").

Design time has been in short supply lately, but I have a couple hours to kill before picking up a rental truck (moving today--hooray!), so I've been playing around with Eagle. I wasn't planning on getting into PCB construction, but now that I am, it seems like it's going to be pretty fun. I wanted to separate the tone section so that I can switch it out easily if I want to, but here is my design for the power amp section. The "INPUT" connection on the left will come from the output of the tone stack. VCC and GND will connect to the power supply board. Q1 is based on the MPF102 layout. From top to bottom the pins are: Drain - Source - Gate. I used the part numbers from the TDA2050 datasheet to make it easier to route everything. I'll update them when I finalize my schematic.


I'll have a nice-sized garage in our new place, which means plenty of tinkering room. This amp might just get built yet!

Quote from: Bakeacake08However, I'm still not sure what makes you think I'd get away with using the MPF102.

I hope this didn't come across as too confrontational and/or abrasive. My son was waking up from a nap, so I didn't take the time to proof read. What I was trying to say was really that I don't really understand how JFET parameters work, or how they break down. I think I get it with BJTs, but I've never used FETs before, so they're still new to me.

Quote from: Rolybut a CRO is normally a better method.

Yeah, I definitely have one of those on my wish list.  I will play around with my DMM until I can find one.


I've attached an updated schematic. I referenced one side of the heater circuit to ground, which I think is what you were explaining I needed to do. I can also get a 6.3V transformer instead and ground the center, if that's better (I remember reading about that somewhere on the internet awhile back). I moved the connection to the power supply to the power amp end and put some filter caps in front of the pre-amp stage. What is the benefit of putting the power supply here? My gut says it has something to do with preventing interference, but I'm not sure. Also, would there be any drawbacks to wiring each section in parallel? That is to say, what if I ran a line from each section straight to the power supply output?


I really appreciate your help with this design. It might be awhile before I actually get to build it as we're looking for a bigger place right now (happily gave up my tinker area for the new baby), but hopefully it gets done sooner than later and I can get on to the next project. 

Quote from: RolyMaybe, and no offense to Shockley but a bias point Vgs of around -3V and Id of 1mA is a bit more realistic with an actual device.

But . . . but the textbook says . . .

I guess I'll go with your actual, real-life experience on this one.   However, I'm still not sure what makes you think I'd get away with using the MPF102. I understand how you worked backwards to get to the 1V signal, but I'm under the impression that exceeding the max drain-gate voltage would do something like blow it up (or whatever happens to JFETs when they fail).


And speaking of headroom, working backwards from the power amp section, the max input is 1V, meaning the max input of the JFET stage is about 1V as its gain is roughly 1. So the max output from the tubes should be around 1V as well, correct? I know that there is some loss in the tone stage (20dB or so, I believe), but how does that relate to the voltage signal? Also, what is the best way to test the output voltage of my pickups? It seems I should know this in order to figure out the output of each stage. My instinct would be to hook up my DMM for AC voltage to the guitar and play a loud chord and hopefully get a reading. Maybe there's a better way?

Quote from: RolyYou will need to select C16 to have a ripple current rating higher than your maximum output current.

That makes sense. How do I calculate the maximum output current? The maximum that my transformer can put out?

Quote from: RolyYou are getting to the point where you need to throw your ideas up against reality and see what sticks.

Thumbs up indeed! Now I just need to explain to my wife why we need all these electronic parts instead of, for instance, fruit. You can live off pop tarts and hot pockets, right?

Okay, I've done some learning about JFET biasing, and here's where I'm at so far:Shockley's equation gives the drain current for a given value of Vgs: Id = Idss * [1 - (Vgs/Vp)]2The datasheet for the mpf102 gives a Vp value of -8V and an Idss of 2mA to 20mA. I assumed 10mA for my calculations.Putting that all into the math machines, a Vgs of -2V should yield a quiescent current of about 5.5mA. As Vgs = Vg - Vs, Vs should be around 19.5V, as we know Vg is set at 17.5V.19.5V / 5.5mA gives us a source resistor value of 3,545ohms, or 3k3, which (I just found out) happens to be what I had when I first added the JFET. Am I on the right track so far? I hope so, cause I'm going to keep going. You said that I could lower the current with a 15k resistor. Thinking it through, changing Rs to 15k (assuming a 19.5V drop across it) would lower the current to about 1.3mA--but this would also affect Vgs to -5V, meaning that Rs would actually be dropping 22.5V (17.5 - [-5]). This appears to be within the specs of the FET, which I think is why you said it shouldn't affect the performance too much. Do I appear to be understanding the calculations correctly? I forgot how much fun math can be to learn.


Also one other note about the mpf102: the datasheet says that the drai, drain-gate voltage max rating is 25V. I can't seem to find a drain-source voltage rating, but I know that I will be running at around 35 VRMS, which will peak at about 50V. Will this FET work in my circuit? I assume I should use the bigger number when figuring maximums, but even the RMS value seems too high.


Thanks again for all your help. Learning is fun!

Oh right. I just realized I did 1M instead of 10M when I did the calculation. Whoops.

Sorry for almost cheating the whales.

That would be great if you could post those files, and maybe some instructions on how to use them. (I tried setting up a file for the 12ax7, but I couldn't figure out what I was supposed to do with the code I had.)


According to my calculations, a .1uF with a10Meg resistor will pass everything over about 1.6Hz. So I'd say you were right about that one. 


I've done a lot of research and found some good videos on JFETs and all the calculations, but I'm still puzzled where you got the 16V figure for Vs. In other words, why does one assume that Vs is a couple volts below Vg? I understand everything about what you said except for how you arrived at the Vs figure. My head is swimming with math right now (Shockley's equation, remembering how to algebra, etc.), so maybe it's simple and I'm just thinking too much. We'll see, I guess.


The power amp is set up directly from the datasheet, so I wasn't too worried about any of those values being off, but I calculated them just for fun and confirmed that they are acceptable.


As far as the safety ground goes, my understanding is that the best place to connect it is from the chassis to the ground of the reservoir capacitor (and all that to earth ground, or course). I was aware of this, and I actually drew it on to one of my hand-written schematics I did at work on a previous draft. Do I need to add a safety ground to the heater supply as well? If so, how would this affect the signal circuit and what sort of things should I be aware of for that?

I've finally found some time to tweak my schematic, so here is the new version with some revisions:- Changed reservoir cap to 2000uF- Removed regulator circuit- Replaced it with its own transformer- Removed R4 (V1b grid resistor)- Changed gain and volume pots to 500k- Added JFET buffer between tone stage and power ampI don't quite get JFETs yet. I understand the basic principle of them, but for some reason understanding all the details is giving me trouble. I found this buffer schematic online, and it seems to be basically the same as a BJT emitter-follower circuit. What should I be looking for as far as actually calculating correct values? Is there maybe a good tutorial somewhere I haven't found yet? I put 10Meg for the bias resistors so the input impedance will be 5Meg. I don't know why I put 3k3 for the source resistor other than that's what I saw on the schematic.


How does the input impedance for a tone stack work? In this case, I guess that it's the sum of the resistors on the bass side, as those are the first ones the signal sees--though I'm positive it's more complicated than that. I also guess that it will be quite low compared to what I need (as you mentioned it will need at least 360k), so it might be reasonable to say that a buffer before the tone stage would do some good as well.


And speaking of these buffers, is a simple version like this good enough, or would it be worth the trouble of adding complexity via a complimentary feedback pair and/or using a constant current source, or something of that nature?


That is certainly not the end of my questions, but it is for right now. Thanks again for all your help, and I hope you had a fantastic holiday season!


Matt

Well first of all, I have to say that I've been studying electrics for a while now, and I am thoroughly impressed with how helpful everyone is. I've read tons of message board replies that say something to the effect of, "You shouldn't do it that way because of this, but if you want to, here's how you would do it." It's very fun to learn with so many willing teachers out there.

@Roly:

I realize that doing my own design is a huge mouthful, but I'm really enjoying the challenge, and right now I'm more interested in learning the theory of how all the different pieces come together. I still might actually build a proven project for my first one, but right now I have a newborn and lack a shop, so it's easier to get the learnin' in.

Anyway, I've done some more homework, and here's what I've come up with. First, I'm not sure where I got 68uF from. I think I did the calculations before I figured out everything that was going to be in my circuit, and missed some decimal places maybe? Anyway, I t = C V, so C_reservoir = I t / V.

(.4A) x (1/120) / 1.75V [5% of 35] ~1900uF, so I'd round to 2000uF. I don't know if I should have used peak or RMS voltage, but this one comes out higher, so I'd go with that.


For the bypass capacitors, I found an old magazine article (http://www.rfcafe.com/references/popular-electronics/bypass-capacitor-jan-1962-popular-electronics.htm) that said Xc should be about 1/10th of the cathode resistor, so in my case Xc = 56. Since the lowest guitar frequency is 80Hz (or maybe 70Hz if I play in drop D, which I do sometimes), I'll pass everything over f=60Hz.

C_bypass = 1 / (2 x pi x 56 x 60) ~ 47uF.


I still might be confused about the mains fuse value, but I might have it: My transformer will drop the voltage by a factor of .3 (36V / 120V = .3, so V_secondary = V_primary x .3), which means it will raise the current by the same ratio (I_primary / .3 = I_secondary. So to figure the primary current draw, I_primary = I_secondary x .3 (I did some equation rearranging on paper that I didn't type out).

So if the secondary current is 400mA, multiplying by .3 will give me a primary current draw of 120mA, and I should choose a fuse based on that number.


I didn't realize the 12au7 was not in production anymore. I found an article comparing different tubes at low voltages (>12 V) where he did a bunch of measurements and determined that it had the best performance. I'm not married to it though, so I will change it to the 12ax7.


I put the 80ohm resistor in there because I figured that would limit the current to the required 150mA. But on reflecting on other schematics I've seen in which they connect directly to the 12.6V (or 6.3V), I suppose that the filament itself might limit the current, so the resistor wouldn't be needed.

I did not calculate the Johnson noise of the circuit. I read about it in one of the many articles/books I have read, but I haven't studied it very deeply at all; I mostly just knew it existed and that I might have to take it into account. But I do appreciate your point on the impedance mismatches. I had not considered that (obviously), but now it jumps out at me while I'm perusing various schematics, so thanks for putting it in my head.

The second grid-bias resistor can be removed because the grid is already referenced to ground via the gain pot.

Speaking of the gain pot, I'm not quite clear about how it loads the anode. I get how the two resistors are in parallel and how that makes 20k. I don't know what that means as far as how the anode is being loaded. Does this "count against" the output impedance of the first triode, or the input impedance of the second?

@phatt

I haven't had a chance to look at your schematics yet (my computer has been weird about not loading webpages lately), but I'm really interested in your 555 design. That's the chip that kinda made everything click for me when I was first learning about transistors. And I appreciate your encouragement about working with high voltages. I actually feel a lot better about it since I started with this project, but I'm really into it now, so I figure I might as well finish it. I'll see if I can get it recorded when I'm done and maybe it'll surprise you and sound sorta good--and by that I mean maybe it will make a sound. 

I started out wanting to build an all-tube amp from the ax84.com, but I was still learning about electricity and circuitry, so the high voltages made me a little nervous, which led me to research solid-state amp design. Then I found out that you could make a pedal using tubes, so I thought it might be cool to have a solid state clean channel and a tube crunch channel. That seemed rather complex for my first design attempt (at this point I had decided that I wanted to try to design something myself), so I settled on building a hybrid amp, which leads me to this post. I have come up with the attached schematic and I was hoping to inquire of people with actual knowledge and experience in the hopes of not killing myself of those around me with my first amp project.


I have a few specific questions, but I would also love to hear any overall comments anyone might have as well.


1) C17 is used as a reservoir cap. I estimated there will be a quiescent current somewhere around 3-400mA. Is this enough capacitance? Too much?


2) For the heater circuit, I saw somewhere online about using a zener diode to drop the voltage inside the operating range of a 12V voltage regulator. Then I added an 80R resistor to supply the required 150mA. Is this the same current that will be going through the zener as well? Also, does this idea even make any sense? I was hoping to be able to use the same (relatively cheap) transformer instead of having to get one just for the heater supply. (*crosses his fingers*)


3) I'm not super familiar with how to choose/implement fuses, so any comments on my current scheme would be great.


4) C2 and C4 are cathode bypass caps. Does 10uF seem like a reasonable value? I know there is such a thing as partially bypassed and fully bypassed, but I don't know how different capacitor values will affect the sound.


5) The tone section is a passive James/Baxandall type circuit. I know that there will be some insertion loss; will scaling down the resistor values by, say, a factor of 10 (and scaling up the caps accordingly) affect this loss at all? From what I understand, this could at least cut down on the Johnson noise, but I'm not very familiar with the concept of insertion loss.


General comments on the design: I chose the TDA2050 because I looked up the schematic for the Marshall 15DFX I used to have and that's what it used, so I figured it would work. That chip has a max voltage rating of 50 volts, so I picked at 36V RMS transformer, which will put out 35 volts after losing 1V in the bridge rectifier. All resistors will be .5W unless otherwise noted. I've read thousands of pages of electrical theory in the past few months, but this is the first non-9V-battery project I'm attempting, so I hope I'm at least on the right track with this.


I greatly appreciate any feedback I you have for me.


(You know, to reduce the amount of distorted ideas I might be throwing out. Waka waka.)


Matt