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 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Thu 11, 2018 6:20 am 
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I have one other question. I was going to post this in another post, but I thought it might get moved to who knows where, and since those of you who have chimed in are probably well suited to answer this, I'll post it here.

In the power supply section, you have the RC networks, forming a voltage divider. But when I try to figure this out on my own using a voltage divider calculation, the voltages that I calculate aren't what are shown in the schematic. So how do you properly calculate the voltages after each resistor?

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 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Thu 11, 2018 1:28 pm 
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TPAairman wrote:
Soooo, how do you go about making the metal that shiny????

As to the original question, I appreciate the suggestions, but I think I'm going to stick to adding gain stages. However just to make sure I'm looking at this the right way, I take it I would want those stages to be higher gain? On one hand that makes sense, but on the other hand, it seems as though that might blow out the sound too much too quickly.

If it's what I think it is not many people will be very happy with the answer.

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 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Thu 11, 2018 4:21 pm 
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TPAairman wrote:
Soooo, how do you go about making the metal that shiny????

Chrome, baby!


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 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Thu 11, 2018 6:32 pm 
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TPAairman wrote:

As to the original question, I appreciate the suggestions, but I think I'm going to stick to adding gain stages. However just to make sure I'm looking at this the right way, I take it I would want those stages to be higher gain? On one hand that makes sense, but on the other hand, it seems as though that might blow out the sound too much too quickly.
Yes, that is what I found, and others here. I think you are right to start with a low-mu tube like the 12AY7 if you have one. And you should only need one more stage to achieve your goal, so that would be one half of a 9-pin duo-triode.

You are asking about power supply voltages. The schematic shows readings under a load, the amp powered up. Also, they were using a vacuum tube voltmeter, while we use DMM's mostly.
If there is a tube rectifier, there is a considerable voltage drop from the tube, that's another factor.
In other words, your calculation is probably right.

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 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Fri 12, 2018 2:36 am 
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westcoastjohn wrote:
The schematic shows readings under a load, the amp powered up.


That makes sense, but I still can't get it to calculate correctly. Even taking into account the plate and cathode resistors, I still can't get the calculations to come out to what the schematic shows. In fact looking at one Fender schematic I have, it shows 190 volts at the plate of the pre-amp tube, but when I do the math, it came out to about 4 volts.

I guess, let me turn the question around - if I were designing a scratch build amp, how would I calculate the values of the resistors I would need in the voltage divider?

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 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Fri 12, 2018 3:33 am 
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What voltage are you starting with from the wall? What do the schematics show? 115 117v?

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 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Fri 12, 2018 3:51 am 
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No, from the power transformer. Look at the one below. You start with 360 volts. You go through a 10K resistor and now it shows 320. Go through a 22K and it shows 280. At the plates of the 12AX7 it shows 190. I'm not even close on that one.

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 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Fri 12, 2018 4:08 am 
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I've always been told that with a 20% tolerance nothing has to be quite exact.

I know it doesn't answer your question but see if this page helps any?
https://robrobinette.com/How_To_Build_a_Tube_Amp.htm

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 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Fri 12, 2018 3:58 pm 
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You sound like a person who has knowledge, is asking the right questions and is willing to learn...but don't get hung up on theory.. I'd say for now just start making the amp with what you have AFA circuits. Theory will get you into the ballpark but actual voltages in a working circuit can be tweaked easy enough to get what you want. Two tubes, the same number, can draw different current values due to many factors...You will learn a lot more by just doing the build and measuring and tweaking as you go. Thru years of doing this I've found that you'll rarely hear the difference in a guitar amp with a 12AX7 that has 150VDC on the plate or 110VDC on the plate. You ear will tell you want you want.


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 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Fri 12, 2018 11:10 pm 
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I get that. The thing is the theory wasn't adding up, and for me, if I don't understand the theory, I'm usually lost as to how much to tweak. Or as I go about making my modifications.

But about the power supply, I threw this one out on an amp building forum that I recently came across, and I see now where I'm going wrong. I was looking at the resistors on the B+ rail combined the tubes and their resistors, as a voltage divider network, which is not the case. But that's why I was coming up with voltage values that were light years away from reality.

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 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Sat 13, 2018 5:27 am 
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Looks like I'm actually not 10% there on this one.

As I understand this, I take the current and how much voltage drop I want, then using Ohm's law, I know what value of resistor. However, what I'm stuck on is how do I know what current value to use? I know on this one, we can look at what's there and figure it out, but if I were designing an amp from scratch, what would I use to calculate the value of the dropping resistors in the B+ rail?

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 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Sat 13, 2018 5:52 am 
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More info here..but you're going deeper than probably 75% of the members here (including myself) can go with theory, design and application of power supplies. Leo just took the RCA tube manual and tweaked it for his start into amp designs..(OK..I maybe took a little poetic license with that... :lol: )
http://diyaudioprojects.com/Technical/Tube-Power-Supplies/


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 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Sat 13, 2018 8:06 am 
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Let me put this a bit differently. I posted this same question at an amp forum that I recently found and long story short, I'm told that I need to use the current at the cathode, since all of the current in the tube has to flow through the cathode. And I know how much voltage I want the given resistor to drop. So what I'm being told over there, repeatedly is just use Ohm's law. Which would be my desired voltage drop divided by the current at the cathode.

What I can't seem to get anyone to understand is I have no idea how much current I should be looking for at the cathode, so therefore I can't use Ohm's law.

One guy keeps saying that for a 12AX7, he just figures 1 mA and it works. Which is great, but what value do I use if I have another tube picked out. I also get responses where people look at the schematic and can tell me the current based on the voltage drop (which I would already know) and what the resistor value is in the schematic.

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 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Sun 14, 2018 11:56 pm 
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After some more research, I got my answer, so I figured I post it here for others that might want to know. It's needed info if you are adding stages to an existing design, as I'm doing here.

We know the supply voltage from the dropping resistor, and we know how much voltage the plate resistor is dropping (supply voltage minus the plate voltage), so we can figure the current on the plate resistor. That's the current for that stage.

So on the dropping resistors, we just have to add up the current of all of the downline stages, since all of them will have current passing through whatever dropping resistor we are looking at, and we know how much voltage we want it to drop, so again, ohms law.

This means that in my case, if I take an existing circuit, and I want to add another stage, but I want to keep the voltages the same on the other stages, I will then have to adjust all of the upline dropping resistors (and maybe add one for this stage) to maintain the same voltage at each node, but accounting for the new total current draw.

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 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Mon 15, 2018 4:26 am 
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OK, my brain is rusty in that department, but that makes sense.
Luckily, plate voltage can be lower on the other tubes and the amp will still amplify, and with overdrive.
Lower plate voltage on the power tube will cause it to go into distortion at a lower volume level, so that is a design feature some guitar players might look for, what they might call 'crunch'. :lol:
Maximum plate voltage is only needed if you need to get maximum performance, the highest possible output, from the tube. The output then stays clean at a higher volume, what they call 'increased headroom'. I think that may be what you are trying to accomplish, so in that case, yes, you will want to adjust for the increased voltage drop from the added tube stage.


When you have more complex circuits with loads in parallel as well as series, you may need to use Kirkhoff's laws of current and voltage.

You may have this already, but for anyone lurking, here's a simple tutorial:

http://www.electronics-tutorials.ws/dcc ... t-law.html

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 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Mon 15, 2018 8:27 am 
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westcoastjohn wrote:
I think that may be what you are trying to accomplish, so in that case, yes, you will want to adjust for the increased voltage drop from the added tube stage.


Yes, that's exactly it. And in digging around on this one, I've found out that the supply voltage at each node of the dropping resistors is not just a somewhat random voltage. It turns out that figure is deliberately picked based on the plate voltage and plate resistors values. I'm still a bit hazy on how they pick those two values - but it's not quite a dart throw either, though it's kind of close. But if I'm understanding this right, we want a given plate current, which is set by the plate resistor. And in order to have that, the resistors has to drop a certain voltage. That voltage drop plus the plate voltage is then the supply voltage we want to start with.

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 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Mon 15, 2018 4:22 pm 
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TPAairman wrote:
As to the original question, I appreciate the suggestions, but I think I'm going to stick to adding gain stages. However just to make sure I'm looking at this the right way, I take it I would want those stages to be higher gain? On one hand that makes sense, but on the other hand, it seems as though that might blow out the sound too much too quickly.
I don't think you would want to add a gain stage. What you need is a "distortion stage". A voltage starved 6AU6 limiter circuit similar to those used in FM tuners comes to mind. You will want to include an input level control to tailor the sound to your liking. I think the best place to insert this would be at the output of one of the 12AY7 input preamp/mixers. Basically what will happen is that the 6AU6 gets overdriven by a variable amount and the output is sent to the rest of the amp through its volume and tone controls. A bypass switch would also come in handy.

Maybe this idea has already been suggested.

Dave


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 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Fri 19, 2018 1:38 am 
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I haven't read every word of this thread, just skimmed, but wanted to add this.

I've eyed this circuit up for YEARS, still haven't built it. I keep thinking I'll do it as a standalone box/preamp with its own power supply.

This is from the famous/legendary Trainwreck pages that Ken Fischer wrote many years ago, Steve Melkisethian of Angela Instruments printed them in his catalog. Anyway, Rob Robinette (mentioned earlier in this thread I think) converted it to html here. Plenty of good stuff here about servicing and tips. Ken was an absolute guru and of course Trainwrecks are legendary.

Scroll down to page 33 of this page and you'll see the Universal Gain Stage Ken designed. Sure looks good to me. There are lots of ways to vary its gain and tone and apply it to an amp circuit.

https://robrobinette.com/The_Trainwreck_Pages.htm

I think you're best off building something simple first, like a tweed Champ 5F1, then going from there. A cranked Champ has a fantastic overdrive tone and may be all you need.

-Kevin

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 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Fri 19, 2018 6:25 pm 
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I recently built a little Champ Amp with a built-in Variac so the B+ can be set anywhere between zero and 340 Volts. This was a real eye opening experiment.

Voltage has almost no affect on gain. When the input and output signals are overlapped an a dual trace scope, they don't start to separate until the voltage gets WAY down (under 100 volts). With the guitar volume set low, there is NO volume change in the amp when the voltage is turned up and down.

What the voltage does affect is the "headroom", how loud it gets before it starts to distort. With the voltage all the way up, bridge and neck pickups in series, guitar volume maxed, the amp is as loud as it can possibly get and its breaking up just a bit for some cool overdrive tones. Set the voltage at 75% and there is almost no perceptible difference. Remember that the Variac is linear, our ears aren't. That is why volume controls use log logarithmic taper potentiometer. Turn the voltage down to 30% and now it's getting both quieter and more distorted. Turn down the guitar volume, it gets less distorted but not much quieter. When you reach the point that it sounds "clean" turning down the guitar volume will make it quieter.

The takeaway is that there is no need to nitpick voltage in a guitar amp. It'll work on just about any voltage you feed it, and it takes BIG changes to make a noticeable difference. Higher voltage is only useful if you want it clean AND loud.

Each stage of the amp [typically] has it's own decoupling capacitor to isolate it from the other stages. The resistors between the stages work with the capacitors to make an effective low-pass filter. It's better to calculate the value of these resistors based on the bandpass of the filter (and just accept whatever voltage drop results) than to pick them based off of voltage drop (which could result in obnoxious "HUMM").


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 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Fri 19, 2018 8:36 pm 
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No, you size the resistors to achieve the operating voltages the stage needs to operate according to the objectives you have, then you choose a capacitor value to eliminate hum.

-David


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