Build Your Own Clone Message Board

So, I was hoping you guys could explain this to me. I'm reading it but it just isn't sticking. I've read it on two sites but I'm not totally getting what is going on. I was hoping maybe hearing it from some of you it might stick or a might go off heaven forbid. A dumbed down version is more than welcome too

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Never heard of it. Links?

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-Mike

You got it Mike

http://www.valvewizard.co.uk/Common_Gain_Stage.pdf

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That's a good read, TN! I just skimmed over it, and couldn't find the "phenomenon" you were talking about. Where's it at?

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EDIT: Page 24 of the PDF.

I don't quite understand it fully, and it can be applied to gain stages consisting of BJTs and FETs as well, but what I take away from it is this; setting up a triode/BJT/FET gain stage with only three resistors will cause some feedback within the device that limits the amount of circuit gain and bandwidth the circuit will provide. Adding a cathode/emitter/source capacitor (i.e. the 25µf/25volt cap on Fender amp triode gain stages, known as an AC Bypass Capacitor) increases gain and bandwidth. You can tailor the frequency response of the gain with the value of the cap.

For examples, check out the fuzz pot on the fuzz face schematic and the boost pot on the stratoblaster schematic. These designs both use a pot as a variable resistor to increase gain by bringing the bypass cap into the circuit as the resistance of the pot goes down.

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MasterDelayer/Reverbrador/Ampaholic/TopJacker

Ah, sorry. I didn't realize it sent you to page one. It's actually pg 24 towards the middle.

Now I'll chew on what Morgan said.

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A quick conceptual explaination till I get to a computer. Tap dancing on a phone sucks.

Basically when you feed the stage a signal, the wiggle of the cathode voltage makes the gain move around. If you start to lock that voltage down (add a bypass cap) it'll stabalize and increase the gain above some frequency depending on the value of the cap.

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-Mike

+1. It helps to visualize the cap as a short for AC and an open for DC.

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So is cathode-current feedback good, bad or neither? I ask because on pg 25 it says "....without it (bypass cap), cathode-current feedback tends to slow down the onset of grid current, giving a smoother and more compressed sound."

The word smoother makes me think someone might want to go after that characteristic, no? By adding the cap you give it a more aggressive clipping it goes on to say.

So in essence cathode-current feedback is not bad or good? Or am I all wet in my thinking here?

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I think the point is what you're trying to achieve, and how it's related to your biasing scheme. By biasing closer to cutoff or saturation, you're allowing for more distortion while sacrificing potential (clean) gain. By biasing in the middle, you're allowing for more clean gain before clipping. Adding the bypass cap is another way to allow for more gain - which will still clip (distort) when it hits the rails. But let's say you weren't going for distortion, and wanted maximum clean headroom. Then you might bias your Q at the center of the load line, and use a bypass cap. Tweaking the value of the bypass cap will allow you to adjust the low-frequency response as well. If you were going for max distortion but wanted that "creamy" sound described, you might bias closer to cutoff or saturation and omit the bypass cap since you won't need as much gain to reach clipping.

See why they call it the "art" of electronics?

Too bad the only art tools I've got is a can of spray paint and a hammer

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Ok, I get that. Makes sense.

Now onward in the article. I think I'll just skip right over that square root math from hell problem he has in there next

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Damn John,

Just put the little blue thingy in the holes
Then put the little trash can thingy in the holes
Then put the little ice cream sandwhiches in the holes

Plug in
Play...

Your making this waaaay to hard

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So build amps the way I do my pedals

Genius !!

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quoted for GENIUS

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First of all cathode current is a good thing. Without cathode current we couldn't get our tubes working! Cathode, plate and grid current all work together to give us amplification.

Second cathode bias is a form of NFB (negative feed back). The cathode current changes in accordance with grid current.

This is important in cathode biased output stages for example (this example applies to preamp tubes though):
You bias your tubes at idle and all is good, lets say you bias to 70% plate dissipation. Now as you start playing and there is no some grid current entering the tube. This causes a decrease in cathode/plate voltage because the cathode current increases. If this did not happen the tubes would overheat and be dissipating too much power (read red plate!).

Common tricks with cathode biased stages is to add caps, Zener diodes, etc to bias/add to cathode resistor. Now a cap charges up and stores a charge, this helps keep the cathode at a constant voltage (as well as increase AC gain). So now when you have a lot of grid current and the cathode current increases and the cathode voltage decreases. The cap sees this and discharges in order to maintain a con stand voltage. The larger the cap the more constant the voltage will be held. This is an issue in output stages when you use too big of a cap (1000uF or so) because the tube's can not can their cathode voltage and as a result the tube's dissipate too much power.

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http://www.pdfelectronics.com/

+1 Once you have a functional circuit, the rest is just seasoning to taste. Understanding what seasons you like and how much to add is the technical and the art part rolled into one. This phenomenon is the technical, how much you want to use it to tailor the sound is art. I can't see how it would ever be bad (damage the circuit), more like desirable/undesirable depending on your goals. There are other ways of dealing with grid current, and the blocking distortion it causes, so while this is involved with it, there are other tools to fix it: grid stopper resistors. http://www.aikenamps.com/InputRes.htm

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-Mike

Thanks guys. I actually understand that. Shocking...for me.

You might see an impendance question next...maybe. Need more coffee.

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Impedance gets a bit complicated. how's your math? It's just algebra & trigonometry, but there are imaginary numbers involved. Basically, impedance has two components - a real component (resistance) and an imaginary component (reactance).

If you only express impedance as a resistance (like we do most of the time!) it's sort of useless information. To illustrate why, let's say I came to your house and said that I had buried a million dollars 100 yards away from your door. Where would you start digging? To find it, you'd have to excavate a ring 200 yards across. But if I told you I had buried the treasure 100 yards DUE EAST of your door, you'd find it and be rich!

Impedance works the same way. You have to imagine it on a graph. The X (horizontal) axis represents resistance. So a 10k ohm resistor would be represented by a point on the X-axis at 10k distance from the center zero point. Reactance (capacitance or inductance) is plotted from the Y-axis. Capacitance is poitive in value, inductance is negative. So if you took a circuit and plotted it, you'd have a phasor (a line) connecting the center point of the graph and the point that represents the meeting of the reactance and resistance values. The angle of that line tells you how much phase shift your incoming signal will experience as it passes thru the circuit. The magnitude (length) of the line tells you the complex impedance in Ohms. It's really quite cool! And doing it this way avoids calculus. Which is a very good thing

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Tell me about it. My daughter just got a degree last Spring from Boston University to teach secondary school mathematics. She's now doing a one year Masters program at SUNY Albany (Hey, DC!! Your alma mater!!), because a Masters degree is pretty much required to land a full-time teaching gig around here. So right now, she's taking a course in some hellaciously obtuse branch of math called "mathematical analysis". From Wikipedia:

"Mathematical analysis is a branch of pure mathematics that includes the theories of differentiation, integration and measure, limits, infinite series, and analytic functions. These theories are often studied in the context of real numbers, complex numbers, and real and complex functions. Analysis may be conventionally distinguished from geometry. However, theories of analysis can be applied to any space of mathematical objects that has a definition of nearness (a topological space) or, more specifically, distance (a metric space)."

Right! I understood that....

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“My favorite programming language is SOLDER” - Bob Pease (RIP)

My Website * My Musical Gear * My DIY Pedals: Pg.1 - Pg.2

Make it Stop!

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Sounds fun

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http://www.pdfelectronics.com/

I will always associate that phrase with the brain-damaged Marines crawling around in Quake 2....

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“My favorite programming language is SOLDER” - Bob Pease (RIP)

My Website * My Musical Gear * My DIY Pedals: Pg.1 - Pg.2