Circuit analysis questions

[caprotesta]

Hello all,

I've ventured into the realm of circuit analysis. I know and can apply Ohm's Law, but I'm having problems integrating what I know into real world situations. For example, I'm trying to figure out how to calculate voltage drops in a simple amplifier schematic:

http://thmq.mysteria.cz/diystompboxes.com/OTT/TT15.jpg

On the schematic, there are four hand-drawn voltages. What calculations must be done to arrive at these figures? My calculations don't jibe with those on the schematic.

I apologize if this is a stupid or overly broad question. I can do simple circuit analysis, but am trying to go something a little more complicated.

Thanks for any/all help!

[R.G.]

Resistors are always - always! - current to voltage converters or voltage to current converters.

What must be done to arrive at these numbers is to know the voltage into that resistor string and the currents out of the nodes, then calculating one of the permutations of V= I*R. If it doesn't work out that way, you either don't know the voltages or currents well enough.

This last is nearly always the case with me. 

[PRR]

> a stupid ..question.

Nobody is born knowing this stuff.

> My calculations don't jibe....

It might be good to show your calculations. There's like 99 wrong-ways. Nobody has time to type-up explanations for the 98 ways you didn't go wrong.

> there are four hand-drawn voltages

These are marked A B C D.

The power transformer is not specified. Therefore we must assume the "325V" is correct.

The 220K resistor draws a current and dissipates a power which I hope you can calculate.

"Being stupid", points B C and D have no DC path to power supply return. Therefore current is zero. Therefor all are at 325V. Done.

Ah, but does point D connects to a point D top-left which goes to some resistors and circles? Then there IS a DC path from power node D to supply return. What is the circle? It could be an open or a short. Since we don't need to buy circles to get opens or shorts, it is probably something in between. But finding the open and short currents gives us some limits on what node D load current *might* be.

Open: infinite resistance
Short: 101.5K resistance

There's two of these resistor-circle things on node D. And overall uncertainty is large. Pencil "50K?" as the maximum load. At the other limit, infinity/zero is probably absurd. In ignorance, it may be wise to pencil "100K?" as a likely load.

Ignoring node C for a moment: we have 325V through 150r+10K+15K, feeding node D loaded with perhaps 100K, perhaps 50K, perhaps much more. Throw-out the 150r for simplicity (it's far smaller than the 10K 15K). 10K+15K feeding 100K, 100K/(100K+25K), node D might be 0.8 times 325V, or 260V.

Hey! 260V with gross approximations is mighty close to the "265V" penciled. Yay!

But we ignored node C loading, and we'd expect node D to sag lower, not higher.

But in the real world, we mostly only care to know to buy 200V 300V or 400V parts. Not the exact voltage, but some upper limit on how strong our parts must be.

I'm being "ignorant" about those circles. I know what they are supposed to be. Even though the drawing does not say "guitar amp", the combination of a 12AX7 vacuum tube, 100K and 1.5K resistors, is seen in most/all older Fender amps and many-many other amps. SO often, that I've bothered to know a little about this set-up. Over any B+ voltage you would want to use, the plate will sit at 70% of the B+ voltage. (This can be derived from 1.5K, 100K, and 12AX7 parameters; but easier to just study EVERY circuit you can get eyes on.)

If plate is 70% then the 100K drops 30%. Or the other way: the tube (and 1.5K) acts-like 2.33 times 100K. So the tube and resistors act like 333K.

Node D has two 333K loads, 167K. Fed from 10K+15K, 167K/(167K+25K) or 0.87 times 325V, gives 285V at node D. That's high, but we have not accounted for node C load.

If node C were loaded with 12AX7and 100K+1.5K.... but it isn't. The cathode bias resistor is 1.2K shared under two sections, so like 2.4K each section. Then there's another 47K under that, like another 94K each section. This is different enough that I would not be sure the circle is a 12AX7. However we can still say the _max_ load (circle shorted) to node C is 98.2K, and more-likely twice of that, 200K.

Now node C has this 200K? load and the 15K to node D at 167K?. 15K+167K= 182K... oh, heck, round it to 200K. Node C is fed 325V through 10K and has two 200K (=100K) loads. Node C is 325V*(100K/(10K+100K)) or 295V.

5V high, but we still have not considered the node B load. In ignorance, we might say the 470r resistors are a clue. But that suggests numbers like 1 Ampere, which is absurd. These circles are different.

Being fairly experienced, I'd guess these circles are different. In fact, EL84? If so, then the "5V" drop shown for the B-C 150r resistor does not add-up. The drop is more like 2V. 2V versus 5V is different, but 323V versus 320V is same-enuff for any practical purpose. Don't take circuit notations too literally.

[Johan]

when you know "Ohm'slaw", studdy "kirchhoff's law"...then "Murphy's law"..
J

[caprotesta]

Wow. Thanks so much for the responses, especially Johan's. It's going to take me a while to digest this. But to answer your question, Johan: yes, it is an EL34.

[Johan]

I think you confused my reply with PRR's, but thanks anyway..

[caprotesta]

Yes. Sorry, I didn't see the break. I appreciate ALL the responses I received, but PRR went above and beyond what I expected and I really appreciate the time spent writing that response. Now it's time to start crunching the numbers!