Coupling caps- can you explain them to me?

[flatfive]

So the higher the value of a coupling cap, more low frequencies get through therefore more bass.

But the higher the value of a bypass cap, fewer low frequencies get through, therefore more treble.

If lowering the value of bypass caps bleeds off more low frequencies to ground, one would assume that a lower value coupling cap would allow more low frequencies through. But this is not the case. Why?

[tubeswell]

Originally Posted by flatfive

But the higher the value of a bypass cap, fewer low frequencies get through, therefore more treble.

No its the other way around - the higher the value of a bypass cap, the more gain there is for more frequencies to get amplified - so a 22uF cap will add overall gain to all the signal (resulting in treble, middle and bass frequencies all getting amplified), whereas a 0.47uF cap will only add gain to the higher frequencies.

[Chuck H]

The reason coupling caps and bypass caps seem to work inversely is that they are not performing in the same circuit. In a plate coupling circuit a cap works as you would expect. Small caps only pass high frequencies and as you increase the cap value you accesss a correspondingly lower frequency. A cathode bypass cap is there to REMOVE AC from the cathode circuit. The presence of AC on the cathode creates local negative feedback. This can improve liearity but at the expense of gain. A bypass cap bleeds AC off of the cathode to ground but leaves the biasing DC on the cathode. So, used in a cathode bypass circuit small values remove high frequencies from the local negative feedback, thus increasing the gain for those frequencies. Increasing bypass cap value produces a correspondingly lower frequency gain increase. The reason the values seem disproportionate (ie: .047uf is a typical coupling cap size but 22uf would be a typical bypass cap size) is because of the two circuits different impedances. The impedance of a circuit is one of the dictating factors for determining a capacitors rolloff frequency.

Chuck

[cbarrow7625]

Is everybody talking with the same terms here? I don't think that "higher" and "lower" values are the best descriptors for caps. "Larger" and "smaller" VALUES for caps is much clearer. Where "larger" = increased capacitance and "smaller" = decreased capacitance.

I have seen a number of times when people say "higher" value, meaning a smaller cap that passes only higher frequencies. High & low are proper for frequencies. To avoid future confusion may I suggest that we standardize on "larger" and "smaller" capacitance values when referring to caps?

[flatfive]

When I say "higher" cap, I mean higher capacitance. But I'm ok with larger and smaller.

[Enzo]

A coupling cap passes the signal along to the next stage. The larger value that cap is, the more lower freqs can pass through it.

In a tube gain stage, the current through the tube causes a voltage drop across the cathode resistor. As the current increases, so does this voltage. That increase in cathode voltage tends to bias the tube to less conductivity. A sort of natural negative feedback. Increasing the currrent through the tube tends to reduce the gain of the tube at the same time.

A bypass cap lets some of the signal bypass that resistor. That means that any signal bypassing it won;t be contributing to that voltage drop, and won't be negating any gain.
So a smaller cap tends to let only highs bypass the resistor, so only highs don;t have their gain negated. The larger the bypass cap, the lower the freq that gets bypassed around that resistor. And thus the lower the freqs that don;t have their gain negated. So the larger the bypass cap, the lower the freqs that get the gain. SO more and more bottom as the value increases.


This may not be a very rigorous description technically, but I hope it makes the concept more understandable.

[EFK]

Enzo thank you for a very clear and simple description!

[flatfive]

Enzo, the only wire that comes out of this cap/resistor goes to ground.

It would seem that any signal (even if it bypassed the resistor) would end up going to ground also. But somehow I don't think this is right.

So this signal that is bypassing the resistor, where is it going?

[Enzo]

Not sure what you mean. The cathode resistor and bypass cap are wired to ground at one end and the cathode of the tube at the other.

The DC tube current flows through the resistor to provide a bias voltage for the tube. The signal would normally do that as well. Adding the caps allows signal of any freq over whatever the cap dictates to be shorted to ground. Any signal thus shorted will not contribute to any voltage forming across the cathode resistor. Only the unbypassed freqs will do that. If they don;t contribute to the cathode voltage, then they don;t participate in gain reduction. SO those freqs are amplified more than the lower freqs which have some of their gain negated by the action of the cathode resistor.

Remember, the tube is not some solid block inside. We can ground the cathode completely with a wire, if we want. The tube will still function. it is not like grounding the cathode - directly or only at certain freqs - is grounding the signal path through the amp. perhaps we should make the understanding that the signal path through the amp is not the same thing as signal passing through a tube. The signal path through a stage is usually in the grid, out the plate, and on to the next stage. But in that process, we put the signal on the grid. The grid is not connected to anything inside the tube, it just exercises its influence over other things to control what comes out of the tube. Just as you can use your fingers to turn the knob on a light dimmer and cause the bulb to change brightness, but your fingers are not connected to the bulb.

The grid affects the voltage at the plate. From that plate we take the varying voltage - which is our ongoing signal - and send it along to the next stage. The signal appears also at the cathode. But it is not conected to either grid or cathode. What we do with the signal that appears at the cathode does however affect what goes on at the plate.

Of course in other circuits like a cathode follower or a phase splitter, we can make use of the signal at the cathode. But in those applications it would generally be counter prodcutive to bypass the cathode then.

[Old Tele man]

...a COUPLING capacitor passes AC-signals through to the next stage.

...a DE-COUPLING or BY-PASS capacitor passes AC-signals through to ground (typically).

[Enzo]

Tele, I could be wrong, but I get the impression he is wrestling with the idea of grounding the AC signal. As if grounding the AC at the cathode would somehow ground off the signal path.

[flatfive]

Thanks for all your help, but I'm just not getting it.

[Enzo]

Well, where are we leaving you in the dust? Are you not getting why it works, or not getting that it works at all? Not getting the distinction that coupling caps (plate to next grid) do something different from bypass caps? (Cathode to ground) Does any part of it make sense yet?

In your post, you used the term "get through" for both situations. Maybe I don;t understand the question.

A coupling cap passes the signal along. Imagine a loudspeaker aiming out the window. We are sending our signal out the window. The larger the speaker the more bottom end we can send out the window. And of course the smaller the speaker the more it sounds like a tweeter.

The bypass cap is not passing the signal along, it is controlling the particular stage response. In post 2 tubeswell mentioned you have the bypass cap thing reversed. The larger the bypass cap value, the more low freq will pass through the stage. More passes through the bypass cap as well, but that doesn;t take away from what passes through the tube.

I tried to explain why a larger cathode cap will make more lows come through the stage than with a smaller cap.

A lower value of bypass cap doesn't bleed off low freqs to ground. Not from your signal path anyway. Without a bypass cap, the cathode resistor all by itself generates some negative feedback. To some extent, the tube fights its own gain. A bypass cap prevents that resistor from developing this gain reducing negative feedback effect, by bypassing the signal influence around the resistor. SO the bypass cap is not bleeding low signal to ground, it is bleeding to ground the cathode resistors ability to fight the tube's gain. A small cap passes only high freqs, so only high freqs get ignored by this natural gain reduction. The larger cap allows lower freqs to also get ignored by the natural gain reduction. The larger the cap, the lower this ignoring goes.

SO you could think of the bypass cap as coupling the signal to the "don;t turn me down" department of the tube. The larger the caps, the lower the freq that won;t get turned down.

[Robert M. Martinelli]

Since My English is rather poor, I'll try to make things clearer with an image...See the ( badly drawn ) attached file.

A capacitor's ability to behave differently at different frequencies is called Capacitive Reactance, indicated on theory books with the term Xc.

You can think a capacitor's Xc as the different resistance the cap opposes to the passage of different frequencies.

The formula to calculate Xc is rather simple : Xc= 1/(2*Pi*F*C) Where Xc is a cap's Capacitive Reactance in Ohms, Pi is 3.1415926..... F is the Frequency in Hertz and C is the cap's capacitance in Farads.

You can clearly see that, being the F factor at the equation divider, the higher the frequency, the lower the capacitor's Xc.

This said, you can have a cap used as an High Pass Filter, HPF ( in series with the signal ) or a Low Pass Filter, LPF ( in parallel with the signal ). In the first case the cap lets all the frequencies above the cutoff frequency ( the frequency at which the signal is attenuated by -3dB, or 0,707 times ) pass through, in the second case the caps shunts to ground all the frequencies above the cutoff frequency ( thus letting the frequencies below the cutoff pass through, hence its name ). With a single cap the HPF/LPF filter slope is -6 dB/Octave ( doubling the frequency the signal's amplitude gets halved ).

Hope this helps

Best regards

Bob

[tubeswell]

WTF - I am equally comfortable with the terms bigger and smaller, and higher and lower, recognising that some people refer to use one or the other - but I get what the OP meant (which is what counts).

Anyhow here is a handy page of a triode gain stage explaining the various bits

Tube Terminology for Dummies

[flatfive]

I have trouble with all this terminology, and am trying to visualize things. Is voltage being applied to the cathode? Looking at the schematic I don't see any. So how is the resistor working? How is the cathode negatively charged in relation to the plate? If the flow of electrons is going through the resistor then it seems that is must be flowing from ground. That would make sense, except now the capacitor wouldn't be sending anything to ground.

[Chuck H]

Originally Posted by flatfive

I have trouble with all this terminology, and am trying to visualize things. Is voltage being applied to the cathode? Looking at the schematic I don't see any. So how is the resistor working? How is the cathode negatively charged in relation to the plate? If the flow of electrons is going through the resistor then it seems that is must be flowing from ground. That would make sense, except now the capacitor wouldn't be sending anything to ground.

This isn't really a good place to ask this sort of question. There is alot of information available and it's really up to you for the most part to learn basic tube operation. It's actually a little insulting because with good info available (web, books, etc.) your demonstrating that our time is not as valuable as yours. At least to you. Of course we want to help, but it's very hard to get the point across when the terminology and concepts go over your head. We keep trying to say the same thing in different ways but it's not translating. I'm sure it's frustrating for you as well.

As for preamp tubes... Electrons move through a tube from the cathode and out the plate. The grid controls the flow. The small resistance placed between the cathode and ground elevates the cathode from 0 volts. This creates a small positive charge on the cathode that makes it positive with respect to the grid, or conversely, the grid negative with respect to the cathode. You could also ground the cathode and apply a bias voltage to the grid of the tube instead as is done with most power tubes. Though it's never done with preamp tubes in guitar amps, the point is that it's basically the same thing. This is the tubes bias. It's there to make the tube behave in a stable manner and sets the stage for tubes primary function as an amplifier. More voltage at the cathode sets the tubes central operating point closer to cutoff and less voltage sets that point closer to saturation.

Now...

When the tube is in operation it is amplifying the signal at the grid. So, not only a small part of the tubes DC operating voltage, but now AC voltage can also appear at the cathode because it's elevated from 0 volts. Whether because of internal negative feedback or additional cathode voltage or both, this AC voltage on the cathode results in a decrease in the tubes amplification potential. So it stands to reason that if we remove the AC voltage from the cathode we can increase the amplification. We still need to keep the tube biased for stable operation. Since capacitors pass AC voltage and block DC voltage we use a capacitor to pass signal from the cathode to ground. The fact these capacitors are typically placed across the cathode resistor is incidental. It's a capaitor to ground removing some AC from THAT part of the circuit, where the presence of AC actually reduces gain.

That's the best I can do.

Chuck

[flatfive]

I knew I should have kept my mouth shut. Believe me, I've tried to study this stuff and it's pretty hard to grasp when every time you read an explanation it introduces another new complex subject. All this effort and I'm no smarter than I was! Now I guess it's time to go back and just play the guitar...

[tubeswell]

Originally Posted by flatfive

I have trouble with all this terminology, and am trying to visualize things. Is voltage being applied to the cathode? Looking at the schematic I don't see any. So how is the resistor working? How is the cathode negatively charged in relation to the plate? If the flow of electrons is going through the resistor then it seems that is must be flowing from ground. That would make sense, except now the capacitor wouldn't be sending anything to ground.

I started my reply but somehow my computer wouldn't let me load it once I'd finished, and now Chuck has chimed in, but here is my 2Cw nevertheless.

I will assume we are talking about a conventional pre-amp triode gain stage (and not anything fancy like a cathode follower or a phase inverter or an output tube)

See if you can read the downloadable chapter on this page. It explains a lot of things

The Valve Wizard

The cathode is heated in order to enable it to more freely release electrons.

A high +ve voltage is applied to the plate (or anode in the UK), and so the electrons being freed at the cathode are attracted to the plate.

On the way from the cathode to the plate the electrons pass by/through the control grid. If there is an a/c signal on the control grid, this is "picked up" by the electric field flowing to the plate (which is at a high voltage) and gets 'acknowledged' by the voltage on the plate which, by virtue of the plate resistor, varies in accordance with the waivering signal now flowing to it via the control grid.

The plate, being at a high +ve voltage now also has a waivering voltage signal in addition to the high voltage. This waivering signal at the plate is bigger than the waivering signal at the grid, because of the plate resistor, which magnifies and reflects the swings in the voltage coming to the plate, allowing the voltage at the plate to vary compared to the fixed voltage on the DC supply side of the plate resistor.

This bigger waivering voltage (we will call it the 'amplified signal') is now bled off the plate with another wire that goes to the grid of the next stage. Only we don't want any direct current/or high voltage on the next stage's grid, so we block it with a coupling cap, that lets the amplified signal pass by but stops the high DC voltage from getting to the grid of the next stage.

Now back to what is happening at the cathode.

The cathode and the control grid need to be set up with their own 'base voltages' in such a way that when a signal is applied to the grid, the electrons will continue to flow from the cathode to the plate no matter what signal is being fed into the grid. This is called 'biasing'.

So we set the cathode at a slightly more positive voltage than the grid (but still far less than the plate) - usually between 1-3V or so. With cathode-biased tubes, this is done by inserting a cathode resistor between the ground return path, and the cathode. So now electrons (that are being drawn from the ground return to go to the plate) pass through the cathode resistor first.

This resistor effectively inhibits the flow of some of the electrons, leaving the cathode end of the resistor (and the cathode) slightly more +vely charged than the ground return path (and the grid).

However, due to the fact that we now have a resistor between the ground return and the cathode, when the much higher plate voltage starts to vary because of the disturbance created by the signal injected from the grid, this has an effect on the voltage at the cathode, which varies slightly up and down in sympathy with the varying voltage in the tube.

This effectively reduces the overall amount of voltage between the cathode and the plate slightly, and hence reduces the amplified signal slightly from what it otherwise might be if the cathode could be held at a steady bias voltage.

So we add a cathode bypass cap in parallel with the cathode resistor. The cathode bypass cap charges slightly on the cathode voltage 'up' swings and discharges on the cathode voltage 'down' swings, keeping the cathode voltage more constant, and therefore enabling a potentially slightly bigger overall voltage to be seen between the cathode and the plate enhancing the amplification of the tube.

The size of the cathode bypass resistor determines which frequencies will get held constant at the cathode. A larger/higher () value cap will hold the cathode voltage constant for more frequencies, and a smaller/lower value cap will only hold cathode voltage constant for the higher frequencies more.

[defaced]

That's a pretty solid, simple, step by step explanation. Had I had something like that when I started looking at tube schematics and thinking to myself "what the flying _ is going on here?", I would have started building amps many years earlier than when I did.

[flatfive]

defaced, I have to agree. tubeswell has made things clearer now. At least I now know that I had the direction of the current backwards. Probably from looking at the 5E3 layout too much. Seeing that the current passes first through the cathode resistor helps a bunch and I had been looking for that information.

And now I see how the bypass capacitor charges and discharges, allowing different frequencies to pass, based on the value of the capacitor.

Chuck H, I'll be re-reading your posts to look for more.

Didn't mean to insult anyone. I will try to learn more basics before starting a thread like this again.

[defaced]

At least I now know that I had the direction of the current backwards.

Stop for a second. Be very clear in your head, and when you're looking at things, that there is a clear and distinct difference between "current flow" and "electron flow". Current flow, which is the "conventional" direction (it is also incorrect, but that's a different story) is that charge flows from positive to negative. Electron flow, which is technically correct but too late to fix it now, is that charge flows from negative to positive. For me, I understand things fundamentally in electron flow, but for looking at a schematic, I look at it in terms of current flow.

When you were originally looking at things, you had current flow correct, but you didn't know that it was backwards from what was actually happening. Now re-read tubeswell's post and note that he explicitly said "electrons", not "current". I'll guess that was intentional.

[Koreth]

I started to type up an explanation, but Chuck H said it clearer than I could, and with fewer words. Though you said you've tried to study this stuff before, I think you should keep at it. If the explanations you find keep introducing more complex concepts, then perhaps you're trying to swallow too much at once. If you don't have a strong grasp of basic electronic theory, you're going to have a harder time studying and manipulating the circuits in tube amps. It's much like playing guitar. If a person doesn't know his chords and scales and similar guitar basics (the theory of playing) he's going to have a much harder time of playing the guitar than if he did. If I had to guess (and just guessing, not asserting), perhaps the sources you've been using to try to study tube amps are assuming you understand more about electronic theory than you do? That would certainly make learning tube amp circuites more difficult than it needs to be.

If you want to keep at it, I highly recommend this article. It is a preview of recently released book written by a board member here. It does a very good job of explaining the basic vacuum tube amplifier, what all the components in said amplifier do and why. There is technical jargon, but it doesn't drown you in the jargon, with the assumption you already know what the jargon means, it takes time to explain each concept. If you have an understanding of Ohm's law, and can do basic algebra (which would probably be needed to fully understand Ohm's law anyway) the article will help you out. A week ago, I was still struggling to understand the basics of a tube amplifier circuit, and how the heck all the parts worked together. I've not even finished reading that article yet, but the way everything is explained, everything is starting to click for me. An hour with that article and I learned more about a tube amp circuit than I did from months of staring at diagrams, and reading articles that were way over my head.

Some patient, unhurried reading of that article will really help you wrap your head around tube amps.

[Enzo]

Good discussions are going on here. Good.

Flat, you said one thing that I want to respond to. you mentioned voltage on the cathode that didn;t seem to be applied from anywhere. I think it was pointed out that it COULD be applied, but it basically makes its own voltage most times. The reason I bring it up is Ohm's Law. Look it up in any electronics material. If you learn nothing else, learn Ohm's Law. I have been in electronics over 50 years, and believe me I use Ohm's Law every day. Not in some cosmic sense like my mom's car uses thermodynamics in its engine, I mean i directly use it to find some voltage or current or resistance.

Ohm's Law is the simple description of the relationship between voltage, current, and resistance. If you know two of those, you can get the third with simple arithmetic.


When current flows through the tube, it also flows through the cathode resistor. The resistor is part of the path the current takes through the circuit. Ohm's Law tells us that when current flows through a resistance, a voltage will appear (or "drop") across that resistor. It is the current through the cathode resistor that creates the voltage across it. Across meaning end to end of it. if one end is grounded, then the other end will have some voltage on it. the polarity of that voltage depends upon the direction the current flows.

AN example:
Ohm's Law says - voltage = current times resistance
When I learned it they said "E = I x R" I think these days they use V instead of E. so V = I x R. And of course you can invert that into I = E/R and so on.

If your tube has a typical 1 milliamp of current flowing (1ma, which is also .001 amp) and your cathode resistor is 1.5k ohms (1500 ohms), then we can use Ohm's Law to determine that the voltage across that resistor will form and be V = .001 x 1500 = 1.5v


You signal through the amp makes the current through the tube vary along with it. That varying current causes a varying voltage to form across the cathdoe resistor. or better stated, the signal causes the otherwise steady cathode voltage to vary.

[cminor9]

flatfive, I wouldn't worry too much for now. What you are asking about shouldn't stop you from assembling an amp or even fixing one. It might prevent you from tuning your preamp differently in some fashion other than trial and error or by following general guidelines (larger value cathode bypass caps for more bass, smaller to attenuate some bass.) It might stop you from designing a circuit in a knowledgeable way.

Think of all the things that were once a mystery to you but now make sense. Many things are very Zen: you don't understand them until you put them into practice, then only once you've done it do you know why. I predict this too will make sense in time.

Keep building amps, and keep playing guitar.

That's my advice, a fellow novice amp builder who tries to grasp the conceptual stuff but doesn't always get it at first. My failure to get it at first is usually due to my impatience and desire to grasp large things before fully grokking the basics.

I have built two complete amps, tinkered with other beginnings of projects, and fixed many with my new-found skills. I learn something each time. Come back in six months and re-read this thread if you still don't get it. I'd bet more will make sense. I bookmark certain threads and come back, and dang it if I don't understand more than the last time I read them.

[Enzo]

Well that is certainly true.

To this day I have these AHA moments where something finally clicks into place. Some concept I never thought through, or something I never fully understood, or something I had flat out wrong.

And a lot of times some smaller detail doesn;t make much sense by itself, but later when you understand the whole process more, it provides the context for the detail to come into focus.

And then there are the times when you thought you were wrong on something for a long time and finally figure out you had it right after all.

And sometimes you get parts but not the whole thing. A stupid example - when I was learning color codes, the first ones I really internalized were yellow and purple. I couldn;t tell you what half the other colors were, but I knew yellow-purple was 4-7. Lemme see, what is 4-7-orange?

[Steve Conner]

Originally Posted by defaced

there is a clear and distinct difference between "current flow" and "electron flow".

This is unfortunately true. All descriptions of circuit operation that you'll read anywhere are written in terms of conventional current, which flows from positive to negative. But the operation of a tube can only be understood in terms of electronic current: electrons spraying out of the heated cathode and making their way towards the plate.

And of course electronic current is the physical reality, and conventional current just a convention. So to be physically consistent we should speak of electrons coming out of the ground rail, flowing through the cathode resistor, out of the cathode, landing on the plate, going through the plate load resistor, finally to be swallowed up by the B+ rail, where the power supply sucks them in like a sort of electronic vacuum cleaner, removes 250 volts of pressure from them and shoves them back into the ground ready for another trip.

But this is the very opposite of the description you'll read in textbooks. It's also arguably less intuitive, because the flow of electrons runs in the opposite direction to the "pressure" (voltage) that drives it, and this was the reason for adopting conventional current in the first place, to get away from this "water flowing uphill" feeling. Defining voltage the other way round would have worked too, but the pioneers of electricity chose to switch the direction of current flow.

So, like all practicing engineers, my mental models of circuit operation work with conventional current, because that's what I was taught. But when thinking of device operation (tubes, transistors, diodes and so on) you must remember to think in terms of electron flow instead.

[Enzo]

I'm with you there. All my life I have been thinking in conventional current - positive voltage seeking ground - all the while realizing that the stuff really works on electron flow which is the other way. That is why the arrows on diodes point the way they do - the reverse of electron flow, but in the direction of conventional current.

[flatfive]

Just when I thought I was getting somewhere...

[defaced]

Don't get frustrated. I've got an engineering degree and was tearing my hair out just a couple of months ago trying to figure out transistors and having this same problem: what's moving in what direction? No site that I could find made it clear where the power supply was or what direction either current or the electrons were going or where the load was, and I sold my EE text book so I was up a creek on that front. Every place used a different convention or left out information that I needed to piece it all together. I finialy got it, but just by staring at schematics and doing a couple of simple tests.

[Chuck H]

In reality I'm just a hack that builds amps. I've never studied electronics and I'm not even able to read calculus formulas because I never learned the language (though I see that changing in my future). And I just want to say to any greenie who has ever been frustrated trying to figure out just what the $%#& is going on inside a tube amp that I wish I had a post like this to read seventeen years ago when I was just starting out. Even with good mechanical and conceptual skills it's been a long road to being able to think outside the box on issues like 'electrons aren't water', the differences between DC and AC voltages and impedance vs. resistance. Some of the fundamental issues here could have saved me alot of time and effort (and painful shocks)

Thank you to Enzo and Steve who are our resident experts on this particular post. I look foreward to your corrections (even if I don't always handle it gracefully).

Chuck

[Steve Conner]

Originally Posted by defaced

Don't get frustrated. I've got an engineering degree and was tearing my hair out just a couple of months ago trying to figure out transistors and having this same problem: what's moving in what direction?

I don't blame you! When it comes to junction transistors, they really are confusing. For a start, they use two kinds of charge carriers - electrons and "holes" - that carry opposite charges and flow in opposite directions at once. This is why they're called "bipolar" transistors, as opposed to MOSFETs and vacuum tubes, which only use one kind of carrier.

To make matters worse, they come in two varieties, NPN and PNP, that are exact mirror images of each other, with the electrons and holes changing places. (To be pedantic, the "N" parts of the device contain electrons, and the "P" regions contain holes. MOSFETs also come in P-channel, where the charge carriers are holes only.)

And as the final insult, schematics of old transistor circuits don't always follow the modern practice, of drawing the circuit so that more positive voltages are towards the top of the page, and conventional current flows from the top of the page towards the bottom.

According to this drawing convention, PNP transistors should always have their emitters pointing towards the top of the page, and the ground rail in a PNP transistor circuit should be at the top of the page, with the battery "dangling" from it by its positive terminal. I find schematics that don't strictly follow this practice almost impossible to understand, and often have to redraw them before I get it.

But, to mentally model the operation of a transistor in a circuit, all you really need is Horowitz and Hill's "Transistor Man". Except for PNP you have to glue his feet to the ceiling, as it were.