Cathode bypass capacitor question

[sand]

Hello, I'm new here and relatively new to tube electronics. I read everything I can get my hands on, books and online info. For some reason weird I love the theory part more than the practical part... and the 'electrons path of travel' thread is great, in spite of the sparks. How can you avoid sparks when we're talking about electricity

I have my own questions, related to electrons flow. I hope I'm not asking too much...

I'm reading Navy module 6, where it talks about cathode biasing. I understand what cathode biasing does, what throws me off is what the module says about the bias capacitor here RF Cafe - Navy Electricity and Electronics Training Series (NEETS), Module 6

Figure 1.23.(A) is pretty obvious. For 1.23.(B) it says that "In view B, the positive-going signal is applied to the grid. This causes increased conduction through the tube, which attempts to drive the cathode to +20 volts. But notice that the top of Ck is still at +10 volts (remember capacitors oppose a change in voltage). The top plate of Ck is, in effect, 10 volts negative in relation to the top of Rk." So far, so good...

Now comes the part I don't really get "The only way that Ck can follow the signal on the top of Rk (+20 volts) is to charge through the tube back to the source, from the source to the lower plate of Ck. When Ck charges through the tube, it acts as the source of current for the cathode. This causes the cathode to remain at +10 volts while the capacitor is charging."

So...why does Ck act "as the source of current for the cathode"? The top of Ck is at +10V, while the top of R is at +20V. Isn't the top of Rk the source of current for the cathode?


Next question... Figure 1.23.(C) says "Under these conditions", which I guess it means in response to what happened in figure (B), "conduction through Rk will decrease". Why is that? Thank you.

[tubeswell]

So...why does Ck act "as the source of current for the cathode"? The top of Ck is at +10V, while the top of R is at +20V. Isn't the top of Rk the source of current for the cathode?

The cap stores 'charge' on a short term basis while the current through the tube (and potential voltage at the cathode) is increasing and then releases it again when the current through the tube (and the potential voltage at the cathode) is decreasing. In this way it 'shunts' AC (at the cathode) to ground. This increases the potential voltage between the cathode and the plate (more than it otherwise might be without the bypass cap there), because without the bypass cap, the cathode voltage would swing up and down slightly in-sympathy with the plate (because the varying (AC) tube current would otherwise cause the voltage dropped across the cathode resistor to increase and decrease respectively with the changes in cathode/tube current) thus reducing the plate-to-cathode voltage slightly, and thus reducing the overall gain. By putting a bypass cap in you are 'anchoring' the cathode voltage allowing the potential plate-to-cathode voltage (and hence overall gain) to be higher on the signal swings.

The size of the cap determines the rate at which it stores and releases 'charge' i.e.: smaller caps work on shorter AC wavelengths/higher frequencies. Hence you use a smaller value cap for 'treble boost' (because it only stores and releases the charge for higher frequencies).

[sand]

Thanks, tubeswell, and sorry if I'm too dense... I understand your explanation of the role and behavior of the bypass cap, I'm afraid I just don't "see" yet how it is possible for the lower voltage of Ck to be the source of current, instead of the higher voltage or Rk :-(

[tubeswell]

They mean that the extra charge in the Ck 'tops-up' the supply of current to the tube via the cathode. The Ck and the Rk are at the same potential at the cathode, but the Ck momentarily (or instantaneously) gets a fresh and fast in-rush of charging current when the tube current increases, and then (more) slowly releases it again when the tube current decreases (like a filter cap does when it is filtering rectified pulses of current)

[sand]

Yeah, that's very likely. I realize it's not easy to describe such an instantaneous and ongoing process. Thanks a lot!

[Enzo]

In some of those threads, we can get into tiny details. Those details matter in the context of the discussion, but not in the real world of working on amps. In my world of fixing them, I care about current, and I know what the relationship between current and voltage will mean in the plate or cathode resistor. That is Ohm's Law after all. At those times, I really don't care or need to know whwich way the electrons move in the wires. I just need to know what voltages i will find and where.

Now I am all for understanding the physics, it can;t hurt and ultimately enhances your understanding of amps. But whenever your eyes start to glaze over, think a moment about what the issue ultimately will mean to the amp. If you come away understanding that that bypass cap tries to keep the cathode voltage steady, you have got the lesson.

When we get into calling the cap a source of current, then you have to back away from that practical level and dive into the theory level.

Tubewells description works for me. MAybe put in different terms, that cap is both a source and sink of current. When the tube current increases, it WANTS to increase the voltage across the resistor. But the cap will be sucking up that extra current until the cap is full. SO the voltage can;t rise, only the cap charge rises.

In your example, the top of Ck is +10, the top of Rk wasn;t +20, it was TRYING to get to +20. It is the charging current of Ck that kept the voltage at +10. SO instead of the current through Rk increasing, the charging current of Ck serves the tube.

And I could be mistaken, but I think we may have just run into that whole electrons versus conventional current thing. If you seem to understand how things are working, but the explanation of the currents seems backwards to you, then you probably DO get it, but don;t realize they are talking conventional current instead of electrons.

[sand]

In some of those threads, we can get into tiny details. Those details matter in the context of the discussion, but not in the real world of working on amps.

I agree, and I love those threads Learning why some component works the way it does is very exciting. Maybe it makes me feel young, in school again

If you come away understanding that that bypass cap tries to keep the cathode voltage steady, you have got the lesson.

Yeah, I got that lesson. However, there are lots of things I still have to learn and because of that I'm not confident in whatever knowledge I already gathered.

[enorbet]

Greetz
If you love understanding more deeply how each component functions Amazon.com: Radiotron Designer's Handbook: F. Langford-Smith: Books is the Holy Grail. If you enjoy seeing a wide variety of circuits and how they relate to sound you may find Amazon.com: Audio Cyclopedia (9780672206757): Howard M. Tremaine: Books as engaging a read as any mystery novel and a lot more useful and satisfying, not to mention idea provoking.

[tedmich]

Greetz
If you love understanding more deeply how each component functions Amazon.com: Radiotron Designer's Handbook: F. Langford-Smith: Books is the Holy Grail. If you enjoy seeing a wide variety of circuits and how they relate to sound you may find Amazon.com: Audio Cyclopedia (9780672206757): Howard M. Tremaine: Books as engaging a read as any mystery novel and a lot more useful and satisfying, not to mention idea provoking.

If you appreciate older classic texts, don't forget to check Pete Millet's treasure trove of older out of print tube/electronics books:
Technical books online