Technical question about cathode biasing

[EtherealWidow]

Ok. So forgive me if this question is a little bit simple, but I'm still just learning how amplifiers work. I'm reading some old NEETS manuals that get very in depth to how exactly electricity works and its applications. I'm at the part about how vacuum tubes work. They have a schematic of a cathode-biased triode. Certainly something I'm familiar with by now, but their triode is battery powered. In their schematic, they have the plate connected to the + terminal of the battery and the cathode connected to the -. Makes sense to me. What doesn't make sense to me is amplifier schematics. In most guitar amp schematics, the plate of a 12AX7 is connected to B+ with a 100K resistor and a coupling capacitor and the cathode is simply connected to ground through a cathode resistor and a cathode bypass cap. My question is this: How is current getting to the cathode? Is it being coming FROM ground? I thought everything goes TO ground. I guess it could make sense if the plate is positive relative to ground and therefore draw current from ground. The P1 theory document from AX84 http://www.ax84.com/p1.html goes into a brief overview of conventional current and then goes on to say that current is flowing FROM the plate TO the cathode to ground. Impossible. The plate would be arcing to the cathode. That's bad. Maybe their wording is just weird or it's just the fact that they're explaining it with conventional current. You guys, help me out. What's going on inside our amplifiers?

 

[shytfayse]

In fact the electrons are flowing from the negative cathode (a lot of free, negatively charged electrons) to the positive plate (the absence of electrons). Somebody out there thought teaching the reverse, conventional current flow, or hole flow (the absence of electrons), from positive to negative, would seem simpler. The grid and screen elements are there to control the flow of electrons through the vacuum. As the grid goes more positive there is more electrons flowing between the cathode and the plate. The inverse is true as the grid goes more negative. There are two common types of biasing - grid and cathode. Biasing a tube really is a means to keep the tube running within it's active state and within the operating parameters of the tube. Without proper biasing the tube can clip (too hot/maximum current) or go into cut-off (too cold/no current) or even run away and blow up. Grid biasing places a fixed negative voltage on the grid that allow the tube to draw idle current between the cathode and plate at a steady rate that is within the tube's operating limits. You see this most often in the push-pull power section of class A/B tube amps. Cathode biasing places a resistor in the cathode circuit to limit the amount of current flowing from cathode to the plate that is within the tube's operating limits. You see this more often on single-ended class A amps. The AX84 community has a lot of single ended amp projects on their board because of thier simplicity and they are all cathode biased.

I know this is quite a bit of stuff to plow through. Keep reading and learning!

- Pat

 

[EtherealWidow]

Hey. Thanks for responding. Since I've posted this I've learned a little more about electronics and applied it to what I know about tube amps. It just really threw me off how in the PT1 article they say that current flows to the plate, through the grid, to the cathode and then to ground, when I know for a fact that current cannot flow from the plate to the cathode. That would severely mess the tube up.

The way I understand amps now is that the ac current off the PT going to the rectifier is changed to a massive positive DC current (essentially a huge vacuum). That is connected to the OT and plates of all the other tubes with voltage dividers between the filter caps. The massive DC current "vacuum" draws current from all the plates feeding the signal from gain stage to gain stage, eventually coming to the power tubes, and then dragging that signal across the primary coil of the OT, inducting that modified and amplified guitar signal to the OT secondary, and then to the speaker.

Am I on target?

 

[shytfayse]

Hey. Thanks for responding. Since I've posted this I've learned a little more about electronics and applied it to what I know about tube amps. It just really threw me off how in the PT1 article they say that current flows to the plate, through the grid, to the cathode and then to ground, when I know for a fact that current cannot flow from the plate to the cathode. That would severely mess the tube up.

It really does not flow from plate to cathode, but, the opposite. You really have to have a grasp of electron flow vs hole flow. You have to know current is electrons, the negatively charges particle of an atom, moving though a conductive media. The practice of looking at current flowing from negative to positive is called actual current flow or electron flow. You can flip that around though. You can visualize the positive side of the circuit as atoms being void or missing electrons, which are called holes. The holes are attacted towards the negative which atoms an abudance of free electrons. This is called hole flow or conventional current flow. Somebody out there, a long time ago, decided to attach the negative side to earth nd/or the chassis as a safety measure, I don't know why, and this IMO is a big reason why conventional current is the most widely accepted method to teach electronics. Flip the coin and attach the positive side to earth and we have actual current flow.

The way I understand amps now is that the ac current off the PT going to the rectifier is changed to a massive positive DC current (essentially a huge vacuum). That is connected to the OT and plates of all the other tubes with voltage dividers between the filter caps. The massive DC current "vacuum" draws current from all the plates feeding the signal from gain stage to gain stage, eventually coming to the power tubes, and then dragging that signal across the primary coil of the OT, inducting that modified and amplified guitar signal to the OT secondary, and then to the speaker.

Am I on target?

Think of voltage as the potential for current flow. A battery can just set there, but without completing the circuit from negative to positive there is no current flow. The higher the voltage - the higher the potential for current to flow. I can't get a 9 volt battery to arc over, between the terminals, in air, but, increase the voltage to 10,000 volts it will probably start to arc through the air. The only vacuum is the absence of matter within a tube.

 

[EtherealWidow]

Right. That's what I'm saying. Conventional current is really a horrible way for explaining how vacuum tubes work, being that electrons in reality flow from negative to positive. When I posted this, I just hadn't known that you could draw current FROM ground. I was always under the impression that it had to flow TO ground. Now I know better.